v>EPA
United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park NC 27711
EPA-450/2-78-042b
October 1978
Air
Stack Sampling
Technical Information
A Collection of
Monographs and Papers
Volume II
1
-------
EPA-450/2-78-042b
Stack Sampling Technical Information
A Collection of Monographs and Papers
Volume II
Emission Standards and Engineering Division
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air, Noise, and Radiation
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
October 1978
-------
This report has been reviewed by the Emission Standards and Engineering
Division, Office of Air Qual ity Planning and Standards, Office of Air, Noise
and Radiation, Environmental Protection Agency, and approved for publica-
tion. Mention of company or product names does not constitute endorsement
by EPA. Copies are available free of charge to Federal employees, current
contractors and grantees, and non-profit organizations - as supplies permit
from the Library Services Office, MD-35, Environmental Protection Agency,
Research Triangle Park, NC 27711; or may be obtained, for a fee, from the
National Technical Information Service, 5285 Port Royal Road, Springfield,
VA 22161.
Publication No. EPA-450/2-78-042b
u
-------
PREFACE
The Clean Air Act of 1970 requires the Administrator of the
Environmental Protection Agency to establish national emission standards
for new stationary sources (Section 111) and hazardous air pollutants
(Section 112). The development of these emission standards required the
concurrent development of reference test methods and procedures. The
reference test methods and procedures are published in the Federal Register
along with the appropriate regulations.
From time to time, questions would surface concerning the methods and
procedures. In many cases, specific studies would be needed to provide
informed, objective answers. The papers and monographs resulting from these
studies were usually distributed to people involved in emission measurement;
a major method of distribution has been the Source Evaluation Society
Newsletter.
To provide a readily available resource for new and experienced personnel,
and to further promote standardized reference methods and procedures, it has
been decided to publish the papers and monographs in a single compendium.
The compendium consists of four volumes. The Table of Contents for all
four volumes is reproduced in each volume for ease of reference.
Congratulations and sincere appreciation to the people who did the
work and took the time to prepare the papers and monographs. For the most
part the work was done because of personal commitments to the development
of objective, standardized methodology, and a firm belief that attention
to the details of stack sampling makes for good data. The foresight of
Mr. Robert L. Ajax, the former Chief of the Emission Measurement Branch and
now the Assistant Director, Emission Standards and Engineering Division, in
providing the atmosphere and encouragement to perform the studies is
gratefully acknowledged. Tbe skill and dedication of Mr. Roger Shigehara,
in providing personal supervision for most of the work, is commended.
Don R. Goodwin
Director
Emission Standards and
Engineering Division
111
-------
VOLUME I
TABLE OF CONTENTS
Method for Calculating Power Plant Emission Rate 1
by R. T. Shigehara, R. M. Neulicht, and W. S. Smith
Emission Correction Factor for Fossil Fuel-Fired Steam 10
Generators (COp Concentration Approach)
by R. M. Neulicht
Derivation of Equations for Calculating Power Plant Emission 20
Rates (02 Based Method - Wet and Dry Measurements)
by R. T. Shigehara and R. M. Neulicht
Summary of F Factor Methods for Determining Emissions from 29
Combustion Sources
by R. T. Shigehara, R. M. Neulicht, W. S. Smith,
and J. W. Peeler
Validating Orsat Analysis Data from Fossil-Fuel-Fired Units 44
by R. T. Shigehara, R. M. Neulicht, and W. S. Smith
A Guideline for Evaluating Compliance Test Results 56
(Isokinetic Sampling Rate Criterion)
by R. T. Shigehara
-------
VOLUME II
TABLE OF CONTENTS
A Type-S Pi tot Tube Calibration Study 1
by Robert F. Vollaro
The Effect of Aerodynamic Interference Between a Type-S 24
Pi tot Tube and Sampling Nozzle on the Value of the
Pi tot Tube Coefficient
by Robert F. Vollaro
The Effects of the Presence of a Probe Sheath on Type-S 30
Pi tot Tube Accuracy
by Robert F. Vollaro
An Evaluation of Single-Velocity Calibration Technique as 48
a Means of Determining Type-S Pitot Tube Coefficients
by Robert F. Vollaro
Guidelines for Type-S Pitot Tube Calibration 63 -
by Robert F. Vollaro
The Effects of Impact Opening Misalignment on the Value of 89
the Type-S Pitot Tube Coefficient
by Robert F. Vollaro
Establishment of a Baseline Coefficient Value for Properly 95
Constructed Type-S Pitot Tubes
by Robert F. Vollaro
A Survey of Commercially Available Instrumentation for the 104
Measurement of Low-Range Gas Velocities
by Robert F. Vollaro
The Use of Type-S Pitot Tubes for the Measurement of Low 122
Velocities
by Robert F. Vollaro
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VOLUME III
TABLE OF CONTENTS
Thermocouple Calibration Procedure Evaluation 1
by Kenneth Alexander
Procedure for Calibrating and Using Dry Gas Volume Meters 10
As Calibration Standards
by P. R. Westlin and R. T. Shigehara
Dry-Gas Volume Meter Calibrations 24
by Martin Wortman, Robert Vollaro, and Peter Westlin
Calibration of Dry Gas Meter at Low Flow Rates 33
by R. T. Shigehara and W. F. Roberts
Calibration of Probe Nozzle Diameter 41
by P. R. Westlin and R. T. Shigehara
Leak Tests for Flexible Bags 45.
by F. C. Biddy and R. T. Shigehara
Adjustments in the EPA Nomograph for Different Pitot Tube 48
Coefficients and Dry Gas Molecular Weights
by R. T. Shigehara
Expansion of EPA Nomograph (Memo) 60
by R. T. Shigehara
EPA Nomograph Adjustments (Memo) 63
by R. T. Shigehara
Graphical Technique for Setting Proportional Sampling 65
Flow Rates
by R. T. Shigehara
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VOLUME IV
TABLE OF CONTENTS
Recommended Procedure for Sample Traverses in Ducts Smaller 1
Than 12 Inches in Diameter
by Robert F. --Vollaro
Guidelines for Sampling in Tapered Stacks 24
by T. 0. Logan and R. T. Shigehara
Considerations for Evaluating Equivalent Stack Sampling 28
Train Metering Systems
by R. T. Shigehara
Evaluation of Metering Systems for Gas-Sampling Trains 40
by M. A. Wortman and R. T. Shigehara
An Evaluation of the Current EPA Method 5 Filtration 49
Temperature-Control Procedure
by Robert F. Vollaro
Laboratory Evaluation of Silica Gel Collection Efficiency 67
Under Varying Temperature and Pressure Conditions
by Peter R. West!in and Fred C. Biddy
Spurious Acid Mist Results Caused by Peroxides in Isopropyl 79
Alcohol Solutions Used in EPA Test Method 8 (Memo)
by Dr. Joseph E. Knoll
Determination of Isopropanol Loss During Method 8 Simulation 80
Tests (Memo)
by Peter R. West!in
Comparison of Emission Results from In-Stack Filter Sampling 82
and EPA Method 5 Sampling
by Peter R. West!in and Robert L. Ajax
EPA Method 5 Sample Train Clean-Up Procedures 98
by Clyde E. Riley
viii
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October 15, 1P75
A TYPE-S PITOT TUBE CALIBRATION STUDY
Robert F. Vollaro
INTRODUCTION
A study in which 51 Type-S pi tot tubes were calibrated against a standard
(Type-P) pi tot tube was recently undertaken in response to growing concern over
reports of pitot calibration work in which certain observers had obtained Type-S
pitot coefficient values consistently below the range 0.83 to 0.87. The 51
Type-S tubes selected for calibration varied a great deal in physical condition
and geometry. Some of the tubes were commercial models, representing various
manufacturers; the rest had been made within the U. S. Environmental Protection
Agency. This paper discusses the calibration study, its results, and its signi-
ficance.
PRELIMINARY CONSIDERATIONS
The following things were done prior to calibration:
1. Each Type-S pitot tube was assigned a permanent identification number.
2. Each pitot tube was assigned an "A" side and a "B" side. Both the A
and B sides were calibrated (see Figure 1).
3. The physical condition of each Type-S pitot tube was evaluated. The
appearance of each tube was described in detail and, when necessary,
sketches were made to supplement the verbal description.
Figure 1. Type-S pitot tube, top view, with A and B sides
marked.
-------
4. The following dimensions of each tube were measured and recorded:
a. Tube length, in inches, measured from the center of the impact
openings to the quick-disconnect fittings (Figure 2, dimension a);
b. Distance between the A-side and B-side impact openings*, expressed
in inches (Figure 2, dimension b);
Figure 2. Measurement of Type-S pitot tube length (dimension "a^") and impact-plane
separation distance (dimension "b").
c.
Length and width, in inches, of A-side and B-side elliptical impact
openings. Since some of the tubes were made of thin-walled stainless
steel and others of heavy-walled material, impact opening dimensions
were measured as shown in Figures 3a (thin-walled) and 3b (heavy-
walled).
T
W
_1_
Figure 3a. Measurement of Type-S pitot
tube impact-opening dimensions (thin-
walled tube).
W
Figure 3b. Measurement of Type-S pitot
tube impact-opening dimensions (heavy-
walled tube).
* Measured with a digital micrometer.
-------
5. The alignment of the A-side and B-side impact openings of each tube
was checked, as follows:
a. First, the tube was examined in end view to determine whether its
impact planes were perpendicular to the transverse tube axis (see
Figure 4a). Micrometer readings (M, and f*L in Figure 4b, below)
were taken to confirm the visual observations.*
TRANSVERSE
TUBE AXIS
-IMPACT
PLANES
-Mi-
Figure 4a. Type-S pilot tube, end
view; impact-opening pjanes per-
pendicular to transverse tube axis.
-MZ-
Figure 4b. Micrometer readings MI
and M2, taken to check impact-plane
alignment with respect to transverse
axis.
b. Second, the tube was examined in top view to determine whether its
impact planes were parallel to the longitudinal tube axis (see
Figure 5a). Micrometer readings (M- and M. in Figure 5b) were taken
to confirm the visual observations.**
LONGITUDINAL
TUBE AXIS
ASIDE PLANE
i
Z_
'
* B "N
"fLX 7 f
s J,4
-------
c. Third, the tube was examined in side view (from both sides), for two
specific types of misalignment: (1) length misalignment (A and B
tubes of unequal length) and (2) planar misalignment (impact opening
center-lines noncoincident). Figures 6a, 6b, and 6c illustrate pro-
perly aligned openings, length misalignment, and planar misalignment,
respectively.
-E
Figure 6a. Type-S pitot tube, side view;
impact-openings properly aligned.
1
0
c ^
A \^ ^/.
Figure 6b. Type-S pitot tube, side
view, showing length misalignment
(dimension "X").
Figure 6c. Type-S pitot tube, side view; show-
Ing planar misalignment (dimension "Y")-
EXPERIMENTAL SET-UP
The calibrations were done in a wind tunnel (see Figure 7) consisting of a cen-
trifugal blower with adjustable speed drive unit, a surge tank, and a long, straight
duct section made of 12 in. i.d. smooth-walled polyvinyl chloride (PVC). The pur-
pose of the surge tank was to dampen pulsations in the blower discharge; the long
straight run of pipe was necessary to ensure the presence of stable, well-developed
flow profiles in the test section. Test section velocities during calibration ranged
from approximately 1500 ft/min to 3500 ft/min; the A and B sides of each Type-S pitot
tube were calibrated at six different velocities within this range, spaced at approxi-
mately equal intervals.
-------
VARIABLE-SPEED
BLOWER
SURGE
TANK
Jr-wlll n i tor
0(
V \ o
\ A TEST
J (J SITE
t
D
I
Figure 7. Pitot tube calibration system.
Two test ports were cut in the test section of the PVC duct, 90° apart.*
One port was cut slightly upstream of the other, to ensure that the impact open-
ings of both the standard pitot tube and the Type-S tube would be in the same plane
during calibration (see Figure 8). To minimize misalignment of the pitot tubes
with respect to the flow (yaw and pitch angles), the tubes were not hand-held;
instead, special holders, properly aligned with the ductwork, were used.
Figure 7, for illustrative purposes only, shows the ports 180° apart.
-------
HOLDER
lol STANDARD
^-PITOTTUBE
>SETSCREWS
OK
IMPACT
PLANE
-FLOW-
Jv
SET SCREW-
TYPE-S
PITOT TUBE
HOLDER
Figure 8. Experimental set-up.
An inclined-vertical gage-oil manometer (Dwyer Model 421-10) was used to
read all AP values. The inclined part of the manometer scale had a range of
0 to 1.0 in. of water, graduated in divisions of 0.01 in. H20. All of the cali-
bration data were within this 0 to 1 in. range; AP readings falling in between
two divisions were read to the nearest 0.005 in. FLO, as shown in Figure 9.
The "Experimental Error Considerations" section of the Appendix discusses the
-------
0.298 in. 0.303 in. 0.308 in. 0.313 in.
1
1
1 0.300 in.
|
U- R£AD -*4*- READ -<
| AS
AS
0.300 in. 0.305 in.
1
0.310 in.
i
1
1
4*- READ
I AS
1 0.310 in
1
1
1
i
1
1
-i-l
|
Figure 9. Reading of AP to the nearest 0.005 in. H20.
implications of reading AP this way.
For convenience, the Tygon lines from both the Type-S and standard pitot
tubes were connected to a pair of panel-mounted 2-way valves, which, in turn,
were connected to the manometer. By opening these valves to the correct posi-
tion, either AP or AP . , could be read without disconnecting any pitot lines.
The calibration standard used in these tests was a Prandtl-type pitot tube,
meeting certain design criteria that ensure its coefficient to be 0.99 +_0.01
o yi
(for velocities above 600 ft/min). *
CALIBRATION PROCEDURES
The following procedures were used to perform the A and B side calibrations
of each Type-S pitot tube:
a. The manometer was cleaned, filled, leveled, and zeroed. All pitot
lines and fittings were leak-checked.
b. The standard pitot tube was inserted into the duct, with its im-
pact opening at the duct center.
c. The valves were opened to the AP ., position.
-------
d. The fan was turned on to setting #1; the flow was allowed to
stablilize.
e. The value of AP , , was read and recorded.
f. The standard pitot tube was withdrawn from the duct.
g. The Type-S pitot tube was inserted into the duct, with its im-
pact opening at the duct center.
h. The valves were positioned to read AP .
i. The value of APg was read and recorded.
j. The Type-S pitot tube was withdrawn from the duct.
k. The standard pitot tube was reinserted into the duct; the valves
were re-positioned to read AP ...
Steps d through k above were repeated at fan settings 12 through #6.
CALCULATIONS
The following formula was used to determine the coefficients of the Type-S
pitot tubes:
/AP_. .
C = C (Standard)/-/^ (Equation 1)
v v - Qr
where:
C = Type-S pitot tube coefficient
C (Standard) = coefficient of standard pitot tube = 0.99
AP £, = standard pitot tube reading (in. H?0)
APg = Type-S pitot tube reading (in. H20)
-------
For each calibration (A or B side), six values of C were computed using
the above formula, i.e., one at each fan setting. From these six C values, an
average coefficient was determined, as follows:
-
C (A or B side) = -^ (Equation 2)
SUMMARY OF RESULTS
A. Preliminary Considerations - The results of the preliminary examinations
and measurements of the 51 Type-S pitot tubes are presented in Table I (see Appendix).
From Table I, it is evident that there was considerable dimensional variation among
the tubes; for example, their lengths varied from 29 in. to 113 in., their impact
plane separation distances (Figure 2, dimension b) ranged from 0.679 in. to 1.079 in.,
and their impact opening sizes ranged from 0.43 in. to 0.59 in. in length and from
0.26 in. to 0.39 in. in width. Table I also shows that 39 of the 51 tubes had seen
at least some field use; 22 of 51 had been used extensively. Finally /Table I shows
that nearly all of the tubes were imperfect geometrically. The four most frequently
observed types of geometric misalignment were as follows:
1. Minor impact-plane misalignemnt (one or both planes) with respect
to the transverse tube axis (88 percent of the tubes).
2. Minor impact-plane misalignment (one or both planes) with respect
to the longitudinal tube axis (61 percent of the tubes).
3. Length misalignment (16 percent of the tubes).
4. Planar misalignment (16 percent of the tubes).
Sixty-seven percent of the tubes exhibited two or more of the above types of misalign-
ment.
-------
10
B. Calibrations - The results of the calibrations are also presented in
Table I. One hundred-two values of C" were obtained (i.e., 51 A-side and 51 B-
side coefficients), ranging from 0.805 to 0.880, with a mean value of 0.848 and
an average deviation of 0.008 (see "Statistical Considerations" section in the
Appendix). Ninety-four of the 102 coefficients (92 percent) fell within the
range 0.83 to 0.87, which is cited in the literature as "normal" for the Type-S
instrument. The average A-to-B-side coefficient difference was 0.005, and 46
of 51 tubes (90 percent) had an A-to-B-side difference of 0.010 or less.
CONCLUSIONS
A recent study in which 51 isolated (i.e., not attached to sample probes)
Type-S pitot tubes were calibrated against a standard pitot tube has demonstrated
the following:
1. It is highly probable that a given Type-S pitot tube will have A-side and
B-side coefficients within the range 0.83 to 0.87 and an A-to-B-side coef-
ficient difference of 0.010 or less. Therefore, in reference to the pre-
viously mentioned studies in which C values consistently below the range
0.83 to 0.87 were obtained (see Introduction), it appears unlikely that
the pitot tubes themselves were responsible for the low coefficient values;
other factors were probably involved. It has recently been learned that
when a Type-S pitot tube is used in the presence of a sampling nozzle,
there must be adequate separation distance between the tube and nozzle, or
they will interfere aerodynamically, causing a reduction in the value of C .
In the studies reporting low coefficients, calibration was done in the
presence of a nozzle; hence, aerodynamic interference is a possible explana-
tion for the consistent departure of the C values from the 0.83 to 0.87 range.
-------
For each calibration (A or B side), six values of C were computed using
the above formula, i.e., one at each fan setting. From these six C values, an
average coefficient was determined, as follows:
6
icp
C (A or B side) = j (Equation 2)
SUMMARY OF RESULTS
A, Preliminary Con s i derations - The results of the preliminary examinations
and measurements of the 51 Type-S pi tot tubes are presented in Table I (see Appendix).
From Table I, it is evident that there was considerable dimensional variation among
the tubes; for example, their lengths varied from 29 in. to 113 in., their impact
plane separation distances (Figure 2, dimension b) ranged from 0.679 in. to 1.079 in.,
and their impact opening sizes ranged from 0.43 in. to 0.59 in. in length and from
0.26 in. to 0.39 in. in width. Table I also shows that 39 of the 51 tubes had seen
at least some field use; 22 of 51 had been used extensively. Finally/Table I shows
that nearly all of the tubes were imperfect geometrically. The four most frequently
observed types of geometric misalignment were as follows:
1. Minor impact-plane misalignemnt (one or both planes) with respect
to the transverse tube axis (88 percent of the tubes).
2. Minor impact-plane misalignment (one or both planes) with respect
to the longitudinal tube axis (61 percent of the tubes).
3. Length misalignment (16 percent of the tubes).
4. Planar misalignment (16 percent of the tubes).
Sixty-seven percent of the tubes exhibited two or more of the above types of misalign-
ment.
-------
10
B- Calibrations - The results of the calibrations are also presented in
Table I. One hundred-two values of (T were obtained (i.e., 51 A-side and 51 B-
side coefficients), ranging from 0.805 to 0.880, with a mean value of 0.848 and
an average deviation of 0.008 (see "Statistical Considerations" section in the
Appendix). Ninety-four of the 102 coefficients (92 percent) fell within the
range 0.83 to 0.87, which is cited in the literature as "normal" for the Type-S
3
instrument. The average A-to-B-side coefficient difference was 0.005, and 46
of 51 tubes (90 percent) had an A-to-B-side difference of 0.010 or less.
CONCLUSIONS
A recent study in which 51 isolated (i.e., not attached to sample probes)
Type-S pi tot tubes were calibrated against a standard pi tot tube has demonstrated
the following:
1. It is highly probable that a given Type-S pitot tube will have A-side and
B-side coefficients within the range 0.83 to 0.87 and an A-to-B-side coef-
ficient difference of 0.010 or less. Therefore, in reference to the pre-
viously mentioned studies in which C values consistently below the range
0.83 to 0.87 were obtained (see Introduction), it appears unlikely that
the pitot tubes themselves were responsible for the low coefficient values;
other factors were probably involved. It has recently been learned that
when a Type-S pitot tube is used in the presence of a sampling nozzle,
there must be adequate separation distance between the tube and nozzle, or
they will interfere aerodynamically, causing a reduction in the value of C .
In the studies reporting low coefficients, calibration was done in the
presence of a nozzle; hence, aerodynamic interference is a possible explana-
tion for the consistent departure of the C values from the 0.83 to 0.87 range.
-------
11
2. Generally speaking, the value of the Type-S pitot tube coefficient
(C ) seems to be relatively unaffected by the following: (a) variations
in tube dimensions (length, impact opening size, etc.), (b) various types
of minor imperfections in tube geometry, and (c) deterioration in the
physical condition of the tube, resulting from field use. It is not
readily apparent, however, why 8 percent of the tubes calibrated in this
study had coefficients outside the range 0.83 to 0.87, or why 10 percent
of them had A-to-B-side coefficient differences greater than 0.010. Im-
pact-opening misalignment may have been a factor, but this cannot be as-
certained without further study. Therefore, although it is likely that
the coefficient of a given Type-S pitot tube will be between 0.83 and
0.87 and that its A-to-B-side coefficient difference will be 0.010 or
w
less, these points are by no means certain and should not be assumed with-
out calibration.
REFERENCES
1. Herrick, R..General Environments Corporation. Springfield, Virginia.
(Unpublished data). 1973.
2. Fluid Meters, Their Theory and Application. Published by the American
Socity of Mechanical Engineers. 5th Edition. New York, 1959.
3. Smith, W. S., W. F. Todd, and R. T. Shigehara. Significance of Errors
in Stack Sampling Measurements. Presented at the Annual Meeting of APCA.
St. Louis, Missouri. June 14-19, 1970.
4. Perry, Robert H., Cecil H. Chi 1 ton, and Sidney D. Kirkpatrick. (editors).
Chemical Engineers' Handbook. Fourth Edition. McGraw-Hill Book Company.
New York, 1963.
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12
APPENDIX
-------
TABLE I: Summary of Results
Pitot
Tube
Number
MMMiHMMM
2 02
2-03
2 05
2-07
2-08
2 09
3-03
3-04
3-05
3-06
3-07
4-02
4-03
* Note:
Pitot Tube Dimensions
Tube
Length
(in.)
30
31
30
29
29
31
40
41
41
43
36
52
51
Impact
Plane
Seoara-
ion(in.)
0.828
0.847
0.798
0.825
0.795
0.857
0.794
0.752
0.775
0.850
0.797
0.820
0.891
Impact Opening
izedn. x 10~2)
A(l x w)
S3 x 33
47 x 28
56 x 31
56 x 33
56 x 33
47 x 28
56 x 31
59 x 38
56 x 31
47 x 28
56 x 33
53 x 34
45 x 27
The angular values listed
B(l x w)
56 x 33
47 x 28
56 x 31
56 x 33
58 x 33
47 x 28
59 x 33
52 x 31
56 x 31
47 x 28
53 x 31
56 x 33
45 x 27
Geometric Misalignment*
Trans-
verse
Axis
5° A
4° A
3° B
2° A
3° A
2° A
5° B
-
2° A
4° A
4° B
2° A
5° A
Longi-
tudinal
Axis
1° B
3° B
4° B
-
4° B
2° B
4° B
6° B
5° B
2° B
5° B
2° B
-
Length
1/16"
-
1/16"
-
1/16"
-
-
-
1/16"
-
-
-
-
Planar
1/16"
-
-
-
1/32"
-
1/32"
-
-
-
-
-
Amount
of
Field
Use**
E
E
E
E
E
N
E
E
E
N
E
E
L
Coefficients
C
P
A-side
0.805
0.849
0.855
0.835
0.821
0.848
0.855
0.858
0.852
0.853
0.862
0.830
0.847
C
P
B-side
0.805
0.855
0.861
0.854
0.815
0.850
0.855
0.864
0.852
0.850
0.859
0.827
0.850
A-to-B
Side
Differ-
ence
-
0.006
0.006
0.019
0.006
0.002
-
0.006
~
0.003
0.003
0.003
0.003
in the "Transverse Axis" and "Longitudinal Axis" columns above refer to the
-I / \ J 1 -1 «J »»-? I-U *< n o -n A r> t- *-rv *-Vi/-iod avOG" f~VlP 1pl~t"PT
total number of degrees by which the .u^~._w r ,.-, ^ -
"A" indicates that only one plane was misaligned, while "B" signifies that both were misaligned The angu-
lar values are estimates, based on micrometer readings taken prior to calibration; the "Mathematical Models
section of the Appendix shows how these estimates were made. The numerical values listed in the Length and
"Planar" misalignment columns above refer, respectively, to dimensions x and y of Figures 6b and 6c.
**Note: The letters E, L, and N stand for "Extensive," "Limited," and "None," respectively.
GO
-------
TABLE I: Summary of Results
(Continued)
! " "
Pitot
Tube
Number
4-05
4-06
4-07
4-08
1 /
| 4-09
p » ,, .
' 4-10
4-11
4-14
4-15
, _,
4-1.6
U i '/
4-18
Pitot Tube
Tube
Length
(in.)
51
50
50
50
51
52
50
50
48
56
35
48
50
Impact
Plane
Separa-
tion (in.)
0.812
0.819
0.871
0.850
0.895
0.776
0.849
0.886
0.812
1.079
1.070
0.821
0.868
Dimensions
Impact Opening
Size (in. xl 0-2)
A(l x w)
58 x 31
47 x 28
47 x 28
47 x 28
45 x 27
56 x 31
47 x 28
48 x 28
47 x 28
55 x 28
56 x 28
48 x 28
47 x 28
B(l x w)
55 x 34
47 x 28
47 x 28
47 x 28
45 x 27
53 x 31
47 x 28
50 x 28
47 x 28
55 x 28
55 x 28
48 x 28
48 x 28
Geometric Misalignment
Trans-
verse
Axis
2° A
1° A
2° A
9° A
4° A
4° A
-
9° B
-
-
6° B
8° B
Longi-
tudinal
Axis
-
3° A
-
2° B
-
6° B
1° B
-
2° B
-
-
1° A
1° B
Length
_
-
-
-
-
-
-
1/16"
-
-
-
-
-
Planar
1/32"
-
_
_
_
_
_
-
-
-
-
-
-
Anount
of
Field
Use
E
N
N
N
L
E
L
L
L
L
E
N
L
. Coefficients
c"
P
A-side
0.815
0.853
0.857
0.857
0.846
0.855
0.854
0.859
0.853
0.845
0.843
0.851
0.850
C
P
B-side
0.815
0.856
0.853
0.853
0.851
0.864
0.853
0.839
0.857
0.841
0.850
0.856
0.851
A-to-B
Side
Differ-
ence
0.003
0.004
0.004
0.005
0.009
0.001
0.020
0.004
0.004
0.007
0.005
0.001
-------
TABLE I: Summary of Results
Pitot
Tube
Number
^^
4-19
4-20
4-21
4-22
5-01
5-02
5-03
5-05
5-06
e. 08
5 09
6-01
Pit
_
Tube
Length
(in.)
MI^«B"*«"«^*"^^
52
54
50
^
48
66
66
66
63
66
66
65
66
76
-
ot Tube 1
_
Impact
Plane
Separa-
-tnnfin^
W^MMM.OT^M
0.941
0.844
0.815
0.818
0.865
0.864
0.858
0.962
0.865
0.875
0.800
0.864
0.679
_^ ^~
Dimensions
___ ^_
Impact Opening
ize fin. x 10 L)
\(1 x w)
44 x 27
4o x /o
55 x 33
47 x 28
47 x 28
47 x 28
47 x 28
45 x 28
47 x 28
47 x 28
53 x 32
47 x 28
55 x 33
Bfl x w)
47 x 28
AS -x 28
56 x 33
47 x 28
47 x 28
i^^^ ^-^ -^
47 x 28
i
47 x 28
45 x 28
47 x 28
47 x 28
56 x 32
47 x 28
53 x 30
,^M> ^
W
Geora
Trans-
verse
Axis
.^« ^ '
"
3° B
3° A
2° A
ii i
5° B
i
9° B
2° A
3° A
1
2° A
3° A
8° B
-i
etric Mis
- i "
Longi-
tudinal
Axis
i
2° A
'
1° B
i -
1° B
1° B
3° B
T5"!
- '
4° B
in '
1° B
L6° B
salignment
Length
1/16"
-
-
-
-
1/16"
-
-
Planar
Offi^-
-
-
-
-
~"
-
-
-
1/32"
Amount
nf
Field
Use
__E
N
E
N
L
L
L
L
N
E
L
E
Coefficients
c"
P
A-side
MMWMM^»
0.834
0.846
0.845
0.848
0.854
0.853
0.848
OR At
. ot-5
0.852
0.845
0.855
0.864
C
P
B-side
0.855
0.853
0.855
0.848
n ftsi
0.842
0.850
0.847
0.849
0.855
0.843
0.850
.880
f-rt "R
A tO o
Side
Differ-
ence
0.021
» - .mi
0.007
0.010
0 001
0.012
0.003
0.001
"0.006
~0.003
0.002
0.005
Om £.
.Olb
-------
(Continued)
Pitot
Tube
Number
6-02
6-03
6-05
6-06
6-07
7-01
7-02
7-03
8-01
8-02
9-01
9-02
Pitot Tube Dimensions
Tube
Length
(in.)
76
76
72
72
72
88
89
93
96
96
113
112
Impact
Plane
Separa-
tion (in.)
0.845
0.818
0.795
0.845
0.831
0.875
0.778
0.884
0.841
0.875
0.906
0.884
Impact Opening
Size(in. x 10~2)
AC1 x w)
'47 x 28
56 x 31
50 x 28
47 x 28
47 x 28
47 x 28
59 x 30
47 x 28
50 x 28
47 x 28
56 x 31
47 x 2b j
B(l x w)
47 x 28
53 x 31
48 x 27
47 x 28
47 x 28
47 x 28
59 x 31
47 x 28
50 x 28
47 x 28
58 x 31
47 x 28
~l
Geometric Misalignment
Trans-
verse
Axis
6° B
4° B
6° A
11° B
6° A
8° B
6° A
5° B
5° B
4° B
1.1° B
11° B
Longi-
tudinal
Axis
_
3° B
5° B
-
-
-
5° B
-
-
1° T
2° B
1° B
Length
_
1/16"
-
-
-
-
-
-
-
-
-
-
Planar
_
-
1/16"
-
-
-
-
-
-
-
1/32"
-
Amount
of
Field
Use
L
E
N
L
L
N
E
E
N
L
E
E
Coefficients
c"
P
A-side
0.844
0.861
0.853
0.846
0.851
0.842
0.840
0.840
0.839
0.847
0.840
0.850
C
P
B-side
0.847
0.848
0.852
0.855
0.845
0.848
0.849
0.843
0.844
0.841
0.843
0.847
A-to-B
Side
Differ-
ence
0.003
0.013
0.001
0.009
0.006
0.006
0.009
0.003
0.005
0.006
0.003
0.003
-------
17
STATISTICAL CONSIDERATIONS
The 102 values of C" (the average A or B side pi tot tube coefficient) ob-
tained by calibration had a mean value of 0.848 and an average deviation of 0.008.
The following formula was used to compute the average deviation:
n
where:
o = average deviation
n = number of coefficients = 102
x" = mean coefficient value = 0.848
x-= individual coefficient value.
About 75 percent of the coefficients were within one average deviation (a) of
the mean; 91 percent were within 2o, and 98 percent were within 4a. Figure A-l
represents this graphically.
-------
18
o
a
LU
K
MEAN VALUE = 0.848
AVERAGE DEVIATION = 0.008
20
10
0.816 0.824
0.832
0.864
0.840 0.848 0.856
AVERAGE TYPE-S PITOT COEFFICIENT
Figure A-1. Frequency distribution of Cp values.
0-872 0.880
-------
19
EXPERIMENTAL ERROR CONSIDERATIONS
It has been shown (Figure 9) that because of the sensitivity limitations of the
gage-oil manometer used in this study, each AP could be read only to the nearest
0.005 in. hLO, thus making the uncertainty of each recorded value of AP about 0.002
to 0.003 in. H?0. The uncertainty of each C value (calculated from two such AP
readings) is, consequently, greater than this. Consider the following example:
Suppose that during calibration, values of APstd and APg equal to 0.300 in.
H?0 and 0.400 in. HLO, respectively, are read and recorded. The recorded value of
C , based on these readings, would then be (by Equation 1):
C = 0.99 / 0.300/0.400 = 0.857
Note that although AP ,, was read and recorded as 0.300 in H,,0 its true value could
have been as high as 0.303 in. H20 or as low as 0.298 in. H20; similarily, A?S
could have been as high as 0.403 in. HpO or as low as 0.398 in. H20. Therefore, the
value of C , recorded as 0.857, could have been (considering only the extreme cases)
as high as 0.99 / 0.303/0.398 = 0.864, or as low as 0.99 / 0.298/0.403 = 0.851. Thus,
because of the sensitivity limitations of the manometer, each value of C is uncertain
by about +_ 0.006. Note, however, that additional uncertainty allowances in C are
still needed, first to cover possible errors by the observer, in reading the manometer
(e.g., because of parallax or incorrect "sight-weighted" averaging of minor flow pul-
sations) and in handling and aligning the pi tot tubes, and second, to account for the
uncertainty of the value of the standard pi tot tube coefficient. Therefore, an ex-
perimental error tolerance of +_ 0.005 (arbitrary) will be assigned to each C value
to cover observer error, and an additional allowance of +_ 0.01 will be made to account
for uncertainty in the value of C (standard). Thus, the total uncertainty of each
recorded value of C is approximately j^0.02.
-------
20
MATHEMATICAL MODELS
In this study four types of impact-plane misalignment with respect to
the transverse and longitudinal tube axes were observed. A mathematical model
of each type, showing how estimates of misalignment angle were made, is pre-
sented below:
1 Transverse Axis Misalignment (one plane only)
In end view, the shape of the pi tot tube can be approximated by
a trapezoid, with bases b and b', and height h (see Figure A-2). The
bases b and b1 correspond to micrometer readings M- and M,, respec-
tively (in inches).
Figure A-2. Transverse axis
misalignment, one plane only.
The height h is approximately 0.375 in., since the pitot tube is made
of 3/8 in. o.d. stainless steel. The "transverse skew angle," a, can
be estimated, making use of the fact that alternate interior angles are
equal, as follows:
M - M
a = arc tan
0.375
-------
21
2. Transverse Axis Misalignment (both planes)
The approximations are similar to those made for case 1 above,
except that the "total" transverse skew angle, (a-, + «2), is mea-
sured (see Figure A-3). The tangent of the sum of two angles is
Figure A-3. Transverse axis
misalignment, both planes.
determined as follows:
tan a, + tan c»2
tan (a-! + <*2) - i + tan c^ tan a
Referring to Figure 3A, tan ^ = c/h, tan «2 = d/h, and b - b1 = c + d.
Substituting these into the formula for tan (c^ + c*2) gives:
* , ± ^ c/h + d/h _ _c_+_d = _b_JLj>'
tan («-, + a2) = i + (c/h)(d/h) " h + cd h + cd
The height, h, will again be 0.375 in., and b - b1 will be the
micrometer difference, M2 - M]. The product, cd, will be negligibly
small unless a-, and ^ are very large; therefore, the expression can be
reduced to:
. h, M9 - M,
, . \ b - b c. \
tan (a-, + «2) = ^ 0.375 ,
-------
3.
22
which is the same formula used in case 1 above. Solving for (ct + a )
gives :
(al +a2] ~~ arC tdn -0751
Longitudinal Axis Misalignment (one plane only)
The mathematical model for this type of misalignment is a trapezoid
(see Figure A-4).
Figure A-4. Longitudinal axis
misalignment, one plane only.
The "longtitudinal skew angle," Bf is determined by the following formula:
6 = arc tan b " b'
h
In the above formula, b and b' are, respectively, micrometer readings
M4 and Mj; h, the height of the trapezoid, is equal to the projected
length of the properly aligned impact opening.
4- Longitudinal Axis Misalignment (both planes)
Using approximations (for the sum of the tangents of two angles)
similar to those in case 1 above, the formula for this type of misalign-
ment (see Figure 5A) is:
-------
23
(31 + 32) = arc tan
M - M
Figure A-5. Longitudinal axis
misalignment, both planes.
* Note- For small values of e, the height of the trapezoid, h, can be
taken to be equal to the projected length of either impact opening,
without appreciable error.
-------
24
THE EFFECT OF AERODYNAMIC INTERFERENCE
BETWEEN A TYPE-S PITOT TUBE AND
SAMPLING NOZZLE ON THE VALUE
OF THE PITOT TUBE COEFFICIENT
Robert F. Vollaro9
Introduction
In source sampling, the Type-S pi tot tube is the instrument most
commonly used to measure stack gas velocity. Before a Type-S pi tot tube
is used in the field, however, its coefficient (C ) should be known. The
value of Cp is usually determined by calibration against a standard pitot
tube, although in many past instances, a "theoretical" C value of 0.85
has been assigned to the pitot tube, without calibration.*
Within the past year, however, certain observers have questioned the
validity of using, in actual field test situations, either an assumed value
of Cp or one obtained by laboratory calibration. It has come to light
that when a Type-S pitot tube is used in its customary source-sampling
configuration (i.e., attached to a probe sheath, with its impact openings
adjacent to a sampling nozzle) the value of C can be significantly reduced.1'2
The reduction in Cp is caused by aerodynamic interference between the pitot
tube and nozzle; this interference phenomenon has been shown to be a downstream
effect; i.e., it causes a decrease in pressure on the static side of the pitot
1 2
tube. ' It is believed that the closely spaced pitot tube and nozzle "...
act somewhat as a venturi. The velocity in the space between the tubes
will be higher than free stream velocity, thereby reducing the static pressure."1
This has been thought to be acceptable because Type-S pitot tube coeffi-
cients generally range from about 0.83 to 0.87 (Reference 4).
Emission Measurement Branch, ESED, OAOPS, EPA, RTP, NC, June 1975
-------
25
Consequently, when aerodynamic interference is present, taking Cp to
*
be equal to either 0.85 or a laboratory calibration value can result in
significant velocity measurement error. Accordingly, in February 1975,
investigators conducted a study to determine (1) the minimum distance needed
between a Type-S pi tot tube and sampling nozzle to prevent aerodynamic inter-
ference, and (2) whether or not this distance requirement is met when sampling
nozzles of various sizes are attached to a probe constructed in compliance
with the current guidelines (APTD-0581)3. This paper discusses the study,
its results, and the significance of these results.
Experimental Set-Up
The effects of aerodynamic interference between a Type-S pi tot tube
and sampling nozzle on the value of C were studied in a wind tunnel with a
test section diameter of 12 inches. The experiments consisted of three sets
of pi tot tube calibrations; in each set, the effects of one particular nozzle
size were investigated. Three nozzle sizes - 1/2-inch, 3/8-inch, and 1/2-inch
(i.d.) - were studied; to ensure representativeness, each nozzle size was
examined at test section velocities approximately equal to those at which
**
the nozzle is ordinarily used in the field.
At the outset of each set of calibrations, a "background" or reference
coefficient for the Type-S pitot tube was obtained by calibrating it, with
the sampling nozzle removed, against a standard pitot tube. Calibration was
then repeated several times with the nozzle in place, varying only the separation
* Unless, of course, calibration was done in the presence of a nozzle.
**
For example, a 1/2-inch nozzle is generally used for isokinetic sampling
when velocities are about 15 ft/sec or less; therefore, 1/2-inch nozzle in-
terference was studied at test section velocites around 12 ft/sec.
-------
26
distance (dimension "X", Figure 1) between the pitot tube and nozzle.
Initially, the gap was set small enough so that the C value obtained
markedly lower than the background value. The gap was then gradually
PITOT TUBE
f SEPARATION
x DISTANCE, inches
ID
NOZZLE
Figure 1. Type-S pitot tube with sampling nozzle in place.
widened until Cp became equal (within experimental error) to the reference
value; the value of "X" at which this occurred was assumed to be the minimum
pitot-nozzle separation distance needed to prevent aerodynamic interference.
Summary of Results
As shown in Table I, for all three nozzle sizes studied, interference ef-
fects become negligible when the gap between the nozzle and pitot tube is about
3/4-inch. It thus becomes clear that the separation distances of 1/2-inch, 7/16-
inch, and 3/8-inch which result, respectively, from attachment of 1/4-inch, 3/8-
inch, and 1/2-inch (i.d.) nozzles to a pitobe assembly constructed according to
the APTD-0581 guidelines, are too small to prevent interference; velocity mea-
surement errors of 6 percent or more are likely to occur at these spacings.
-------
27
Conclusions
A study of the effect of aerodynamic interference between a Type-S
pitot tube and sampling nozzle on the value of the pitot tube coefficient
(C ) has demonstrated the following:
1. When a Type-S pitot tube is used in the presence of a sampling noz-
zle, there should be at least 3/4-inch free-space between the tube and nozzle,
in order to prevent reduction in the effective value of Cp as a result of
aerodynamic interference.
2. When sampling nozzles (1/4-inch i.d. and larger) are attached to
pitobe assemblies constructed according to the guidelines in APTD-0581, the
resulting pitot tube-nozzle separation distances will be much less than 3/4-
inch, and hence insufficient to prevent interference.
3. When pitot tube-nozzle interference is present, the use of either
an assumed value of 0.85 for C or a laboratory calibration value (i.e., one
which was not obtained in the presence of a nozzle) can result in velocity
measurement errors of 6 percent or more.
Recommendations
To improve the accuracy of velocity measurements made with a Type-S
pitot tube when it is used in the presence of a sampling nozzle, either of
the following can be done:
1. Construct pitobes so as to provide a separation distance of at
least 3/4-inch between the nozzle and pitot tube, with the largest size
sampling nozzle (1/2-inch i.d.) in place. This necessitates that the
-------
Table I. SUMMARY OF RESULTS'
Case
1/4-in. nozzle
gap=l/2-in.
1/4-in. nozzle
gap=19/32-in.
1/4-in. nozzle
gap=3/4-in.
3/8-in. nozzle
gap=7/16-in.
3/8-in. nozzle
gap=19/32-in.
3/8-in. nozzle
gap=ll/16-in.
1/2-in. nozzle
gap=7/16-in.
1/2-in. nozzle
gap=5/8-in.
1/2-in. nozzle
gap=ll/16-in.
1/2-in. nozzle
gap=25/32-in.
Reference
value of
Type-S pi tot
coefficient
(Cp*)
0.815
0.815
0.815
0.814
0.814
0.814
0.822
0.822
0.822
0.822
Actual value
of Type-S
pi tot coeffi-
cient
(Cp)
0.768
0.793
0.808
0.749
0.769
0.811
0.789
0.772
0, 788
0.812
True velocity
measured by
standard pi tot
tube,
ft/mi n
2,290
2,315
2,320
1,455
1,470
1,510
680
645
660
690
Apparent
velocity
based on
Cp* value,
ft/mi n
2,430
2,375
2,340
1,585
1,560
1,515
705
690
690
690
Velocity
error
from
interference, %
6.1
2.6
0.9
8.9
6.1
0.3
3.7
7.0
4.4
0.0
r-o
Co
Each listed value of Cp* and Cp is the average of three independent determinations. All velocity values were
calculated using the pitot tube equation (Equation 2-2 of Reference #5).
-------
29
pi tot tube be welded to the probe sheath with short pieces of 5/8-inch
(o.d.) stainless steel tubing, instead of the 1/4-inch pieces currently
specified in APTD-0581.
2. Calibrate Type-S pitot tubes after installation in the pitobe
assembly and in the presence of various size sampling nozzles. The
calibration should be performed in the velocity range associated with
the respective nozzle sizes.
References
1. Davini, R. J. and D. G. DeCoursin. Progress Report No. 7; Particulate
Sampling Strategies for Mechanically Disturbed and Cyclonic Flow;
Period January 1 - January 31, 1974. Prepared for EPA by FluiDyne
Engineering Corporation; Minneapolis, Minnesota. February 19, 1974.
2. Vollaro, R. F. and P. R. Westlin. Environmental Protection Agency;
Durham, N. C.. January 1974. (Unpublished data)
3. Martin, R. M.. Construction Details of Isokinetic Source-Sampling Equip-
ment. Environmental Protection Agency Publication Number APTD-0581.
April 1971.
4. Smith, W. S., W. F. Todd, and R. T. Shigehara. Significance of Errors
in Stack Sampling Measurements. (Presented at the Annual Meeting of
APCA. St. Louis, Missouri. June 14-19, 1970)
5. Standards of Performance for New Stationary Sources. Federal Register.
36 (247) December 23, 1971.
-------
30
THE EFFECTS OF THE PRESENCE OF A PROBE SHEATH
ON TYPE-S PITOT. TUBE ACCURACY
Robert F. Vollaro*
INTRODUCTION
A number of recent studies have demonstrated that the coefficient (C )
of a Type-S pi tot tube can be substantially lowered when the tube is used^s
a component of a pitobe assembly (i.e., when the tube is attached to a sample
probe equipped with a nozzle and thermocouple).1'2'3 Complex aerodynamic in-
teractions between the pi tot tube and the other components of the assembly are
believed to be responsible for the reduction in the value of C . The effects
of the presence of a sampling nozzle and thermocouple on C have been exten-
sively studied.1'2 Considerably less is known, however, about the way in which
a probe sheath affects Cp. Certain observers have reported reductions of up to
5 percent in the value of Cp. believed to be directly attributable to the pres-
ence of a probe sheath.1'4 However, none of the studies cited has discovered
the exact nature (mechanism) of this reduction; therefore, it has not been possi-
ble, to date, to adjust for it properly during calibration and field use of
pitobe assemblies.
When it is attached to a sample probe, a Type-S pitot tube is ordinarily
attached in such a way that the center of the sampling nozzle (when in place) will
be in line with the center of the pitot tube impact openings (Figure 1). This
will, in most cases, leave a distance of 2 to 4 inches between the center of the
pitot tube openings and the leading edge of the sample probe (dimension »y",
Figure 1). Some observers are of the opinion that a 2-to-4-inch separation dis-
tance is insufficient to prevent aerodynamic interactions between the probe and
pi tot tube; hence, they believe aerodynamic interference to be the cause of the
* Emission Measurement Branch, ESED, OAQPS, EPA, RTP, NC, November 1975
-------
31
SAMPLE PROBE
TYPE-S
PITOTTUBE
NOZZLE
Figure 1. Type-S pitot tube, attached to sample probe.
1-in. CYLINDRICAL
PROBE SHEATH
TYPE-S
PITOTTUBE
Figure 2. Type-S pitot tube and 1-in. cylindrical probe sheath (Experiment # 1).
-------
32
probe sheath effect. Others believe, however, that a distance of / ^ -1 inches
between the probe and pi tot tube openings is more than adequate to eliminate
pitot tube-probe sheath interference; and that the pseudo-high velocity head
(AP) readings and consequent decrease in the value of C are the result of a
reduction in the effective cross-sectional area of the duct caused by the probe
sheath. The question of the nature of the probe sheath effect has not been re-
solved chiefly because all of the studies reporting a sheath effect were done
in small calibration ducts, in which cross-section blockage could have been
significant. ' In view of this, experiments were recently undertaken to deter-
mine the nature and magnitude of the probe sheath effect. This paper presents
the results of the experiments and discusses their significance.
EXPERIMENT #1
SET-UP AND PROCEDURE
The first experiment was done in a wind tunnel having a test-section dia-
meter of 12 inches. At the outset of the experiment, the fan was turned on,
generating a test-section velocity of about 2500 ft/min. A Type-S pitot tube was
inserted 7 inches into the duct, and aligned so that the pitot impact openings were
perpendicular to the direction of flow. The pitot tube was connected to an in-
clined manometer, and a velocity head (AP$) reading was taken and recorded. Next,
the effects of the presence of a probe sheath on the value of AP were observed.
A 1-inch (diameter) cylindrical probe sheath was inserted into the duct, alongside
the Type-S pitot tube (see Figure 2). The only variable for the remainder of the
experiment was the distance between the center of the pitot tube impact openings
and the leading edge of the probe sheath (dimension "y", Figure 2). Velocity head
readings were taken at six different values of "y", i.e., y = 6, 5, 4, 3, 2, and 1
inches. Three experimental runs were performed.
-------
33
RESULTS
The results of the experiment are presented in Table I. Table I shows that
as the distance "y" decreased, the manometer reading steadily increased. In the
range y = 2 inches to y = 4 inches (which is the range of pitot-probe sheath
separation in most pitobe assemblies), the probe sheath caused velocity measure-
ment errors of 1.4 to 4.4 percent.
An attempt was made, based on the results of Experiment #1, to deduce the
nature of the probe sheath effect. The hypothesis that the effect is caused by
a reduction in the cross-sectional area of the duct by the probe sheath (see In-
troduction) was tested. A projected-area model of the probe sheath was made as
shown in Figure 3. For each value of "y". the percentage theoretical blockage
of the duct cross-section was calculated, based on the model, as follows:
Percentage
theoretical =
blockage
100 (Equation 1)
Where:
1 = length of sheath segment inside the duct, in.
w = projected width of sheath segment, in.
p
A = cross-sectional area of the calibration duct, in.
According to this model, a 1:1 correspondence would be expected to exist between
percentage theoretical blockage (i.e., the percentage decrease in the duct area)
and the percentage increase in gas velocity, based on the equation of continuity
for steady flow:
Q = AeV (Equation 2)
Where:
Q = total gas volumetric flow rate, ft /sec
-------
Table I. DATA FROM EXPERIMENT #1
Distance
(y).
in.
-
6
5
4
3
2
1
Velocity *
head
(APS), in. H20
0.536
0.536
0.540
0.550
0.562
0.583
0.625
S&PI **
0.732
0.732
0.735
0.742
0.750
0.764
0.791
Increase
in velocity, %
_
0.0
0.4
1.4
2.5
4.4
8.1
* Average of three experimental runs.
** Proportional to velocity.
WIND TUNNEL
DUCT
PERCENTAGE THEORETICAL f_. lxw 1
QI nrienrr " *~
BLOCKAGE
L DUCT AREAJ
xlOO
Figure 3. Projected-area model of probe sheath.
-------
35
p
A = effective duct cross-sectional area, ft
V = velocity of flowing gas stream, ft/sec
In actuality, however, the experimental results do not show this correspondence
(see Table II and Figure 4). For values of "y" greater than or equal to 3 inches,
the increase in velocity was consistently less than the increase predicted by
Equation 2; at y = 2 inches, the theoretical and actual values were approximately
equal; at y = 1 inch, the actual increase in velocity surpassed the theoretical.
The following suggestions were put forth as possible explanations of these results:
1. The sheath effect is not a blockage effect at all, but is caused in-
stead by aerodynamic interference between the pi tot tube and probe sheath;
hence, it is independent of sheath segment size, and is a function only of
of the separation distance, "y".
2. The effect is a combination of blockage and aerodynamic interference in
varying relative proportions, with the former effect dominating at the
larger values of "y" (^.3 inches), and the latter beginning to become im-
portant at smaller values of "y". A possible reason that, for y >_ 3 inches,
the actual increase in velocity was less than the theoretical, is that the
probe sheath is a cylinder, not a rectangular solid, and therefore does not
actually effectively block that percentage of the duct cross-section pre-
dicted by the rectangular projected-area model.
Therefore, based on the results of Experiment #1, it was not possible to come to a
definite conclusion about the nature of the probe sheath effect.
EXPERIMENT #2
SET-UP AND PROCEDURE
Experiment #2 was essentially the same as experiment #1, except that the
-------
36
Table II.
PROJECTED-AREA, EQUATION OP
OF DATA FROM EXPERIMENT #]
riVJTI'Y MOPFL
Distance
(y).
in.
6
5
4
3
2
1
__ __
Theoretical
j Blockage, %*
r~ _
0.9
1.7
2.6
3.4
4.3
5.2
Theoretical
increase in
velocity, %**
-
0.9
1.7
2.6
3.4
4.3
5.2
Actual
increase in
velocity, %
1 ^^"I"I^^^^«MI
0.0
0.4
1.4
2.5
4.4
8.1
* From projected-area model of sheath segment.
** Predicted by equation of continuity, Q = AeV.
EQUATION OF CONTINUITY
AeV
3.0 4.5 6.0
INCREASE IN VELOCITY, percent
Sent #G;aphi°al rePresentation of Projected-area, equation of continuity model of data from Ex-
-------
37
cylindrical probe sheath was replaced by a 1-inch rectangular solid (see Figure 5).
The purpose of this experiment was to test hypothesis "b" of the preceding section,
i.e., that the sheath effect is a combination of blockage and aerodynamic inter-
ference, in varying relative proportions, depending on the value of "y". As in
Experiment #1, velocity head readings were taken at values of "y" equal to 6, 5, 4,
3, 2 and 1 inches; three experimental runs were performed.
RESULTS
The results of experiment #2 are presented in Table III. Once again, a con-
sistent increase in the velocity head readings was observed as the value of "y"
decreased. This time, however, the velocity measurement errors ranged from 2.8 to
7.8 percent in the normal pitot - sheath separation range (i.e., y = 2 to 4 inches);
these errors are considerably greater than those obtained in experiment #1 with the
cylindrical sheath.
As in experiment #1, a projected-area model was made for each value of "y",
and a comparison was made between the actual increase in velocity and the theoreti-
cal increase predicted by the model and the equation of continuity. The results of
this endeavor are presented in Table IV and in Figure 6. It is evident from these
results that the correspondence between the percentage theoretical blockage and
the actual velocity increase was nearly 1:1 for values of "y" greater than or equal
to 3 inches. However, for y = 2 inches and y = 1 inch, the actual velocity in-
crease surpassed the theoretical. From this it can be concluded that hypothesis #2
above, that the probe sheath effect is a combination of blockage and aerodynamic
interference in varying relative proportions (depending upon the value of "y"), is
valid. At y >_ 3 inches, the blockage effect is dominant and aerodynamic interference
is minimal; for "y" values less than 3 inches, the total sheath effect becomes a
-------
38
1-rn. RECTANGULAR..
? SOLID >^
Figure 5. Type-S pitot tube and 1-in. rectangular solid (Experiment # 2).
Table III. DATA FROM EXPERIMENT #2
Distance
(y).
in.
-
6
5
4
3
2
1
Velocity*
head
(APS), in. H20
0.522
0.527
0.540
0.550
0.564
0.605
0.663
/&P^~**
0.722
0.726
0.735
0.742
0.751
0.778
0.814
Increase
in velocity,
%
^
0.6
1.8
2.8
4.0
7.8
12.7
*Average of three experimental runs.
**Proportional to velocity.
-------
Table IV.
39
PROJECTED-AREA, EQUATION OF CONTINUITY MODEL
OF DATA FROM EXPERIMENT # 2
Distance
(y),
in.
6
5
4
3
2
1
Theoretical
blockage, %*
0.9
1.7
2.6
3.4
4.3
5.2
Theoretical
increase in
velocity, %**
0.9
1.7
2.6
3.4
4.3
5.2
Actual
increase in
velocity, %
0.6
1.8
2.8
4.0
7.8
12.7
*From projected-area model of sheath segment.
**Predicted by equation of continuity, Q = Ae V.
EQUATION OF CONTINUITY
Q = A.V
INCREASE IN VELOCITY, percent
9 10 11 12 13
Figure 6. Graphical representation of projected-area, equation of continuity model of data from Experiment
#2.
-------
40
combination of aerodynamic interference and blockage. Ncce that .-v ,,,,. blockage
and aerodynamic interference components of the total sheath efft^; "-ro, at
corresponding "y" values, more pronounced with the rectangular solid of experiment
#2 than with the cylindrical sheath of experiment #1. The blockage effect was
greater because a rectangular solid more closely follows the projected-area, con-
tinuity equation model than does a cylindrical sheath; the likely reason that the
aerodynamic effect was greater is that different wakes are formed by sharp-edged,
rectangular solids and smooth, cylindrical solids.
EXPERIMENT #3
SET-UP AND PROCEDURE
Experiment #3 was similar to experiment #1, except that it was done in an
18-inch diameter wind tunnel, and a 2.5-inch cylinder was used instead of a 1-inch
cylindrical sheath. The purpose of this experiment was to observe the effects of
the presence of an external sheath on the value of C . Many pitobe assemblies have
an external sheath enclosing the sample probe, pitot tube, and thermocouple (see
Figure 7); the external sheath will usually have a diameter of about 2 inches. In
this experiment, data were taken at values of "y" ranging from 10 inches to 1 inch,
at 1-inch intervals. Three experimental runs were made.
RESULTS
The results of experiment #3 are presented in Tables V and VI and in Figure 8.
The results are quite consistent with those obtained in experiment #1. Figure 8
shows that for values of "y" l3 inches, the actual velocity increase was consist-
ently less than the increase predicted by the projected-area, continuity equation
model. At y = 2 inches, the actual and theoretical velocity increases were nearly
coincident, and at y = 1 inch, the actual increase surpassed the theoretical. This
-------
EXTERNAL
SHEATH
TYPE-S
PITOTTUBE-
SAMPLING
NOZZLE
Figure 7. Pitobe assembly with external sheath.
-------
42
Table V. DATA FROM EXPERIMENT #3
Distance
(y),
in.
-
10
9
8
7
6
5
4
3
2
1
Velocity*
head
(APS), in. H20
0.325
0.327
, 0.331
0.336
0.342
0.349
0.355
0.363
0.376
0.392
0.419
/APS **
0.570
0.572
0.575
0.580
0.585
0.591
0.596
0.602
0.613
0.626
0.648
Increase
in velocity,
%
-
0.4
0.9
1.7
2.6
3.7
4.6
5.6
7.5
9.8
13.4
*Average of three experimental runs.
**Proportional to velocity.
Table VI. PROJECTED-AREA, EQUATION OF CONTINUITY MODEL
OF DATA FROM EXPERIMENT # 3
Distance
(y).
in.
10
9
8
7
6
5
4
3
2
1
Theoreti cal
blockage, %*
1.9
2.9
3.9
4.9
5.9
6.9
7.9
8.9
9.8
10.8
Theoretical
increase in
velocity, %**
1.9
2.9
3.9
4.9
5.9
6.9
7.9
8.9
9.8
10.8
Actual
increase in
velocity, %
0.4
0.9
1.7
2.6
3.7
4.6
5.6
7.5
9.8
13.4
*From projected-area model of sheath segment.
**Predicted by equation of continuity, Q = Ae V.
-------
43
EQUATION OF CONTINUITY
= AeV
6 8
INCREASE IN VELOCITY, percent
Figure 8. Graphical representation of projected-area, equation of continuity model of data from Experiment
#3. -.
-------
44
reaffirms the findings of experiments #1 and #2 that blockage is the primary
effect for "y" values of 3 inches or more, and that aerodynamic interference
begins to be important when y is less than 3 inches.
CONCLUSIONS
A recent study of the effects of the presence of a probe sheath on Type-S
pi tot tube accuracy has demonstrated the following:
1. If a Type-S pitot tube is attached to a sample probe (with or without
an external sheath), and is used to measure stack gas velocities in a small duct
('v 12 to 36 inches in diameter), pseudo-high velocity head (AP) readings may be
obtained, resulting from significant partial blockage of the duct cross-section
by the probe sheath. The actual percentage increase in the stack gas velocity
caused by blockage (which approximately equals the percentage decrease in C , the
Type-S pitot tube coefficient) will, for cylindrical sheaths, be less than the
theoretical increase predicted by a rectangular projected-area model (e.g., Figure
3) and the equation of continuity for steady flow. It appears safe to conclude
that the actual decrease in the value of C will be less than 1 percent when:
(a) the theoretical blockage is 2 percent or less for pitobe assemblies without
external sheaths; and (b) the theoretical blockage is 3 percent or less for pitobe
assemblies with external sheaths (see Figure 9).
2. When a Type-S pitot tube is attached to a sample probe, if there is inade-
quate separation distance between the center of the pitot tube impact openings and
the leading edge of the probe, aerodynamic interference will occur, causing a re-
duction in the effective value of the Type-S pitot tube coefficient (see Figure 10).
It has been demonstrated that this interference is minimal for pitot tube - probe
sheath separation distances greater than or equal to 3 inches; hence, it will only
-------
45
PITOBE ASSEMBLIES
WITH EXTERNAL
SHEATHS
o
o 3
o
PITOBE ASSEMBLIES
WfflfOUT EXTERNAL
; SHEATHS
DECREASE IN Cp DUE TO BLOCKAGE, percent
Figure 9 Percentage theoretical blockage versus actual decrease in Cp
due to blockage, for pitobe assemblies with and without external
sheaths.
-------
46
I
u
<
oc
<
X
ta
o
oc
EXPERIMENT #1
(NO EXTERNAL
SHEATH)
EXPERIMENT #3
(EXTERNAL SHEATH)
1234
DECREASE IN Cp DUE TO AERODYNAMIC INTERFERENCE , percent
Figure 10. Pitot tube-probe sheath separation (distance "y") versus percentage decrease in Cp due to
aerodynamic interference, for pitobe assemblies with and without external sheaths.
-------
47
be an important consideration for pitobe assemblies that have less than 3 inches
*
between the pitot tube openings and the leading edge of the sheath.
REFERENCES
1. Gnyp, A. W., C. C. St. Pierre, D. S. Smith, D. Mozzon, and J. Steiner.
An Experimental Investigation of the Effect of Pitot Tube-Sampling Probe
Configurations on the Magnitude of the S-Type Pitot Tube Coefficient for
Comnercially Available Source Sampling Probes. Prepared by the University
of Windsor for the Ministry of the Environment. Toronto, Canada. February,
1975.
2. Vollaro, R. F. The Effect of Aerodynamic Interference Between a Type-S
Pitot Tube and Sampling Nozzle on the Value of the Pitot Tube Coefficient.
U. S. Environmental Protection Agency, Emission Measurement Branch, Research
Triangle Park, N. C. February,1975.
3. Davini, R. J. and D. 6. DeCoursin. Progress Report No. 7: Particulate
Sampling Strategies for Mechanically Disturbed and Cyclonic Flow, January 1 -
31, 1974. Prepared by FluiDyne Engineering Corporation, Minneapolis, Minn.,
for U. S. Environmental Protection Agency. Research Triangle Park, N. C.
February,1974.
4. Davini, R. J. and H. A. Hanson. Progress Report No. 11: Particulate Sampling
Strategies for Mechanically Disturbed and Cyclonic Flow; Period May 1 -31,
1974. Prepared by FluiDyne Engineering Corporation, Minneapolis, Minn., for
U. S. Environmental Protection Agency. Research Triangle Park, N. C. June,1974,
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48
AN EVALUATION OF SINGLE-VELOCITY
CALIBRATION TECHNIQUE AS A
MEANS OF DETERMINING
TYPE-S PITOT TUBE COEFFICIENTS
Robert F. Vollaro*
INTRODUCTION
"Method 2 - Determination of Stack Gas Velocity and Volumetric Flow
Rate (Type-S Pitot Tube)," promulgated in the December 23, 1971, Federal
Register, specifies that during calibration of Type-S pi tot tubes, "...
the velocity of the flowing gas stream should be varied over the normal
working range." The results of a number of recent pitot calibration
studies indicate, however, that acceptable values of C (the Type-S pitot
tube coefficient) can often be obtained by single-velocity calibration,
at the approximate midpoint of the normal working range.2'3'6 The purpose
of this paper is, in light of the results obtained in the studies cited,
to evaluate the single-velocity calibration technique as a means of deter-
mining Type-S pitot tube coefficients.
STUDY 1
EXPERIMENTAL METHOD
A study was recently conducted in which 51 isolated Type-S pitot
tubes (i.e., the tubes alone, not attached to sample probes) were calibrated
o
against a standard pitot tube. The calibrations were done in a wind tun-
nel having a test-section diameter of 12 inches. Prior to calibration, one
leg of each Type-S pitot tube was marked "A", and the other, "B". Each
Type-S tube was then calibrated, first with the A-side impact opening facing
Emission Measurement Branch, ESED, OAQPS, EPA, RTP, NC, March 1976
-------
49
the flow, and then with the B-side opening facing the flow. During each
A- and B-side calibration, data were taken at six different test-section
velocities, ranging from about 1500 to 3500 ft/min, and spaced at approxi-
mately equal intervals over this range. At each of the six velocity set-
tings, the value of C , the Type-S pi tot tube coefficient, was calculated,
as follows:
(Eq.
Where:
C = Type-S pi tot tube coefficient
C (std) = coefficient of standard pi tot tube
P
AP ., = standard pi tot tube reading, in. H,,0
AP = Type-S pi tot tube reading, in. H,>0
METHOD OF DATA ANALYSIS
The data from Study 1 were analyzed in the following manner:
1. For each of the 102 (i.e., 51 A-side and 51 B-side) calibrations,
a, the percentage variation in the value of C over the velocity
range from 1500 to 3500 ft/min, was calculated using the following
equation:
- Cn(»)~
Percentage Variation in C = a =
P
P
x 100 (Eq. 2)
Where:
C (h) = highest value of C obtained
C (A) = lowest value of C obtained
-------
50
2.
3.
A graph (histogram) was then constructed, showing the frequency
distribution of the 102 values of a.
For each of the 102 calibrations, the percentage deviation of
Cp(h) from Cp (the value of C obtained at 3000 ft/min, the
approximate midpoint of the normal working range) was calculated
as follows:
Percentage
deviation ,
of C (h) from C
x 100
(Eq. 3)
A histogram was then constructed, showing the frequency distribu-
tion of the 102 values of a.
For each of the 102 calibrations, the percentage deviation of
C (J() from C was determined as follows:
Percentage
deviation i
of C (1) from C
x 100 (Eq. 4)
A histogram was then constructed, showing the frequency distribu-
tion of the 102 values of e.
RESULTS OF DATA ANALYSIS
The results of the analysis of the data from Study 1 are presented
graphically in Figures 1, 2, and 3; in view of the fact that a large number
(51) of randomly-selected pi tot tubes were calibrated in this study, these
results can be considered typical of the Type-S instrument. Figure 1 shows
-------
51
that the mean value of a for the velocity range from 1500 to 3500 ft/min
was 1.4 percent; about 90 percent of the a values were 2.2 percent or less,
and 99 percent of them were 2.6 percent or less. Figures 2 and 3 show that
*
in general, the percentage deviations of C (h) and C U) from C were small;
the calculated values of a and 3 ranged from 0.0 to 1.9 percent, and
averaged 0.7 percent. Analysis of the data from Study 1 has therefore
demonstrated that the coefficient of a given isolated Type-S pi tot tube will
vary by 2.6 percent or less over the velocity range from 1500 to 3500 ft/min,
and that the value of C at any velocity point within this range will be
within +_ 2 percent of C , the coefficient value at 3000 ft/min.
STUDY 2
EXPERIMENTAL METHOD
In June 1974, the University of Windsor conducted a study in which six
commercially available Type-S pitot tubes (representing five different
manufacturers) were calibrated against a standard pitot tube. The calibra-
tions were done in a wind tunnel having a 30- x 30-inch rectangular test sec-
tion. A total of 47 calibration runs were performed with the six Type-S
pitot tubes. During each run, calibration data were taken at 10 or more
different, regularly spaced test-section velocities covering the range from
600 to 4200 ft/min. At each velocity setting, the value of the Type-S pitot
tube coefficient was calculated (Equation 1). Note that isolated pitot
tubes were calibrated in only 2 of the 47 runs; in the other 45 runs, the
tubes were calibrated in various "pitobe assembly" configurations (i.e.,
while attached to sample probes equipped with nozzles or thermocouples
-------
52
20
MEAN VALUE = 1.4%
AVERAGE DEVIATION =0.4% ..
15
10
5
0 0.2
PERCENTAGE VARIATION IN Cp
(a)
PERCENTAGE DEVIATION OF Cp(h) FROM Cp"
(a)
Figure 1. Frequency distribution of a values; Study 1; ve-
locity range 1500 to 3500 ft/min.
Figure 2. Frequency distribution of a values; Study 1; ve-
locity range 1500 to 3500 ft/min.
MEAN VALUE = 0.7X
AVERAGE DEVIATION - 0.4% ,
PERCENTAGE DEVIATION OF Cp ( £} FROM Cp*
1(3)
Figure 3. Frequency distribution of (3 values; Study 1; ve-
locity range 1500 to 3500 ft/min.
-------
53
or both). Aerodynamic interference effects, caused by the nozzle and/or
thermocouple elements being too close to the Type-S pi tot tube impact
openings, were present in many of the pitobe assemblies; consequently,
many of the values of C obtained by calibration were considerably below the
range (0.83 to 0.87) considered "normal" for the Type-S instrument. In 9
of the 45 pitobe assembly runs, sample gas was drawn through the nozzle (at
a number of different flow rates) during calibration.
METHOD OF DATA ANALYSIS
The data from Study 2 were analyzed in the following manner:
1. Considering only those data falling within the velocity range
covered in Study 1 (^ 1500 to 3500 ft/min), values of a, a, and e
were calculated for each of the 45 pitobe assembly runs, using,
respectively, equations 2, 3, and 4 above. Histograms, showing
the frequency distributions of the a, a, e values, were then
constructed and compared against the a, a, and e distributions
from the first study.
2. Considering only those data falling within the "normal working
range" (^ 1000 to 5000 ft/min), values of a, a, 6 were calculated
for each of the 47 runs, using Equations 2, 3, and 4, respectively.
Histograms were then constructed, showing the a, a, and e fre-
quency distributions.
RESULTS OF DATA ANALYSIS
The results of the analysis of the data from Study 2 are presented
graphically in Figures 4 through 9. Figure 4 shows that the mean value of
-------
:or the 45 pitobe asssn^ly rui.s, in the vorjcity range from 1500 to
3500 ft/min, was 1.5 percent; more than 90 percent of the a values were
2.5 percent or less. This is in excellent agreement with the results ob-
tained in Study 1 (see Figure 1), in which the mean value of a was 1.4
percent, and 99 percent of the a values were 2.6 percent or less. Figures
5 and 6 show that the mean values of a and 3 were 0.5 percent and 1.0
percent, respectively. These results also compare favorably with those
obtained in the first study (see Figures 2 and 3). It is significant that
the data from the pitobe assembly runs of Study 2 agree with the data from
Study 1, in which isolated Type-S pi tot tubes were calibrated; this indicates
that the configuration in which Type-S pi tot tubes are calibrated apparently
has little or no effect on the values of a, a, and e.
Figure 7 shows that the mean value of a (for all runs) in the velocity
range from 1000 to 4200 ft/min was 2.4 percent, and that 98 percent of the
a values were 5.4 percent or less. Figure 8 shows that most of the values
of a for the 1000 to 4200 ft/min range were small; the average value of a
was 0.7 percent; 98 percent of the a values were 2.2 percent or less. Figure
9 shows that the values of g were slightly greater than the a values; the e
values ranged from 0.0 to 5.2 percent, and averaged 1.6 percent; approxi-
mately 90 percent of the p values were 3 percent or less. Thus, extending
the velocity range downward from 1500 to 1000 ft/min, and upward from 3500 to
4200 ft/min, produced only a 1 percent increase in the average value of a; also,
there was essentially no change in the average value of a, and only a slight
(0.9%) increase in the average value of 0. It therefore appears safe to conclude
that had the calibrations been done by single-velocity technique at the midpoint
-------
55
a
UJ
s
PERCENTAGE VARIATION IN C
(a)
2.5 3.0,
P
«.S 1.1! 1.5 2.0/ 2.5 3.0
PERCENTAGE DEVIATION OF Cp(£l FROM Cp*
3.5
Figure 4. Frequency distribution of a values; Study 2; veloci-
ty range 1500 to 3500 ft/mi n; pitobe assembly runs only.
Figure 6. Frequency distribution of J3 values; Study 2; velocity range
1500 to 3500 ft/min; pitobe assembly runs only.
MEAN VALUE = 0.5%
AVERAGE DEVIATION = 0.4%
0.1 0.5 0.9 1.3 1.7
PERCENTAGE DEVIATION OF Cp(h) FROM Cp*
(CC]
Figure 5. Frequency distribution of a values; Study 2; velocity range 1500
to 3500 ft/min; pitobe assembly runs only.
-------
56
MEAN VALUE = 1.6%
AVERAGE DEVIATION - 0.9%
PERCENTAGE VARIATION IN Cp
U)
Figure 7. Frequencydistribution of a values; Study 2; velocity
range 1000 to 4200 ft/min.
PERCENTAGE DEVIATION OF Cp (fl FROM Cp"
(0)
Figure 9. Frequency distribution of (3 values; Study 2- ve-
locity range 1000 to 4200 ft/min.
6
3
1.2 1.7 2.2
PERCENTAGE DEVIATION OF Cp(h) FROM Cp"
(a)
Figure 8. Frequency distribution of (rvalues; Study 2; velocity range 1000
to 4200 ft/min.
-------
57
of the normal working range (^ 3000 ft/min), better than 90 percent of the Cp
values obtained would have been valid to within + 3 percent over the entire
1000 to 4200 ft/min range.
STUDY 3
EXPERIMENTAL METHOD
North Carolina State University (NCSU) recently conducted a study in
456
which six Type-S pitot tubes were calibrated against a standard pitot tube. ' '
The calibrations were done in the NCSU low-speed wind tunnel. More than 60
calibration runs were performed using the six Type-S pitot tubes; data from
59 of the runs (considered sufficient to represent the study) will be
analyzed. During each run, calibration data were taken at five or more
regularly spaced test-section velocities, covering the range from 600 to
6000 ft/min. At each velocity setting, the value of the Type-S pitot tube
coefficient was calculated (Equation 1). In 12 of the 59 runs, isolated
pitot tubes were calibrated; in the other 47 runs, pitobe assemblies, some
of which had aerodynamic interference problems, were calibrated. In 10 of
the 47 pitobe assembly runs, sample gas was drawn through the nozzle during
calibration.
METHOD OF DATA ANALYSIS
The data from Study 3 were analyzed in the following manner:
1. Considering only those data falling within the velocity range covered
in Study 1 (~ 1500 to 3500 ft/min), values of a, a, and 3 were cal-
culated for each of the 47 pitobe assembly runs, using, respectively,
-------
58
equations 2, 3, and 4 above. Histograms, showing the frequency
distribution of the o, a, and 6 values, were then constructed
and compared against the a, a, and 3 distributions from the first
study.
2. Considering only those data falling within the normal working range
(^ 1000 to 5000 ft/min), values of a, a, and 3 were calculated for
each of the 59 runs, using (respectively) Equations 2, 3, and 4.
Histograms were then constructed showing the a, a, and 3 frequency
distributions.
RESULTS OF DATA ANALYSIS
The results of the analysis of the data from Study 3 are presented
graphically in Figures 10 through 15. Figure 10 shows that the mean value
of a for the 47 pitobe assembly runs, in the velocity range from 1500 to
3500 ft/min, was 1.2 percent; approximately 90 percent of the a values were
3 percent or less. Figures 11 and 12 show that the average values of » and
3 were, respectively, 0.4 percent and 0.9 percent. These results compare
favorably with those obtained in Study 1, once again indicating that the
values of 0, a, and e are apparently independent of the configuration in
which the pi tot tubes are calibrated.
Figure 13 shows that the mean value of a (for all runs) in the velocity
range from 1000 to 5000 ft/min was 2.2 percent; approximately 98 percent of
the a values were 5.2 percent or less. Note that the distribution of a values
obtained in Study 3 is almost identical to the a distribution obtained for
the 1000 to 4200 ft/min range in Study 2 (compare Figures 13 and 7); this
-------
59
MEAN VALUE =
AVERAGE DEVIATION -0.9%
MEAN VALUE-0.4%
AVERAGE DEVIATION * 0.2S
PERCENTAGE VARIATION IN Cp
(a)
0.2 0.4 0.6 0.8 1.0 1J 1.4 1.S 1.8
PERCENTAGE DEVIATION OF Cp(h) FROM Cp'
I a)
end Figure 11. Frequency distribution of (lvalues; Study 3; velocity range 1500
Fioure 10 Frequency distribution of o values; Study 3; velocity range iouu to 3500 ft/min; pitobe assembly runs only.
to 3500 ft/min; pitobe assembly runs only.
MEAN VALUE »O.S*
AVERAGE DEVIATION - 0.9%
24
21
18
15
12
0; 0.2
1.2i
MEAN VALUE-2.2%
AVERAGE DEVIATION - I.IK _
2.2'
3.2
4.2
5.2
PERCENTAGE DEVIATION OF CpUl FROM Cp*
(0)
Figure 12- Frequency distribution of Rvalues; Study 3; velocity range 1500
to 3500 ft/min; pitobe assembly runs only.
PERCENTAGE VARIATION IN Cp
(o)
Figure 13. Frequency distribution of a values; Study 3; veloci-
ty range 1000 to 5000 ft/min.
-------
60
indicates that in the velocity range from 4200 to 5000 ft/min, the amount
of variation in the value of Cp is negligibly small. Figure 14 (compare
Fig. 8) shows that the average value of a was 1.0 percent; 98 percent of
the a values were 3.1 percent or less. Figure 15 (compare Fig. 9) shows
that the values of e ranged from 0.0 to 4.4 percent, and averaged 1.2
percent; about 90 percent of the e values were 2.8 percent or less.
Analysis of the data from Study 3 has, therefore, yielded results con-
sistent with those obtained in Studies 1 and 2.
CONCLUSIONS
Careful analysis of data from three recent pi tot tube calibration studies
has demonstrated the following:
1. The coefficient of a given Type-S pitot tube can be expected to
vary by 5 percent or less over the normal working velocity range
from 1000 to 5000 ft/min. This should be true whether the Type-S
pitot tube is calibrated alone or as a component or a pitobe
assembly.
2. For a given Type-S pitot tube, the value of C at any point
within the normal working range can be expected to be within about
± 3 percent of Cp , the value at 3000 ft/min. This,also,should
be true for isolated pitot tubes as well as for pitobe assemblies.
Hence, values of Cp obtained by single-velocity calibration at the
approximate midpoint of the normal working range will, in general,
be valid to within +_ 3 percent over the entire range.
-------
61
24
2V
18
12
MEAN VALUE = 1.0%
AVERAGE DEVIATION-0.7%
\
Ol 0.3 1.0 1-7 2-« 11 3J
PERCENTAGE DEVIATION OF Cp(h) FROM Cp*
lot)
Figure 14. Frequency distribution of a values; Study 3; velocity range 1000
to 5000 ft/min.
ME AN VALUE-1.2*
AVERAGE DEVIATION -0.1%
0.4 1.1 M> Z* "
PERCENTAGE DEVIATION OF CpU) FROM Cp*
Figure 15. Frequency distribution of (3 values; Study 3; velocity range 1000
to 5000 ft/min.
-------
62
REFERENCES
1. Standards of Performance for New Stationary Sources.
36(247). December 23, 1971.
2. Vollaro, R. F. A Type-S Pi tot Tube Calibration Study. U. S. Environ-
mental Protection Agency, Emission Measurement Branch. Research Triangle
Park, N. C. July 1974.
3. Gnyp, A. W.s C. C. St. Pierre, D. S. Smith, D. Mozzon, and J. Steiner.
An Experimental Investigation of the Effect of Pi tot Tube-Sampling Probe
Configurations on the Magnitude of the S-Type Pitot Tube Coefficient for
Commercially Available Source Sampling Probes. Prepared by the University
of Windsor for the Ministry of the Environment. Toronto, Canada.
February 1975.
4. DeJarnette, F. R. and J. C. Williams, III. A Study of the Effects of
Probe Design, Construction, and Use on the Accuracy of Type-S Pitot Tubes;
Progress Report No. 2. Submitted to U. S. Environmental Protection Agency
by the Mechanical and Aerospace Engineering Department of North Carolina
State University. Raleigh, N. C. October 10, 1974.
5. Terry, Ellen W. and Herbert E. Moretz. Effects of Geometry and Inter-
ference on the Accuracy of S-Type Pitot Tubes. Submitted to U. S. Environ-
mental Protection Agency by the Mechanical and Aerospace Engineering
Department of North Carolina State University. Raleigh, N.C. April 1975.
6. Terry, Ellen W. and Herbert E. Moretz. First Annual Report on the Effects
of Geometry and Interference on the Accuracy of S-Type Pitot Tubes. Sub-
mitted to U. S. Environmental Protection Agency by the Mechanical and
Aerospace Engineering Department of North Carolina State University.
Raleigh, N. C. August 1975.
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63
GUIDELINES FOR TYPE-S PITOT TUBE CALIBRATION
Robert F. Vollaro*
INTRODUCTION
In source sampling, the Type-S pi tot tube is the instrument most com-
monly used to measure stack gas velocity. Before a Type-S instrument is
used in the field, its coefficient (Cp) must be determined by calibration
against a standard pitot tube. The current Type-S pitot calibration auide-
l« (Fe_deral Reaister, 12/23/71) specify that calibration be done b?mea-
suring velocity head ". . . at some point in a flowing gas stream with
both a Type-S pitot tube and standard pitot tube. Calibration should be
done in the laboratory, and the velocity of the flowing gas stream should
be varied over the normal working range . . ."'
A number of recent studies2'3'4'5 have demonstrated, however that
more detailed and systematic calibration guidelines are needed to ensure
uniformity of application by different observers, which is essential for
the obtainment of accurate, reproducible results. The purpose of this paper
is, based on these recent findings, to present practical, comprehensive
guidelines for the calibration of Type-S pitot tubes.
A SUMMARY OF FACTORS WHICH CAN AFFECT THE VALUE OF Cp
A number of recent studies have shown that when a Type-S pitot tube is
used as a component of a pitobe assembly (i.e., when the tube is attached
to a sample probe equipped with appropriate nozzle and thermocouple)
aerodynamic interactions between the pitot tube and the other components of
the assembly can cause a substantial lowering in the value of the pitot
coefficient (cp).^,4,5 At present, there are four specific known types
of aerodynamic effect associated with pitobe assemblies. These have been
spoken of extensively in the references cited; nevertheless, it will be ad-
vantageous to briefly review them at this time.
<]) Effect of Sampling Nozzle - If there is insufficient separation
distance between a Type-S pitot tube and sampling nozzle, the
tube and nozzle will interfere aerodynamically, resulting in a 3
to 9 percent lowering of Cp. It has been demonstrated that a-
bout 3/4 inch free space between the pitot tube and nozzle will
essentially eliminate this effect.2
(2) Effect of ThexmocQuple - When a thermocouple wire is attached to
a Type-S pitot tube and placed in such a way that the tip of the
wire is in line with the centerline of the pitot tube impact
openings, aerodynamic interference occurs, causing a sharp re-
duction in the value of Cp (up to 9 percent).3
(3) Effect of Probe Sheath. (I) - in small ducts J(* 12 to 36 in-
ches in diameter)., a reduction in Cp (up to 4 percent) can occur,
resulting from reduction of the effective cross-sectional area
of the duct by the probe sheath.5
(4) Effect..of Probe Sheath, (II) - If a pitobe assembly is ronstruc-
ted in such a way that the distance from the renter of the pitot
* Emission Measurement Branch, ESED, OAQPS, EPA, RTP, NC. Presented at 1st
Annual Meeting, Source Evaluation Society, Dayton, Ohio, September 18, 1975
Published in Source Evaluation Society Newsletter 1(1), January 1976
-------
64
tube impact openings to the leading edge of the probe sheath is
less than 3. inches, a slight reduction (up to 3 percent) in C0
can occur.
It is therefore evident that if comprehensive Type-S pitot tube Cali-
bration guidelines are to be written, each of the above effects must be
taken into consideration.
OF TYP_Ejj_PITQT_TUBF COEFFICIENTS
Apparatus and Experimental Set-Up
A. Flow System - Calibration shall be done in a flow system (see
Figure 1) having the following essential design features:
(1) The flowing gas stream must be confined to a definite
cross-sectional area, either circular or rectangular
For circular cross-sections, the minimum duct diameter
shall be 30.5 cm (12 inches); for rectangular cross-sec-
tions, the width (shorter side) shall be at least 25.4 cm
(10 inches).
(2) The cross-sectional area of the flow system must be con-
stant over a distance of 10 or more duct diameters. For
rectangular cross-sections, use an equivalent diameter,
calculated as follows, to determine the number of duct
diameters:
2 LW
De = (L + W) (Equation 1)
Where:
Dg = Equivalent diameter
L = Length
W = Width
To ensure the presence of stable, well-developed flow
patterns at the calibration site ("test section") the
site shall be located at least 8 duct diameters down-
stream and 2 diameters upstream from the nearest distur-
bances.
(3) The flow system shall have the capacity to generate a
test-section velocity around 3000 ft/min, which is the
approximate midpoint of the "normal working range"
(^ 1000 to 5000 ft/min). This velocity must be'constant
with time, to guarantee steady flow during calibration.
-------
65
o
CO
|
-------
66
Note that Type-S pi tot tube coefficients obtained by sin-
gle-velocity calibration at the midpoint of the normal
working range will generally be valid to within +_ 2 per-
cent over the entire range./ If a more precise Correla-
tion between Cp and velocity is desired, the flow system
shall have the capacity to generate a number of distinct,
time-invariant test-section velocities, covering the nor-
mal working range, and calibration data shall be taken at
regular velocity intervals between 1000 and 5000 ft/min
(see Section II).
(4) Two entry ports, one each for the standard and Type-S
pi tot tubes, shall be cut in the system test section.
The standard pi tot tube entry port shall be located
slightly downstream of the Type-S port, so that the stan-
dard and Type-S impact openings will lie in the same
plane during calibration. To facilitate alignment of the
pitot tubes during calibration, it is advisable that the
test section be constructed of plexiglas or some other
transparent material.
B. Calibration Standard - A standard pitot tube shall be used to
calibrate the Type-S pitot tube. The standard pitot tube
shall have a known coefficient, obtained either directly from
the National Bureau of Standards in Gaithersburg, Maryland, or
by calibration against another pitot tube with a known (NBS
traceable) coefficient. Alternatively, a modified ellipsoidal
nose pitot-static tube, designed according to the criteria
illustrated in Figure 2, can be used. This is the only type of
standard pitot tube recommended by the British Standards
Institute (B.S.I.) for use without individual calibration.6
Note that the coefficient of the ellipsoidal nose tube is a
function of the stem/static hole distance; therefore, Figure 3
should be used as a guide in determining its precise coeffici-
ent value.*
C. Differential Pressure Gauge - An inclined manometer, or equi-
valent device, shall be used to measure velocity head (AP).
The gauge shall be capable of measuring AP to within + 0.13 mm
H20 (0.005 in. H20).
D- Pitot Lines - Flexible lines, made of Tygon or similar tubing,
and equipped with appropriate fittings (preferably quick-dis-
connect type) shall serve to connect the pitot tubes to the
differential pressure gauge.
See "Addendum"
-------
67
!
o
o
c
-------
68
1.0Q6r
1.004 -
1.002
1.000
0.998
0.996
0.994
0.992
0.990
°-02 0.04 0.06 0.08 0.10 0.12
0.14
JD
nD
Figure 3. Effect of stem/static hole distance on basic coefficient, C0, of standard pilot-static tubes
with ellipsoids! nose.
-------
69
II Calibration Procedure* - The Type-S pi tot tube shall be assigned a
HeT^T^ridiFtTTTcrtion number. This number shall be permanently
e .
marked or engraved on the body of the tube. Also, one leg of the
tube shall be marked "A," and the other, "B." To obtain calibration
data for the A and B sides of the tube, proceed as follows:
A Make sure that the manometer** is properly filled and that the
oil is free from contamination. Irspect and leak-check all
pi tot lines; repair or replace if necessary.
B Level and zero the manometer. Turn on the fan and allow the
flow to stabilize; if single-velocity technique is used, the
test section velocity should be about 3000 ft/min. If multi-
velocity calibration technique is used, begin with a test-
section velocity of about 1000 ft/min. Seal the Type-S entry
port.
C Position the standard pitot tube at the calibration point
(determined as outlined in sections IV and V), and align it
so that its tip is pointed directly into the flow. Particular
care should be taken in aligning the tube, to avoid yaw and
pitch angles. Make sure that the entry port surrounding the
tube is properly sealed.
D Read AP tH and record its value in a data table, similar either
to the one shown in Figure 4a (single-velocity calibration), or
the one shown in Figure 4b (multi -velocity calibration). Re-
move the standard pitot tube from the duct and disconnect it
from the manometer. Seal the standard entry port.
E Connect the Type-S pitot tube to the manometer. Open the Type-S
entry port. Insert and align the Type-S pitot tube so that its
"A" side impact opening is at the same point as was the standard
pitot tube, and is pointed directly into the flow. Make sure
that the entry port surrounding the tube is properly sealed.
F Read APS and enter its value in the data table. Remove the
Type-S pitot tube from the duct and disconnect it from the mano-
meter.
G. Repeat steps C through F above, until three sets of velocity
head readings have been obtained.
*~ltote~that"this procedure is a general one, and must not be used without
first referring to the specific considerations presented in sections IV,
V, VI, and VII.
** If used; otherwise, check differential pressure gauge to be sure that it
is operating properly.
-------
70
cc
LU
a «=
a
LU LU
CO L_
^- cc
H- ca
o
z
o
f-
cc
CO
CJ
a
CO
a.
LU CJ
a
a.
CJ
.
J2. «M
n. 3:
,
If
O
cc
CM
«
3
LU
0
CO
, n.
CJ
P
CALIBRATIO
LU
a
CO
CO
*
I"?
> n.
LU CJ
a
a.
O '
3
^ CM
a. OC
<" .5
5^
S =
<3 c
C3
z.
CC
-
CM
cn
ca
UJ
a
CO
LU
CO
CO
00
cc
o
_o
a>
LU
CO
CO
cc
o
It
z
a
<
LU
O
LU
C3
CC
LU
CO
LU
ra
CJ
a
CO
"n.
I"
-------
71
<
CJ
1 6
a
LU LU
CO J-
=3 <
K OC
-
O
h;
a.
«
0
}
DC
ca
_i
C_9
LU
a
CO
CO
C3
"*
c
> ~~°-
Ul CJ
a
-~
u
v> CM
o- -x.
c
_ _
afs:
< e
z .
3 O
cc z
C3
sr
LU
CO
CM
n
<
CM
CO
ca
^
CM
n
u
^,
CM
n
O
LU
03
t-
=3
II
O
u
o
o S
LU
CO
LU
a
O 3=
tf O
cc <
UJ LU
-------
72
H. If single-velocity calibration technique is being used, repeat
steps C through G above for the B-side of the Type-S pi tot
tube. If multi-velocity technique is employed, repeat steps C
through G for each of the remaining A-side velocity settings,
and then calibrate the B-side of the pi tot tube in same manner
as side "A."
III.Calculations
A. For single-velocity calibrations, perform the following calcu-
lations :
1. For each of the 6 pairs of velocity head readings (i.e. 3
from side A and 3 from side B) obtained in section II
above, calculate the value of the Type-S pitot tube coef-
ficient, as follows:
APstd
Cp(s) = Cp(std)y/ APS (Equation 2)
Where:
C (s) = Type-S pitot tube coefficient
C (std) = Standard pitot tube coefficient
& std = Velocity head, measured by standard pitot
tube, cm H20 (in. H20)
AP = Velocity head, measured by Type-S pitot
tube, cm H20 (in. HgO).
2. Calculate C~p (side A), the mean A-side coefficient, and
Cp (side B), the mean B-side coefficient; calculate the
difference between these two average values.
3. Calculate the deviation of each of the three A-side values
of Cp(s) from Cp (sj_de A), and the deviation of each B-side
value of Cp(s)from Cp (side B). Use the following equa-
tion :
Deviation = C (s) - C (A or B) (Equation 3)
4. Calculate a, the average deviation from the mean, for both
the A and B sides of the pitot tube. Use the following
-------
73
3
y I
c (side A or B) = ]_ysJ_l_S (A^r J^L (Equation 4)
3
5. Use the pi tot tube if and only if the difference between
(TD (side A) and Cp (side B) is ^0.01, and if the A and B
side average deviations, calculated using Equation 4, are
0.01 or less. When a Type-S pi tot tube fails to meet
either of these criteria, it generally indicates that the
tube has been improperly constructed.
B. For multi-velocity calibrations, perform the following calcu-
lations :
1. At each A-side velocity setting, use equation 2 to calcu-
late the three values of CB(S) corresponding to runs #1,
2, and 3(see Figure 4b). Calculate Cp, the average
(mean) of these three Cp(s) values.
2. For each C_ calculated in step 1 above, use equation 4 to
determine 5, the average deviation from the mean.
3. Repeat steps 1 and 2 above for the B-side of the pi tot
tube.
4. Calculate the average test-section velocity, in feet per
minute, corresponding to each A and B-side fan setting.
Use the following equation:
Where:
v~ = KC T APstd (Equation 5)
PM '
v = Average test-section velocity at the
particular fan setting, ft/min.
K = 5130 (constant)
C = Coefficient of standard pi tot tube.
T = Temperature of flowing gas stseam
during calibration (ambient), °R.
-------
74
P = Barometric pressure during calibration,
in. Hg.
M = Molecular weight of air = 29.0.
= Average of the three standard pi tot tube
readings at the particular fan setting,
in. H20.
5. Using the Cp values and their corresponding "v values as
ordered data pairs, construct a plot of Cn versus 7; plot
both the A-side and B-side data on a single set of co-or-
dinate axes (see Figure 5).
6. Use the pitot tube if and only if: (a) all of the A and
B-side average deviations, calculated using Equation 4,
are <_ 0.01, and (b) the difference between the A and B--
side, curves (see Figure 5) is £0.01 for any given value
of v between 1000 and 5000 ft/min,
IV- Specific Considerations Pertaining to Calibration .of Isolated
Type-S Pitot Tubes' ~~~
When an isolated Type-S pitot tube is to be calibrated, se-
lect a calibration point at or near the center of the duct, and
follow the procedures outlined iri^ sections II and_ III above. The
coefficients so obtained, i.e., Cp (side A) and CD (side B), will
be valid for the measurement of stack gas velocities between 1000
and 5000 ft/min, so long as the isolated pitot tube is used.7 If,
however, the pitot tube is used as a component of a pitobe assem-
bly, the isolated coefficient values may or may not apply; this is
discussed more fully in section V.
V. Pitobe Assemblies
Generally speaking, when a Type-S pitot tube is used as a com-
ponent of a pitobe assembly, its A and B-side coefficients will
differ appreciably from their respective isolated values, due to
aerodynamic interactions among the assembly components. In fact,
the isolated and assembly coefficient values will only be the same
if the ssc.--br.-is constructed according to the following specifi-
cations:
A. To minimize aerodynamic interactions between the pitot tube and
sampling nozzle there must be a separation distance (free-
space) of at least 3/4 inch between the nozzle and pitot tube,
with the largest size nozzle (usually 1/2 inch, i.d.) in place
(see Figure 6a).
B. To minimize aerodynamic interactions between the thermocouple
-------
0.860,
0.850
0.840
0.830
0.820
B-SIDE
DATA^
NOTE- FOR ALL VALUES OF iJBETWEEN 1000 AND 5000 ft/min, THE
DIFFERENCE, "d" = |Cp(A) Cp(B}| .MUST BE <0.01.
1000
2000
4000
5000
3000
TJ, ft/min.
Figure 5. Typical calibration curve, multi-velocity calibration.
6000
-------
TYPE-SPITOTTUBE
X>3/4in. FOR Dn =1/2 in.
Dn^~
SAMPLING NOZZLE
c
Figure 6a. Minimum pitot-nozzle separation needed to prevent interference.
-------
77
and pi tot tube, the thermocouple wire must be mounted on the
pitot tube in such a way that the tip of the wire is in line
with, but at least 3/4 inch from, the center of the pitot
tube impact openings (see Figure 6b).
C. To eliminate pitot tube-probe sheath interference, there must
be at least 3 inches between the leading edge of the probe and
the center of the pitot tube impact openings (see Figure 6c).
For those assemblies which either (1) meet requirements A
through C above but have unknown isolated coefficients, or (2)
fail to meet requirements A through C, use the procedures outlined
in sections II and III, in conjunction with the following special
considerations, to determine the A and B-side coefficients of the
Type-S pitot tube:
1. Although it is preferable that the calibration point be located
at or near the center of the duct, insertion of a probe sheath
into a small duct may cause significant cross-sectional area
blockage, and yield incorrect coefficient values. Therefore,
to minimize the blockage effect, the calibration point may be
a few inches off-center if necessary. To keep the actual re-
duction in Cp due to blockage below 1 percent, it is necessary
that the theoretical blockage, as determined by a projected-
area model of the probe sheath, be 2 percent or less of the
duct cross-sectional area for assemblies without external
sheaths (see Figure 7a) and 3 percent or less for assemblies
with external sheaths (Figure 7b).5
2. For pitobe assemblies in which pitot tube-nozzle interference
is a factor (i.e., those in which the pitot-nozzle separation
distance is less than 3/4 inch* with a 1/2 inch nozzle in
place), the value of Cp will depend somewhat on the amount of
free-space between the tube and nozzle2>3; -jn these in-
stances, separate calibrations should be performed with each
of the commonly-used nozzle sizes in place. Note that single-
velocity calibration technique will be acceptable for this
purpose, even though the larger nozzle sizes (> 1/4 inch) are
not ordinarily used for isokinetic sampling at velocities a-
round 3000 ft/min (the calibration velocity); data from a re-
cent study have shown that for pitobe assemblies with 3/8 inch
and 1/2 inch nozzles in place, Cp does not change appreciably
with velocity over the normal working range.3 If multi-velo-
city calibration technique is used, it is reco;:r,iended, for the
sake of simplicity that each nozzle size be studied only in
Note carefully that a thermocouple wire ettaci',r-d to a Type-S pitot tube
can cause a reduction in the effective a::.;'jnt of frce-sr^ce bstv/een the
pitot tube and nozzle, if the wire is situated vL-t.-'c-en the ii,he and noz-
zle.
-------
IHtHMUUUUKLE ^X>^
)'C
K
Z>3/4in.
TYPE-SPITOTTUBE
1
SAW
ft
PLEP
fl
ROBE
00
Figure 6b. Proper thermocouple placement to prevent interference.
-------
TYPE-S PITOT TUBE
C
SAMPLE PROBE
-Y>3in.-
Figure 6c. Minimum pilot-sample probe separation needed to prevent interference.
-------
PERCENT
THEORETICAL
BLOCKAGE
00
o
X W
1
DUCTAREAj
x100
Figure 7. Projected-area models for typical pitobe assemblies.
-------
81
that part of the normal working range in which it is ordinarily
used for isokinetic sampling (see Figure 8); however, calibra-
tion data covering the entire range may be taken for each noz-
zle size, if desired.
VI. Recall bration and Field Use
A. The Type-S pi tot tube shall be calibrated before its initial
use. Thereafter, if the tube has been significantly damaged by
field use (for example, if the impact openings are bent out of
shape, cut, nicked, or noticeably misaligned), it shall be re-
paired if possible, recalibrated, and replaced if necessary.
B. When the Type-S pi tot tube is used in the field, the appro-
priate A or B-side coefficient shall be used to perform velo-
city calculations, depending upon which side of the pi tot tube
is pointed toward the flow. For tubes calibrated by single-
velocity technique, this coefficient will ordinarily be either
Cp (side A) or Cp (side B), and for tubes calibrated by multi-
velocity technique, it will be the appropriate value of t,
taken from the calibration curve (note, however, the important
exception in section VII).
VII. The Use of Pitobe Assemblies to Sample Small Ducts
v *
When sampling a small duct ("-12 to 36 inches in diameter)
with a pitobe assembly, the probe sheath can block a significant
part of the duct cross-section, causing a reduction in the value
of Cp. Therefore, in certain instances it may be necessary, prior
to sampling, to make adjustments in the coefficient values ob-
tained by calibration. To determine whether or not these adjust-
ments are necessary, proceed as follows:
A. Make a projected-area model of the pitobe assembly, with the
Type-S pitot tube impact openings positioned at the center of
the duct (see Figure 9a). This model represents the approxi-
mate "average blockage" of the duct cross-section which will
occur during a sample traverse. Although the actual blockage
will be less than this for sample points close to the near
stack wall and more than this for points close to the far wall,
the model approximates the average condition.
B. Calculate the theoretical average blockage by taking the ratio
of the projected area of the probe sheath (in.2) to the cross
sectional area of the duct (in. ), and multiplying by 100. If
the theoretical blockage is either 2 percent or less for an
assembly without an external sheath, or 3 percent or less for
an assembly with an external sheath, the decrease in Cp will
be less than 1 percent5 and no adjustment in the pitot tube
coefficients will be necessary. If the theoretical blockaae
See "Addendum"
-------
82
0.840
0.830
0.820
lu
0.810
0.800
0.790
r
'/rin. ! 3/8 in.
NOZZLE! NOZZLE
1000
"T
'/«in.
NOZZLE
1/8 in.
NOZZLE
*-
2000 3000
v. ft/min.
4000
5000
Figure 8. Typical multi-velocity calibration curve far pilube arstmblies.
-------
00
CO
PERCENT
THEORETICAL
BLOCKAGE
x100
Figure 9a. Typical projected-area model for sampling
of small ducts with pitobe assemblies.
-------
84
exceeds these limits, apply corrections to the pi tot tube coef-
ficients as shown in Figure 9b; extrapolate if necessary.
REFERENCES
1. Standards of Performance for New Stationary Sources. Federal Register.
36 (247). December 23, 1971.
2. Vollaro, R. F. The Effect of Aerodynamic Interference Between a Type-S
Pi tot Tube and Sampling Nozzle on the Value of the Pi tot Tube Coeffici-
ent. Environmental Protection Agency. Durham, N. C. February, 1975.
3. Gnyp, A. W., C. C. St. Pierre, D. S. Smith, D. Mozzon, and J. Steiner.
An Experimental Investigation of the Effect of Pi tot Tube - Sampling
Probe Configurations on the Magnitude of the S-Type Pi tot Tube Coeffi-
cient for Commercially Available Source Sampling Probes. Prepared for
the Ministry of the Environment. Toronto, Canada. University of
Windsor. February, 1975.
4. Vollaro, R. F. The Effects of the Presence of a Sampling Nozzle, Ther-
mocouple, and Probe Sheath on Type-S Pitot Tube Accuracy. Environmen-
tal Protection Agency. Durham, N. C. June, 1975.
5. Vollaro, R. F. The Effects of The Presence of a Probe Sheath on
Type-S Pitot Tube Accuracy. Environmental Protection Agency. Durham,
N. C. August, 1975.
6. The Measurement of Fluid Flow in Pipes. British Standards Institute.
London, England. BS 1042 (1971).
7. Vollaro, R. F. An Evaluation of Single-Velocity Calibration Technique
as a Means of Determining Type-S Pitot Tube Coefficients. Environ-
mental Protection Agency. Durham, N. C. August, 1975.
-------
85
LU
IS
U
a
CJ
DC
a
LLJ
31
2!/2 in. CYLINDRICAL MODEL;
USE FOR ASSEMBLIES WITH
EXTERNAL SHEATH.
1 in. CYLINDRICAL MODEL;
USE FOR ASSEMBLIES WITH
NO EXTERNAL SHEATH.
J J__
1 2 3
DECREASE IN PITOT TUBE COEFFICIENT, percent
Figure 9b. Adjustment of type-S pitot tube coefficients to account for blockage effects (small ducts).
-------
12/1/75
86
ADDENDUM TO
"GUIDELINES FOR TYPE-S
PITOT TUBE
CALIBRATION"
I. ALTERNATIVE CALIBRATION STANDARD
According to Section I of these guidelines, it is preferable for the
pitot tube which is used as the calibration standard to have a known coeffi-
cient, obtained either directly from the National Bureau of Standards, or by
calibration against another pitot tube having a known (NBS traceable) coeffi-
cient. Alternatively, an ellipsoidal-nosed pitot static tube, designed
according to the criteria illustrated in Figure 2, can be used as the calibra-
tion standard. The ellipsoidal-nosed pitot static tube is the only type of
standard pitot tube recommended by the British Standards Institute for use
without individual calibration. Despite the excellence of its design, however,
the ellipsoidal-nosed tube is not always carried as a stock item by pitot manu-
facturers; thus, in practice, it may be difficult or expensive to obtain one.
One of the most readily-available types of standard pitot tube is the
Prandtl hemispherical-nosed tube (see Figure A-l). The coefficient of a Prandtl
hemispherical-nosed tube, when designed according to the criteria illustrated in
Figure A-l, will be 0.99 j^O.Ol; its precise coefficient value cannot be deter-
mined without individual calibration, however. From a technical standpoint,
this means that calibrations done using a hemispherical-nosed pitot tube as the
standard will generally not be as accurate as those done using an ellipsoidal-
nosed pitot tube as the standard. Nevertheless, the Prandtl hemispherical-
nosed pitot tube (designed according to Figure A-l) will, because of its availa-
bility and low cost, be considered to be an acceptable alternative to the
ellipsoidal-nosed pitot static tube for use as a calibration standard.
II. ADDENDUM TO SECTION VII (SAMPLING IN SMALL DUCTS)
The usefulness of the projected-area model described in Section VII and
illustrated in Figure 9a is somewhat limited, in that it applies only to: (a)
circular or rectangular cross-sections, having diameters (or equivalent dia-
meters) between 12 and 36 inches; (b) pitobe assemblies having "normal" pitot
tube-probe sheath separation distances (dimension "y", Figure 6c) i.e.,
2"^ y <..4."'The model may or may not be representative of the actual blockage,
when duct diameters less than 12 inches are encountered, or when the pitot tube-
probe sheath separation distances are outside the normal range. When these
situations occur, it is recommended that the following be done:
(a) Abnormal pitot tube - probe sheath spacings
For abnormal pitot tube-probe sheath separation distances, it
will be necessary to make a separate projected-area model at each
traverse point, either along one of the diameters or along one of the
rows, depending upon whether the cross-section is circular or rectan-
gular; in each model, the sampling nozzle opening should be centered
-------
87
around the traverse point in question. The average theoretical
blockage should then be calculated, based on these models, as
follows:
Average
theoretical =
blockage
r?«n"n1
. nAd J
x 100 (Equation 1-A)
Where: n = Number of traverse points on a row or diameter.
Jl = Length of sheath segment inside duct, at the particular
traverse point, (inches).
W = Width of sheath segment at the particular traverse point
(inches).
0
A. = Cross-sectional area of duct, (in ).
Figure 9b should then be used to determine whether or not an adjustment
in the value of Cp is necessary.
(b) Ducts smaller than 12 inches in diameter
At the present time, comprehensive guidelines are being prepared,
by which representative sample traverses can be done in very small
(D < 12 inches) ducts.
-------
8 Static holes
0.02 to 0.04 in.
Impact
opening
0.4D
oo
CD
Figure A-l. Prandtl Hemispherical-Nosed Standard Pi tot Tube
-------
October 7, 1976
89
THE EFFECTS OF IMPACT OPENING MISALIGNMENT
ON THE VALUE OF THE TYPE-S
PITOT TUBE COEFFICIENT
Robert F. Vollaro
United States Environmental Protection Agency
INTRODUCTION
In source-sampling, stack gas velocity is usually measured with a Type-S pitot
tube. Before a Type-S pitot tube is used in the field, its coefficient (Cp)should
be determined by calibration against a standard pitot tube (1). When a Type-S
pitot tube is used in the field, it is subject to considerable abuse; it is often
exposed to hot, corrosive gas streams, and can incur damage when it is trans-
ported to and from field test sites. As a result of this abuse, the impact open-
ings of the pitot tube can become misaligned, which, in turn, can cause a change
in the value of C . Consequently, the pitot tube may need to be recalibrated af-
ter it has been used in the field. Experiments were recently done to determine
quantitatively the effects of impact opening misalignment on the value of Cp.
This paper presents the results of these experiments and discusses their signifi-
cance .
EXPERIMENTAL METHOD
At the outset of the study, a Type-S pitot tub* having properly aligned impact
openings(as a new pitot tube would have, prior to field use) was constructed and
calibrated against a standard pifot tube, thus providing a "reference" coeffi-
cient, C (ref). Subsequently, a number of special Type-S pitot tubes, having
misaligned impact openings, ware constructed and calibrated; the effects of each
type of impact opening misalignment on the value of C were carefully noted. In
all, six different types of misalignment were studied; these six types are con-
sidered representative of misalignment problems which can result from field-use,
because they were selected baa,ed upon observations made in a previous study (2),
in which 51 Type-S pitot tubes (many of which had been used extensively in the
field) were carefully examinee! in top, side, and end-views prior to calibration.
A description of each of the s.ix types of misalignment is given below:
Type 1; Transverse axis misalignment (one opening only).
When the pitot tube is examined in end-view, one impact opening is observed to be
offset by an angle, a, from its properly aligned position with respect to the
transverse tube axis (Figure ],A) Three test cases were considered: a - 3°,
a - 7°, and a - 12°. In each case, data were taken first with the misaligned
opening pointed into the flow, and then pointed away from the flow.
Type 2; Transverse axis misalignment (both openings).
In end-view, both impact openings are offset, at angles otj_ and 02, from their
properly aligned positions with respect to the transverse axis (Figure IB).
Three test cases were considered: a^ - 1*2 ~ 2°; <*i - 0.2 ~ ^*» and al * °2 * ^°
Type 3; Longitudinal axis misalignment (one opening only).
In top-view, one impact opening of the pitot tube is offset by an angle, 0, from
its properly aligned position with respect to the longitudinal tube axis
-------
9Q
(Figures 1C, ID). Note that the angle 3 is arbitrarily defined as negative (-)
when the impact opening is tilted upward as shown in Figure 1C, and positive (+)
when the opening is tilted downward, as in Figure ID. A total of ten test cases
were considered, five with the misaligned opening pointed toward the flow
(3 - -2°, -5°, -8°, +5° and +8°), and five with the misaligned opening opposite
the flow (0 - +2°, +5°, +8°, -58 and -8°).
Type 4; Longitudinal axis misalignment (both openings).
In top-view, both openings are offset, at angles 3i and 02, from their properly
aligned positions with respect to the longitudinal axis (Figure IE). Four test
cases were considered: (Bi - -5°, 02 " +5°)J <81 " ~8°» e2 " +8°>;
(0! - +5°, 02 - -5°); and (6X - +8°, 02 - -8°),
Type 5: Length misalignment.
When the pitot tube is examined in side-view, one leg is observed to be longer
than the other, by an amount, "z" (Figure IF). Three test cases were considered:
z - 1/16 in; z - 1/8 in; and z - 5/32 in.
Type 6; Planar misalignment.
When the pitot tube is examined in side-view, the centerlines of the two legs are
observed to be non-coincident, by an amount, 'V" (Figure 3£). Three test cases
were considered: w - 1/32 in; w - 1/16 in; and w - 1/9 in.
EXPERIMENTAL RESULTS
The results of the experiments are presented in Table I. These results will now
be discussed on a case-by-case, basis t
Reference coefficients; The 4 and B sides of the geometrically perfect Type-S
pitot tube had identical reference coefficient values of 0.846.
Type 1: No significant change, (i.e., greater than 1 percent) from Cp (ref) was
observed for any of the test oases, either with the misaligned opening facing
the flow or opposite the flow.
Type 2; Again no significant change from Cp (ref) was observed for any of the
test cases.
Type 3: With the misaligned impact opening facing the flow, the following re-
sults were obtained: (a) for 0 f. -2°, C remained essentially equal to Cg (ref);
(b) for 0 - -5°, Cp increased to 0.860; Xc) for 0 = -8% Cp increased to 6.867;
(d) for 0 - +5°, Cp decreased to 0.834; (e) for 0 - +8*, Cp decreased to 0.828.
With the misaligned opening oppo3ite the flow, no change from Cp (ref) was ob-
served for any of the five test cases. Thus, it appears that for 0 values up to
8°, longitudinal axis misalignment effects are "one-sided," occurring only when
the misaligned opening faces the flow; the effects are also directional, in that
Cp increases for negative values of 0 and decreases for positive 0 values.
Type 4; For the test case (0! = -5°, 02 = +5°), a Cj value of 0.857 was ob-
tained; for (0! - -8°, 02 - +8°). Cp increased to 0.863; for (0! - +5°, 02 - -5°),
Cp decreased to 0.831; for (01-+80, 02 - -8°), C decreased to 0.828. These re-
sults are consistent, both in magnitude and direction, with those observed for
-------
91
Type 3 misalignment, indicating once again that the flow only
longitudinal axis misalignment of the impact opening which faces the flow.
Type 5; With the shorter leg of the pitot tube facing the flow, the following
results were obtained: for z ° 1/16 in, there was no change from C (ref) ; for
z - 1/8 in, Cp decreased to 0.837; for z = 5/32 in, Cp decreased to 0.828. With
the longer leg facing the flow, Cp remained essentially equal to C (ref), for
all three values of z. Therefore, it is apparent that for values of z up to
1/8 in, length misalignment effects are one-sided, occurring only when the
shorter tube faces the flow.
Type 6; For w - 1/32 in, Cp decreased to 0.838; for w - 1/16 in, Cp decreased to
0.820; for w - 1/8 in, a C value of 0.779 was obtained. These results show that
Cp is very sensitive to planar misalignment; for w values as small as 1/8 inch,
Cp can decrease by up to 8 percent.
CONCLUSIONS
A recent study of the effects of impact opening misalignment on the value of the
Type-S pitot tube coefficient has demonstrated the following:
(1) The value of Cp is unaf fete ted by transverse axie misalignment, for values
of a up to about 10*.
(2) Longitudinal axis misalignment effects first begin to cause a significant
(> 1%) change in Cp when 0 is abput 5°; for 0-8°, Cp can change by 2 to 3 per-
cent. Longitudinal axis effeqts are one-sided, in that they affect Cp only when
a misaligned opening faces the flow. The effects are also directional; negative
@ values increase Cp, and positive 3 values decrease Cp.
(3) Length misalignment first; causes a significant change (decrease) in the
value of Cp when z is about 1/8 inch; the effect is one-sided, occurring only
when the shorter Ie8 °f the pitot tube faces the flow.
(4) Planar misalignment first; causes a significant change (decrease) in the
value of Cp when w is about 1/32 inch; for w 1/8 inch, Cp can be lowered by as
much as 8 percent.
In view of the above, it can be concluded that if the impact opening misalignment
of a given Type-S pitot tube is eevere enough to cause a significant change in Cp
(i.e., if a > 10°, 3 > 5°, z > 1/8 in, or w > 1/32 in), this should be easily de-
tectable by examining the tube in top, side, and end-views. Therefore, a Type-S
pitot tube need not be recalibrated after field use, provided that it passes a
careful visual inspection.
REFERENCES
1. Standards of Performance for New Stationary Sources. Federal Register,
36(247). December 23, 1971.
2. Vollaro, R. F. A Type-S Pitot Tube Calibration Study. U. S. Environmental
-------
Protection Agency, Emission Measurement Branch. Research Triangle Park, N. C.
July 1974.
-------
93
TRANSVERSE
TUBE AXIS
K'
I /
I /
w
LONGITUDINAL
TUBE AXIS,
FLOW f
(0
(E)
(F)
(G)
Figure 1. Types o^ misalignment studied.
*-:
-------
94
TABLE I: SUMMARY OF RESULTS
Type of
Misalign-
ment
1
(Figure 1A)
Test Case
a = 3°, facing flow
a = 3°, opposite flow
a = 7°A facing flow
a - 7°, opposite flow
a -12° , facing flow
a =12°, opposite flow
2
(Figure IB)
al = a2 - 2°
«1 = <*2 = 4°
al = "2 - 6°
3
(Figures
1C, ID)
@ = -2°, facing flow
3 = -5°, facing flow
6 = -8°. facing flow ;
0 = +5°, facing flow
3 = +8°, facing flow
3 -5° , opposite flow
3 -8° . opposite flow
3 - +2°. opposite flow
3 « +5°, opposite- flow
3 - +8°, opposite flow
4
(Figure IE)
pl = -5°. P2 - +5°
pl - -8°, P? - +8°
P! - +5°, f*2 - -5°
^1 - +8°, ^2 * -8°
5
(Figure IF)
z = 1/16", short faring flow
z = 1/16", long facing flow
z = 1/8" .short facing flow
z = 1/8" .long facing flow
z = 5/32". short facing flow
z = 5/32", long facing flow
6
(Figure 1G)
w - 1/32"
w = 1/16"
w - 1/8"
C
P
0.844
0.844
0.846
0.846
0.846
0.844
Cp(ref)
0.846
Percentage Change*
from
Cp(ref)
0.2
0.2
0.0
0.0
0.0
0.2
0.850
0.850
0.844
.0.853
0.860
0.867
0.834
0.828
0.845
0.846
0.848
0.848
0.845
0.846
0.5
O.5
0.2
0.846
0.8
1-.7
2.5
1.4
2.1
0.2
0.0
0.2
0.2
0.2
0.857
0.863
0.831
0.828
0.846
1.3
2.0
1.8
2.1
O.RAA
n.s&Q
n R17
n.ssn
n.a,?R
0,8.4.8
0.846
0.2
0.4
1.1
0.5
2.1
0.2
0.838
0.820
0.779
0.846
1.0
3.1
7.9
Percentage Change m
from C (ref) "
Cp(ref)
x.100
-------
95 4/20/77
ESTABLISHMENT OF A BASELINE
COEFFICIENT VALUE FOR
PROPERLY CONSTRUCTED
TYPE-S PITOT TUBES
Robert F. Vollaro
INTRODUCTION
Experiments were recently done in which 14 isolated Type-S pitot tubes
(Figure 1) were calibrated against a standard pitot tube. During construction
of the 14 Type-S pitot tubes, special care was taken to ensure that the face
openings of each tube were in proper alignment (see Figure 2). The purpose
of the experiments was to try to establish a baseline coefficient value for
properly constructed isolated Type-S pitot tubes; if this could be done, it
would then be possible, in certain instances, to forego calibration and to
assign coefficient values to Type-S pitot tubes on the basis of tube geometry.
This paper presents the results of the experiments and discusses their signif-
icance.
EXPERIMENTAL METHOD
Prior to calibration, the face-opening alignment of each Type-S pitot tube
was carefully checked by examining the tube in top, side, and end-views;
micrometer readings were taken (as described in Reference 2) to confirm the
alignment of the face openings with respect to the traverse and longitudinal
tube axes. Each of the 14 Type-S pitot tubes met the alignment specifications
of Figure 2. The port-to-port spacing (dimension "X," Figure 1) and the
external tubing diameter (dimension "d," Figure 1) of each Type-S pitot tube,
were also measured and recorded.
The calibrations were performed in a 12-inch-diameter wind tunnel in
An "isolated" Type-S pitot tube is any Type-S pitot tube that is not used in
ombination with other source-sampling components (probe, nozzle, thermocouple)
-------
96
Figure 1. Isolated Tvpe-S pilot tube.
TRANSVERSE I
TUBE AXIS
'.»- FACI _».
I OPENING I
PLANES |
(l)
A SIDE PLANE
LONGITUDINAL
TUBE AXIS
B-SIDE PLANE
(k)
(c)
Figure 2. Properly constructed isolated Type-S pilot tube shown in: (a) end-view;
face openings perpendicular to transverse axis; (b) top-view; openings parallel to
longitudinal axis; (cl side-view; both legs of equal length and centerlines coincident
when viewed from both sides.
-------
97
which ambient (70°F) air was flowing. During calibration, the air velocity
was held constant at about 2800 ft/min. Each Type-S pi tot tube was calibrated,
first with the "A" side facing the flow, and then with the "B" side facing the
flow. The procedures for single-velocity calibration of Type-S pi tot tubes,
outlined in Reference 1, were used throughout. Calibration data for each
pitot tube were recorded in a data table similar to the one shown in Figure 3.
METHOD OF DATA ANALYSIS
For each Type-S pitot tube, the calibration data were analyzed as outlined
in (1) through (7) below.
1. For each of the six pairs of differential pressure readings, i.e.,
three from side A and three from side B (see Figure 3), the value of
the Type-S pitot tube coefficient was calculated as follows:
c
C(s) = CCstdj-^p. (Equation 1)
where: Cp(s) = individual A or B-side value of the Type-S pitot tube
coefficient
C (std) = standard pitot tube coefficient = 0.99
APstd = differen"tial pressure reading, made with standard pitot
tube
APs = differential pressure reading, made with Type-S pitot
tube
2. The mean coefficient values for the A and B sides of each Type-S pitot
tube were calculated using the following equation:
-------
98
PITOTTUBE IDENTIFICATION NUMBER:.
CALIBRATED BY:
.DATE:.
RUN NO.
1
2
3
"A" SIDE CALIBRATION
ARstd
(in. H20)
Us)
(in. H20)
C"p (SIDE A)
Cp(s)
DEVIATION
Cp(s)-fp
RUN NO.
1
2
3
"B" SIDE CALIBRATION
ARstd
(in. H20)
^P(s)
(in. H20)
Cp (SIDE B)
CpW
DEVIATION
Cp($)-C"p
Figure 3. Calibration data table.
-------
99
3
E C ( }
C"(A or B) = 1E (Equation 2)
where: C (A or B) = mean coefficient value for the A or B side of the
Type-S pi tot tube
C (s) = individual A or B-side coefficient value
3. A histogram was constructed showing the frequency distribution of the
28 (i.e., 14 A-side and 14 B-side) values of C" .
4. The average deviation of the C (s) values from C" was calculated
for both the A and B sides of each Type-S pitot tube as follows:
a =
(Equation 3)
where: a = average deviation of the C (s) values from the mean A or B
side coefficient
C (s) = individual A or B-side coefficient value
C (A or B) ?= mean A or B-side coefficient value
5. A histogram was constructed showing the frequency distribution of
the a values.
6. The absolute value of the difference between the mean A and B-side
coefficients was calculated for each Type-S pitot tube as follows:
Cp(A) - Cp(B)| (Equation 4)
-------
TOO
Table I. Summary of results.
Pilot tube
identification
number
A
1
B
2
C
3
0
4
E
5
6
7
t
9
External
tubing
diameter, d
(in.)
0.375
0.375
0.375
0.375
0.375
0.375
0.375
0.375
0.375
0.375
0.250
0.250
0.188
0.181
Port-to-port
separation,3
in terms
in tubing diameter
2.26
2.2d
2.3d
2.3d
2.5d
2.Sd
2.6d
2.5d
2-td
2.6d
2.6d
2.6d
2.8d
2.7d
Cp (side Al
0.842
0.846
0.845
0.849
0.841
0.845
0.847
0.851
0.844
0.843
0.842
0.842
0.847
0.850
Cp (side B)
0.841
0.844
0.839
0.842
0.845
0.847
0.846
0.846
0.144
0.836
0.842
0.845
0.154
0.846
A-to-B
side
difference
(7!
0.001
0.002
0.006
0.007
0.004
0.002
0.001
0.005
0.000
0.007
1.000
1.003
0.007
0.004
Average
deviation (a)
Side A
0.002
0.001
0.002
0.001
0.001
0.001
0.002
0.000
0001
0.001
0.001
0.001
O.D02
0.001
SideB
0.002
0.001
0.001
0.001
0.001
0.001
0.000
0.005
0.001
0.001
0.000
0.002
0.001
0.002
Dimension "X," Figu» 1.
0»33 0.836 0.139 0.142 0.145 O.Mt 0.151 0-154 0857
Figure 4. Frequency distribution of Cp value
-------
101
where: y = absolute value of the difference between the mean A and B
side coefficient values
C (A) and C_(B) = mean A and B-side coefficient values, respec-
tively
7. A histogram was constructed showing the frequency distribution of the
Y values.
RESULTS OF DATA ANALYSIS
The results of the data analysis are presented in Table I and in Figures
4 through 6. Figure 4 shows that the average of the 28 C" values was 0.845,
with an average deviation of 0.003; all of the C" values were between 0.836
and 0.854. Figure 5 shows that the values of average deviation (a) for the A
and B sides of the 14 Type-S pitot tubes ranged from 0.000 to 0.005, averaging
0.001; this indicates that the readings obtained during calibration with the
individual Type-S pitot tubes were very consistent and reproducible. Figure 6
shows that the mean A -to -B side coefficient difference for the 14 Type-S
pitot tubes was 0.004, with an average deviation of 0.002; none of the Type-S
pitot tubes had an A -to -B side difference greater than 0.007.
These results compare favorably with those obtained in other recent studies.2'3
In the studies cited, C" values ranging from 0.841 to 0.853, and A -to -B side
coefficient differences of less than 0.01 were obtained for those Type-S pitot
tubes (a total of four tubes in all) having properly aligned face openings (see
Table II).
CONCLUSIONS
This study has satisfactorily demonstrated that isolated Type-S pitot tubes,
constructed according to the specifications of Figure 2, will consistently have
-------
102
MEAN VALUE "0.004
AVERAGE DEVIATION * 0.002
I.M1 «.M2 0.083 UM 0.005 O.OM U07
AVERAGE DEVIATION (a)
Figure 5. Frequency distribution of a values.
I.M2 U04 1.006
A-TO-B SIDE COEFFICIENT DIFFERENCE (y]
Figure 6. Frequency distribution of 7 values.
Table II. Calibration coefficients of properly constructed Type-S pitot tuba
(from References 2 and 3).
Study
number
2
2
2
3
Pitot tube
number
4-1 S
4-16
4-20
Ref.
External
tukirq
diameter (d)
in.
0.375
0.375
0.375
0.375
Pon-to-part
separation
in terms of
tubing diameter
2.94
2.9d
2.3d
2.4d
Cp (side A)
0.845
O.M3
0.846
0.846
Cp (side B)
0.841
0.850
0.853
O.S46
A-to-B
stte
difference
(7»
0.804
0.007
OJM7
0.600
-------
103
coefficient values between 0.84 and 0.85. Also, these pitot tubes will have
A -to -B side coefficient differences of less than 0.01. These findings are
in accord with the results of other studies. In view of this, it is justifiable
to assign baseline coefficient values of 0.84 or 0.85 to properly constructed
isolated Type-S pitot tubes ,without calibration. It should be carefully noted,
however, that the findings and conclusions of this study are limited to those
tubes having external tubing diameter ("d") values of between 3/16 and 3/8 inches,
and having port-to-port spacings of between 2.2d and 2.9d.
REFERENCES
1. Vollaro, R. F. Guidelines for Type-S Pitot Tube Calibration. Presented
at the First Annual Meeting of The Source Evaluation Society, Dayton, Ohio,
September 18, 1975.
2. Vollaro, R. F. A Type-S Pitot Tube Calibration Study. U. S. Environmental
Protection Agency, Emission Measurement Branch. Research Triangle Park, N. C.
July, 1974.
3. Vollaro, R. F. The Effects of Impact Opening Misalignment on the Value
of the Type-S Pitot Tube Coefficient. Presented at The Fourth Annual Conference
on Energy and the Environment, Cincinnati, Ohio. October 7, 1976.
Note: A recent study has shown that slight misalignments of the face openings
of a Type-S pitot tube will not affect the baseline coefficient value of the pitot
tube. Thus, a Type-S pitot tube need not conform exactly to the specifications of
Figure 2 in order to be considered "properly constructed"; consult Reference 3 for
details.
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104 1/3/77
A SURVEY OF COMMERCIALLY AVAILABLE INSTRUMENTATION
FOR THE MEASUREMENT OF LOW-RANGE GAS VELOCITIES
Robert F. Vollaro
INTRODUCTION
Gas velocities in industrial smokestacks and ducts typically range from
about 1000 to 5000 ft/min; velocities in this range can be measured satis-
factorily with a Type-S pi tot tube and gauge-oil manometer. Stacks are
occasionally encountered, however, in which the velocities are consistently
below 1000 ft/min. Measurement of gas velocity is less straightforward below
1000 ft/min than in the 1000 to 5000 ft/min range, because most gauge-oil
manometers are not sensitive enough to give accurate low-range readings. The
purpose of this paper is to evaluate several commercially available instru-
ments which are capable of measuring gas velocities below 1000 ft/min.
SURVEY OF LOW-RANGE VELOCITY INSTRUMENTATION
The following paragraphs provide a brief description and evaluation of
11 commercially available instruments, along with cost data. A summary of
the descriptive information is presented in Table 1.
1. Instrument and Manufacturer: Inclined manometer, Model 125-AV
(Figure 1) manufactured by Dwyer Instruments, Inc., Michigan City, Indiana.
a. Operating principle - A differential pressure signal from a
primary sensing element (e.g., a Type-S pitot tube) causes a positive dis-
placement of gauge fluid along a calibrated, inclined scale.
b. Velocity range - The full-scale range of the manometer is
0 to 1 in. water column; the scale divisions are 0.005 in. H20. The manometer
is readable to the nearest 0.003 in. H20.
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105
Tablel. LOW-RANGE VELOCITY INSTRUMENTATION
Instrument and
manufacturer
Inclined Manometer *
Model 125-AV
Dwyer Instruments, Inc.
Micromanometer *
Model 10133
Thermo-systems, Inc.
Microtector *
Hook Gauge
Dwyer Instruments, Inc.
Electronic Manometer *
Model 1023
Datametrics, Inc.
Mechanical
Vane Anemometer
Davis Instrument Co.
Extended Range
Propeller Anemometer
R.M. Young Co.
Hot-wire Anemometer
Model VT-1 610
Thermo-Systems, Inc.
Hot-film Wedge Sensor
Model 1234-H
Thermo-Systems, Inc.
Fluidic Velocity; Sensor
Model 308 R
Fluidynamic Devices, Ltd.
Stack Velocity Sampler *
Model GSM-1D5K
Teledyne Hastings- Raydist
Differential Pressure *
Transmitter
Brandt Industries, Inc.
Lower
velocity
limit, ft/min
700
700 in field
400 in lab
700 in field
100 in lab
700 in field
100 in lab
70
75
30
60
200
100
150
Temperature
range
Same as
primary
sensor
Same as
primary
sensor
Same as
primary
sensor
Same as
primary
sensor
To 250° F
(est.)
To180°Ffor
continuous
duty
To 212°F
To 570°F
To 450° F
Same as
primary
sensor
Same as
primary
sensor
Resistance
to
particulate
Same as
primary
sensor
Same as
primary
sensor
Same as
primary
sensor
Same as
primary
sensor
Fair
Fair
Fair
to
good
Good
Fair
to
good
Excellent
Excellent
Applications
Industrial stacks, ducts, vents;
also lab applications; air or
non-air streams
Lab applications; limited use
in industrial stacks, ducts,
vents; air or non-air streams
Lab applications; limited use
in industrial stacks, ducts,
vents; air or non-air streams
Lab applications; limited use
in industrial stacks, ducts,
vents; air or non-air streams
Industrial vents and grilles;
special calibration needed for
non-air streams
Roof monitors and vents;, spe-
cial calibration needed for
non-air streams
Industrial stacks, vents, ducts;
lab applications; special cali-
bration needed for
non-air streams
Industrial stacks, vents, ducts,
_ lab applications; special cali-
bration needed for
non-air streams
Industrial stacks, vents,
ducts; air or non-air streams
Industrial stacks, vents,
ducts; air or non-air streams
Industrial stacks, vents,
ducts; air or non-air streams
*Must be used in conjunction with a Type - S pitot tube or other appropriate primary sensing element.
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106
Figure 1. Dwyer inclined manometer, model 125-AV, connect-
ed to a Type-S pitot tube.
MICROMETER
DIAL
Figure 2. Thermo-Systems micromanometer, model 10133,
connected to a Type-S pitot tube.
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107
c. Temperature range - The operating temperature range of the
manometer is the same as that of the primary sensing element.
d. Resistance to particulate matter - Governed by particulate
resistance of primary sensor.
e. Evaluation - As previously noted, the manometer has scale divi-
sions of 0.005 in. H20, and is readable to the nearest 0.003 in. HpO. Thus,
it has greater sensitivity than most inclined manometers, which have 0.01 in.
H20 divisions and are readable to 0.005 in. HpO. Therefore, with the 125-AV,
accuracy of better than 10 percent in velocity head (AP) readings can be
ensured, provided that the manometer is not used to measure values of AP
lower than about 0.03 in. H20 (which corresponds to a velocity of about
700 ft/min for air flowing at 70°F).
f. Cost - Approximately $125.
2. Instrument and Manufacturer: Micromanometer, Model 10133 (Figure 2),
manufactured by Thermo-Systems, Inc., St. Paul, Minnesota.
a. Operating principle - A differential pressure signal from a
primary sensing element causes a displacement of gauge fluid along a
calibrated, inclined scale.
b. Velocity range - The full-scale range of the micromanometer is
0 to 1.2 in. water column. The scale divisions are 0.01 in FLO, but the
instrument has a micrometer dial, making it possible to read velocity head
to the nearest 0.001 in. H20.
c. Temperature range - Governed by the primary sensing element.
d. Resistance to particulate matter - Same as primary sensor.
e. Evaluation - The Model 10133 micro-manometer is better suited
for laboratory work than for source-sampling applications, particularly at
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108
velocities below 700 ft/min. The reason is that the performance of the
manometer is adversely affected by flow pulsations, vibrations, etc. Even
when it is in a vibration-free environment, the instrument cannot be used to
read AP values below 0.01 in. hLO, if AP readings within +_ 10 percent of true
are desired.
f. Cost - $200 or less (estimated).
3. Instrument and Manufacturer: Micro-tector Hook Gauge (Figure 3),
manufactured by Dwyer Instruments, Inc., Michigan City, Indiana.
a. Operating principle - A differential pressure signal from a
primary sensing element causes a slight displacement of gauge fluid. A
metal "hook" mounted in a micrometer barrel is carefully lowered until its
point "just" contacts the gauge fluid. The instant of contact with the fluid
is detected by completion of a low-power AC circuit. On indication of contact,
the operator stops lowering the hook, and reads the micrometer to determine AP.
b. Velocity range - The full-scale range of the gauge is 0 to 2 In.
water column. The micrometer scale is readable to the nearest 0.00025 in. H20.
c. Temperature range - Governed by primary sensing element.
d. Resistance to particulate matter - Same as primary sensor.
e. Evaluation - The manufacturer's estimated readability (to the
nearest 0.00025 in. HpO) implies that one should be able to read AP values
as low as 0.0025 in. 1^0 with +_ 10 percent confidence. In practice, however,
this readability is only possible if the instrument is perfectly leveled and
used in an absolutely vibration-free environment. Generally, the hook gauge
will not be a useful field instrument for measuring velocities lower than
about 700 ft/min.
f. Cost - About $200.
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109
J MICROMETER
~\^ BARREL
HOOK
Figure 3. Dwyer microtector hook gauge, connected
to a Type-S pitot tube.
TRANSDUCER
Figure 4. Datametrics electronic manometer, model 1023, connected to a Type-S
pitot tube.
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no
4. Instrument and Manufacturer: Electronic Manometer, Type 1023
(Figure 4), manufactured by Datametrics, Inc., Wilmington, Massachusetts.
a. Operating principle - A differential pressure signal from a
primary sensing element is converted to an electrical signal by transducers.
The output signal can, if desired, be read on a digital voltmeter or
recording chart.
b. Velocity range - The manometer is useful over a wide range of
velocities because of its multi-scale readout system. The least sensitive
scale is 0 to 100 in. water column, and the most sensitive is 0 to 0.01 in.
HJD, full-scale. The rated accuracy of the manometer is 2 percent of full-
scale for all operating ranges.
c. Temperature range - Governed by the primary sensing element.
d. Resistance to particulate matter - Same as primary sensor.
e. Evaluation - The 1023 manometer is a high-precision instrument;
if zeroed with a digital voltmeter, it is capable of measuring velocity heads
as low as 0.001 in. H20 with acceptable accuracy. Note, however, that read-
ings made on the most sensitive (0 to 0.01 in. H20) scale are adversely affected
by connecting-line vibrations; thus, the lines from the primary sensor to the
transducer must be perfectly still during use in this range. The manometer is,
therefore, better suited for laboratory, rather than field, applications for
measuring AP values below 0.01 in. H,,0.
f. Cost - About $1000.
5. Instrument and Manufacturer: Mechanical Vane Anemometer (Figure 5),
manufactured by Davis Instrument Co., Baltimore, Maryland.
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m
a. Operating principle - A gas stream flowing through the anemometer
(see Figure 5), causes the propeller blades to rotate. The propeller rpm is
proportional to the velocity of the flowing gas. The readout is in linear
feet; dividing this readout by the total measurement time gives the gas
velocity in ft/min.
b. Velocity range - The anemometer can measure velocities between
70 and 5000 ft/min with acceptable accuracy.
c. Temperature range - (The author does not have a reliable estimate
of the instrument's temperature capabilities; however, there seems to be no
reason why the anemometer could not be used in gas streams as hot as 200 or
250°F.)
d. Resistance to particulate matter - The propeller blades provide
fairly good resistance to particulate matter, especially when the instrument
is used for brief periods of time.
e. Evaluation - A mechanical vane anemometer is best suited for
making a "quick check" of the exit velocities from a vent or grille. The
anemometer is calibrated for use in air streams; special calibration is needed
for use in non-air streams. Although the anemometer can accurately measure
velocities in the 70 to 700 ft/min range (out of range of most primary sensor-
manometer combinations), the instrument can only be used for a short time
before it must be stopped, reset, and restarted manually. Thus, the anemometer
is not easily adaptable for use in source-sampling applications.
f. Cost - Estimated at $100 or less.
6. Instrument and Manufacturer: Extended Range Propeller Vane Anemometer
(Figure 6), manufactured by R. M. Young Co., Traverse City, Michigan.
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112
Figure 5. Mechanical vane anemometer,
manufactured by Davis Instruments, Inc.
ABS THERMOPLASTIC
PROPELLER
TO RECORDER
Figure 6. Extended-range propeller vane anemometer, manu-
factured by R.M. Young Company.
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113
a. Operating principle - Flowing gas causes the propeller (see
Figure 6) to turn at a rate proportional to the gas velocity. The propeller
shaft is coupled to a d.c. generator. The generator output is an analog
voltage, proportional to shaft rpm. The output signal is monitored contin-
uously by means of a recording chart.
b. Velocity range - The velocity range for the anemometer is 75
to 6000 ft/min; 75 ft/min is the threshold velocity at which the propeller
begins to turn.
c. Temperature range - With an ABS thermoplastic propeller, the
anemometer can be used continuously in gas streams as hot as 180°F and,
intermittently, in streams as hot as 300°F.
d. Resistance to participate matter - The propeller blades provide
fairly good resistance to particulate matter.
e. Evaluation - Because it cannot be used for extended periods of
time at temperatures above 180°F, the anemometer is of limited value for
source-sampling applications. It would probably be useful for continuous
velocity measurement in roof monitors. The anemometer is calibrated for use
in air streams; special calibration is required for use in non-air streams.
f. Cost - About $700 with recording chart.
7. Instrument and Manufacturer: Velocity Transducer, Model 1610,
manufactured by Thertno-Sys terns, Inc., Minneapolis, Minnesota.
a. Operating principle - The VT-1610 measures the velocity of a
flowing gas stream by sensing the cooling effect of the stream as it moves
over the heated surface of the sensor, the "hot-wire" principle. The output
signals from the sensor are electrical and non-linear. A signal conditioner
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114
is available to linearize the output. The output signals are temperature
compensated so that the readings will be in ft/min, corrected to 70°F.
b. Velocity range - The instrument can measure velocities as low
as 30 ft/min (on the low scale) and as high as 12,000 ft/min (on the high
scale), with acceptable accuracy (+ 2 percent).
c. Temperature range - The instrument can be used in gas streams
as hot as 212°F.
d. Resistance to particulate matter - Unlike many hot-wire devices,
the VT-1610 sensor is ruggedized and has fairly good resistance to partic-
ulate matter.
e. Evaluation - It appears that the VT-1610 would be most suitable
for short-term use in low-temperature air streams, particularly when velocities
are too low (under 700 ft/min) to be measured with most primary sensor-
manometer combinations. If used continuously in a dusty environment, the
instrument will tend to foul after several hours. The sensor is calibrated
for use in air streams; special calibration is required if it is to be used
in non-air streams.
f. Cost - About $1000 for sensor and signal conditioner.
8. Instrument and Manufacturer: Wedge Hot-Film Sensor, Model 1234-H
(Figure 8), manufactured by Thermo-Systerns, Inc., Minneapolis, Minnesota.
a. Operating principle - The 1234-H measures the velocity of a flow-
ing gas stream by sensing the cooling effect of the stream as it moves over
the heated sensor surface, the "hot-film" principle. The output signal is
electrical and can be read continuously on a recording chart, if desired.
When used with a temperature compensator, the readout will be in ft/min,
corrected to 70°F.
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115
\ -
SENSOR
Figure 7. Thermo-Systems hot-wire anemometer, model VT-1610.
SENSOR
Figure 8. Thermo-Systems hot-film wedge sensor, model 1234 H.
-------
116
b. Velocity range - The sensor can measure velocities as low as
60 ft/min (on the low-scale) or as high as 12,000 ft/min (on high-scale),
with acceptable accuracy (+_2 percent).
c. Temperature range - The sensor can be used in gas streams as
hot as 570°F.
d. Resistance to particulate matter - The sensor is ruggedized
and offers good resistance to particulate matter.
e. Evaluation - The 1234-H is best suited for short-term use in
air streams, particularly when velocities are too low to be measured with
primary sensor-manometer combinations. It may prove to be useful for measur-
ing total flow rate from roof monitors, because several sensors, positioned
at different points along a roof vent, can be connected to a multi-channel
readout system. Like the VT-1610, the 1234-H requires special calibration
for use in non-air streams.
f. Cost - About $1500, for one temperature-compensated sensor and
readout system; about $500 for each additional sensor.
9. Instrument and Manufacturer: Fluidic Velocity Sensor, Model 308R
(Figure 9), manufactured by FluiDynamics, Ltd., Ontario, Canada.
a. Operating principle - The following description refers to
Figure 9: A free jet of supply fluid (air or N2) is issued from a nozzle
(point B), and impinges on two pick-up ports (point C). At zero cross-flow
velocity, the differential pressure across the pick-up ports is zero. Any
cross-flow causes the supply air jet to deflect, yielding a differential
pressure signal proportional to the velocity. The output signal can be read
with a differential pressure gauge or transducer.
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117
TO
SUPPLY FLUH)
DIFFERENTIAL PRESSURE
GAUGE
Figure 9. Fluidic velocity sensor, model 308R, manufactured by FluiDynamic
Devices, Ltd.
PURGE
GAS
Figure 10. Stack velocity sampler, model GSM-1 D5K manufactured by Teledyne
Hastings Raydist.
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118
b. Velocity range - The sensor has a full-scale velocity range
of 0 to 3600 ft/min. The accuracy of the sensor is about + 3 percent for
velocities above 600 ft/min, and +_ 5 to 10 percent for velocities below
600 ft/min.
c. Temperature range - The sensor can be used in gas streams as
hot as 450°F.
d. Resistance to particulate matter - The sensor has fairly good
resistance to particulate matter.
e. Evaluation - One of the outstanding features of the sensor is
that it has a linear, high-amplitude output signal, even at low velocities.
For example, when the cross-flow velocity (v ) is 600 ft/min, the sensor
\+
output is about 12 in. H20; at VG = 200 ft/min, the output is about 4 in. H20.
Note, however, that the sensor is difficult to zero; for this reason, its
accuracy falls off appreciably for v < 200 ft/min. The sensing head is
\f
mounted on a cylindrical probe, making it convenient to use in source-sampling
applications. The sensor can be used in non-air streams, provided that the
gas density is known.
f. Cost - About $2000.
10. Instrument and Manufacturer: Stack Velocity Sampler, Model GSM-1D5K
(Figure 10), manufactured by Hastings-Raydist, Hampton, Virginia.
a. Operating principle - The following description refers to
Figure 10: At zero cross-flow, supply fluid (air or N2) is continually purged
at equal rates, out of both impact openings of the Type-S pi tot tube. Any
cross-flow velocity causes a back-pressure against the purge gas, at point A.
The back-pressure signal is proportional to the fluid velocity; thermoelectric
-------
119
sensors (transducers) interpret and convert this signal. The output voltage
from the transducers is linear over about 90 percent of the scale; output
voltage can be read with a digital voltmeter or recording chart, if desired.
b. Velocity range - The velocity range is 0 to 1500 ft/min, full
scale. The lower limit of readability is about 100 ft/min.
c. Temperature range - The instrument is operable at all tempera-
tures at which a Type-S pi tot tube can be used.
d. Resistance to particulate matter - The continuous-purge principle
of the sensor gives it excellent resistance to particulate matter.
e. Evaluation - The most outstanding feature of the Hastings
instrument is that it works with a Type-S pi tot tube; thus, it is easily
adaptable for use with conventional source-sampling equipment. The volt-
meter on the control panel is adequate for reading velocities between 200 and
1500 ft/min. To read accurately velocities between 100 and 200 ft/min, a
digital voltmeter or sensitive chart recorder is needed. The sensor can be
used in non-air streams if the density of the gas is known. Note that the
instrument must be calibrated exactly as it is to be used, because different
calibration curves will be obtained for different pi tot tube and connecting-
line lengths.
f. Cost - About $1500 to $2000.
11. Instrument and Manufacturer: Differential Pressure Transmitter
(Figure 11), manufactured by Brandt Industries, Inc., Raleigh, North Carolina.
a. Operating principle - The following description refers to
Figure 11: Supply fluid (air on Np) exhausts equally out of both sides of
the pitot tube at zero cross-flow velocity. Any cross-flow will cause a
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120
Figure 11. Differential pressure transmitter, series 200; manufactured by Brandt Industries. Inc.
-------
121
back-pressure against the purge gas at point A. The magnitude of the back-
pressure signal is proportional to the fluid velocity. Transducers receive
and convert the back-pressure signal. The output signal from the transducers
is pneumatic and linear; the output can be read continuously on a pneumatic
recorder if desired.
b. Velocity range - The full-scale range of the transmitter is 0
to 0.05 in. water column. The accuracy of the transmitter is estimated at
+ 2 percent of span.
c. Temperature Range - The transmitter can be used at any tempera-
ture at which the primary sensing element (pitot tube or other sensor) can
be used.
d. Resistance to particulate matter - The continuous-purge action
of the supply fluid gives the sensor excellent resistance to particulate
matter.
e. Evaluation - The Brandt transmitter is a versatile device; it
can be used as a single-point sensor, or adapted for multipoint sensing
(e.g., it can be used with a pitot "rake"). The transmitter is easily adapt-
able for use with conventional source-sampling equipment. An especially
attractive feature of the transmitter is a damping control, which allows
true, time-integrated average velocity head readings to be made. Velocities
as low as 150 to 200 ft/min can be read with acceptable accuracy. The instru-
ment can be used in non-air streams if the gas density is known. One draw-
back of the instrument is that there is a practical upper-limit (30 ft) on
the length of the connecting lines; note, also, that the connecting lines
are somewhat vibration-sensitive and should be still when measurements are
made.
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January 19, 1977
122
THE USE OF TYPE-S PITOT TUBES FOR THE MEASUREMENT
OF LOW VELOCITIES
Robert F. Vollaro
INTRODUCTION
In source-sampling, stack gas velocity (Vs) is usually measured with
a Type-S pi tot tube. Before a Type-S pi tot tube is used for this purpose,
its coefficient, C (s), must be determined by calibration against a
standard pi tot tube. A recent study1 has demonstrated that, in most
instances, calibration at a single velocity of about 3000 ft/min is
satisfactory, for both isolated Type-S pitot tubes* and pitobe assemblies;**
Type-S pitot tubes calibrated by this method can be used, with acceptable
accuracy, to measure velocities as low as 1000 ft/min. In most field
applications, stack gas velocities will be above 1000 ft/min; however,
sources having lower velocities are occasionally encountered. It is,
therefore, desirable to know if calibration coefficients obtained at 3000
ft/min are sufficiently accurate for use at velocities below 1000 ft/min,
or whether special calibration in the low range is necessary. Accordingly,
investigators recently conducted experiments in which several isolated Type-S
pitot tubes and pitobe assemblies were calibrated at velocities ranging from
about 400 to 1000 ft/min. This paper reports the results of these experi-
ments and discusses their significance.
EXPERIMENT 1
Experimental Method
In the first experiment, the coefficients of 12 isolated Type-S pitot
tubes were determined by calibration against a standard pitot tube. The
*An isolated Type-S pitot tube is any Type-S pitot tube that is calibrated
or used alone.
**A pitobe assembly is any Type-S pitot tube that is calibrated or used
while attached to a sampling probe, equipped with a nozzle or thermo-
couple or both.
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123
calibrations were done in a wind tunnel having a test-section diameter of
12 in. For each Type-S pitot tube, calibration data were taken at four
different test-section velocities between 400 and 1000 ft/min, spaced at
approximately equal intervals over this range. During calibration, velocity
head (AP) signals from the standard and Type-S pitot tubes were continuously
monitored by an electronic manometer and chart-recorder combination
(Figure 1). The flow in the wind tunnel was normal, time-invariant turbulent
2
flow; therefore, it was possible to take accurate average AP readings from
the chart recordings (see Appendix A). Figure 2 shows a segment of a
typical chart recording.
Method of Data Analysis
Each of the 12 Type-S pitot tubes had been calibrated at velocities
o
ranging from 1500 to 3500 ft/min in a previous study, providing a point of
reference from which to evaluate the data taken in Experiment 1. The data
from Experiment 1 were analyzed as follows:
1. For each of the 12 Type-S pitot tubes, the value of the pitot
coefficient was calculated, at each velocity setting; the following equation
was used:
C (s) = 0.99 / -^ (Equation 1)
Where:
C (s) = Type-S pitot tube coefficient
0.99 = Standard pitot tube coefficient, constant to within + 0.5
percent for the measurement of velocities as low as
3 ft/sec (Reference 7).
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124
ELECTRONIC
MANOMETER
Figure 1. Electronic manometer and chart-recorder combination,
connected to a Type-S pitot tube.
-------
125
en
O
'a.
£
ra
0>
O
O)
u_
-------
126
AP = Average velocity head signal from standard pi tot tube
std
(taken from chart recording), in. H20
AP = Average velocity head signal from Type-S pi tot tube
(from chart recording), in. H20
2. The data from Experiment 1 were correlated with the "reference"
data (from previous study, in the velocity range 1500 to 3500 ft/min). The
percentage deviation of each Cp(s) value from Cp,* its reference coefficient
value at 3000 ft/min, was calculated as follows:
Percentage deviation
of
_ Cp
Cp(s) from Cp
X 100 (Equation 2)
3. A histogram was constructed showing the frequency distribution of
the 4> values.
4. The mean and maximum values of 4. were "adjusted," by adding 2 percent
to them, to cover possible random errors in interpreting the chart recordings
(see Appendix A).
5. The "adjusted" mean and maximum values of 4 were substituted into
the velocity and isokinetic error equations (see Appendix B). Note that
the reason for considering the isokinetic error equation as well as the
velocity error equation is that isolated Type-S pi tot tubes having known Cp
values are sometimes used as components of pitobe assemblies in which
aerodynamic interactions among the components are minimized by proper
intercomponent spacing; in such cases, the Cp values of the isolated pi tot
Q
tube and pitobe assembly are equal.
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127
Results of Data Analysis
The results of the data analysis are presented in Figure 3 and Table 1.
Figure 3 shows that, in general, there was little variance between the
*
values of C(s) and C ; the mean value of was 2.1 percent, with an
average deviation of 1.3 percent. The maximum value of was 6.3 percent,
and 90 percent of the $ values were 5 percent or less. Adding 2 percent
to the mean and maximum values of to cover random errors in reading the
chart recordings gives an "adjusted" mean value of 4.1 percent, and an
adjusted maximum value of 8.3 percent. Substituting these adjusted $
values into the velocity and isokinetic error equations (Equations B-5
and B-6 in Appendix B) gives the following results (see Table 1): (1) for
= 4.1 percent, velocity measurements will be within +_ 6.6 percent of
true, and isokinetic adjustments within 7.0 percent of true, 99.6 percent
of the'time; (2) for $ = 8.3 percent, velocity measurements will be within
9.8 percent of true, and isokinetic adjustments within 10.1 percent of
true, 99.6 percent of the time. Therefore, it can be concluded that when
isolated Type-S pi tot tubes (or interference-free pitobe assemblies
constructed from them) are used to measure velocities in the range from
*
400 to 1000 ft/min, values of C obtained by single-velocity calibration
at 3000 ft/min can be used without introducing serious error; velocity
readings and isokinetic adjustments (if applicable) will be within
+_ 10 percent of true.
EXPERIMENT 2
Experimental Method
In the second experiment, six different pitobe assemblies (three with
thermocouple and nozzle and three with sampling nozzle only) were calibrated
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128
MEAN VALUE = 2.1%
AVERAGE DEVIATION = 1.3%
PERCENTAGE DEVIATION
Figure 3. Frequency distribution of
velocity range3/- 400 to 1000ft/min.
_
values from experiment 1?
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129
Table 1. SUMMARY OF PROBABLE VELOCITY
AND ISOKINETIC ERRORS; EXPERIMENTS 1 AND 2
Quantity,
%
Mean value of
0 or 7
Adjusted mean value
of 0 or 7
Maximum probable
velocity error*
Maximum probable
isokinetic error*
Maximum value of
0 or 7
Adjusted maximum
value of 0 or 7
Maximum probable
velocity error**
Maximum probable
isokinetic error**
Experiment 1
2.1
4.1
6.6
7.0
6.3
8.3
9.8
10.1
Experiment 2
2.4
4.4
6.8
7.2
5.7
7.7
9.3
9.6
"using ad justed mean 0 or 7 value
*using adjusted maximum 0 or 7 value
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130
against a standard pi tot tube. The intercomponent spacings of each of
the assemblies failed to meet the minimum requirements necessary to prevent
aerodynamic interference. The calibrations were done in the same wind
tunnel that was used in Experiment 1. Each assembly was calibrated first
at a test-section velocity of 2200 ft/min (maximum for the wind tunnel) and
then at four different test-section velocities, spaced at approximately
equal intervals over the range from about 475 to 1150 ft/min. All AP
signals were monitored with the electronic manometer and chart-recorder
combination illustrated in Figure 1.
Method of Data Analysis
The data from Experiment 2 were analyzed as follows:
1. For each assembly, Equation 1 was used to calculate the values of
C_(s), first at the "reference" velocity (2200 ft/min) and then at each
of the four velocity settings between 475 and 1150 ft/min.
2. The percentage deviation of each low-range value of C (s) from its
reference coefficient, C (ref), was calculated as follows:
Percentage deviation
Cn(s) - Cn(ref)
Of = y =
Cp(s) from Cp(ref)
Cp(ref)
x 100 (Equation 3)
3. A histogram was constructed, showing the frequency distribution of
the Y values.
4. The mean and maximum values of y were "adjusted," by adding
2 percent to them, to cover possible random errors in interpreting the
chart recordings (see Appendix A).
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131
5. The "adjusted" mean and maximum values of y were substituted into the
velocity and isokinetic error equations (see Appendix B).
Results of Data Analysis
The results of the data analysis are presented in Figure 4 and Table 1.
Figure 4 shows that, generally, there was little variance between the values
of CL(s) and C (ref); the mean value of y was 2.4 percent, with an average
deviation of 1.4 percent. The maximum value of y was 5.7 percent, and about
90 percent of the Y values were within 5 percent. These results are nearly
identical to those obtained with isolated Type-S pitot tubes in Experiment 1
(compare Figures 3 and 4). Adding 2 percent of the mean and maximum values
of Y> to cover random errors in reading the chart recordings, gives an
"adjusted" mean value of 4.4 percent, and an adjusted maximum value of 7.7
percent. Substituting these adjusted y values into the velocity and isokinetic
error equations gives the following results (see Table 1): (1) for y = 4.4
percent, velocity measurements will be within 6.8 percent of true, and
isokinetic adjustments within 7.2 percent of true, 99.6 percent of the time;
(2) for y = 7.7 percent, velocity measurements will be within .9.3 percent of
true and isokinetic adjustments within 9.6' percent of true, 99.6 percent of
the time. Therefore, when pitobe assemblies having aerodynamic interference
problems are used to measure velocities in the range from 475 to 1150 ft/min,
the use of coefficient values obtained by single-velocity calibration at
3000 ft/min will not introduce serious error; velocity readings and iso-
kinetic adjustments will be within +; 10 percent of true. Note that
this conclusion is justifiable, even though the reference coefficients in
Experiment 2 were determined at 2200 ft/min, rather than 3000 ft/min; analysis
of data from two recent studies, ' in which 47 and 45 (respectively) pitobe
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132
10
MEAN VALUE = 2.4%
AVERAGE DEVIATION = 1.4%
1.0 2.4 3.8 5.2 6.6
PERCENTAGE DEVIATION OF Cp(s) FROM Cp(ref), (y)
Figure 4. Frequency distribution of 7 values from experiment 2; velocity
range: 475 to 1150 ft/min.
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133
assemblies were calibrated, shows the average difference (p) between the
values of C (s) at 2201
(see Figures 5 and 6).
values of C (s) at 2200 ft/min and 3000 ft/min to be 0.5 percent or less
CONCLUSIONS
It has been demonstrated that, in the velocity range from about 400
to 1000 ft/min, the value of the Type-S pitot tube coefficient, C (s),
will generally be within 2 to 5 percent of C *, the coefficient value at
3000 ft/min; occasional differences of 6 to 8 percent between C (s) and
C * can be expected. These points have been demonstrated for isolated
Type-S pitot tubes and pitobe assemblies. It has also been shown that
these deviations from C * are sufficiently small to warrant the use of
C * values to measure velocities in the 400 to 1000 ft/min range.** Thus,
special calibration of Type-S pitot tubes in the 400 to 1000 ft/min velo-
city range is unnecessary.
** It is assumed, of course, that a differential pressure gauge capable of
reading AP to within +_ 10 percent, is available. Gauge oil manometers
generally cannot meet this requirement at velocity head (AP) values below
0.03 in. H20.
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134
MEAN VALUE = 0.2%
AVERAGE DEVIATION = 0.2%
PERCENTAGE DEVIATION QF Cp{s) AT 2200ft/rain
jFRQHCp(s) AT, 3000 ft/min,p
Figure 5. Frequency distribution of p values for 47 pitobe assembly calibration
runs, based on data from recent North Carolina State University study.
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135
MEAN VALUE = 0.5%
i AVERAGE DEVIATION = 0.3%
02 0.5 0.8 1.1 1.4 1.7
PERCENTAGE DEVIATION OF Cp(s) AT 2200 ft/min FROM Cp(s) AT 3000 ft/min, p
Figure 6. Frequency distribution of p values for 45 pitobe assembly calibration
runs, based on data from recent University of Windsor study.
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136
REFERENCES
1. Vollaro, R. F. An Evaluation of Single-Velocity Calibration
Technique as a Means of Determining Type-S Pitot Tube Coefficients.
U. S. Environmental Protection Agency, Emission Measurement Branch.
Research Triangle Park, N. C. August, 1975.
2. McCabe, Warren L., and Julian C. Smith. Unit Operations of Chemical
Engineering. New York, McGraw-Hill Book Company, Inc., 1956.
p. 40-41.
3. Vollaro, R. F. A Type-S Pitot Tube Calibration Study. U. S.
Environmental Protection Agency, Emission Measurement Branch.
Research Triangle Park, N. C. July, 1974.
4. Terry, Ellen W.,and Herbert E. Moretz. First Annual Report on the
Effects of Geometry and Interference on the Accuracy of S-Type
Pitot Tubes. (Submitted to U. S. Environmental Protection Agency
by the Mechanical and Aerospace Engineering Department of North
Carolina State University. Raleigh, N. C. August 1975.)
5. Gnyp, A. W., C. C. St. Pierre, D. S. Smith, D. Mozzon, and J. Steiner.
An Experimental Investigation of the Effect of Pitot Tube-Sampling
Probe Configurations on the Magnitude of the S-Type Pitot Tube Coefficient
for Commercially Available Source Sampling Probes. (Prepared by
the University of Windsor for the Ministry of the Environment.
Toronto, Canada. February 1975.)
6. Shigehara, R. T., W. F. Todd, and W. S. Smith. Significance of
Errors in Stack Sampling Measurements. Presented at Annual Meeting
of Air Pollution Control Association. St. Louis, Mo. June, 1970.
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137
7. Ower, E., and R. C. Pankhurst. The Measurement of Air Flow. London,
Pergamon Press, 1966. p. 35.
8. Vollaro, R. F. Guidelines for Type-S Pitot Tube Calibration.
(Presented at the First Annual Source Evaluation Society Meeting.
Dayton, 0. September 18, 1975.)
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138
APPENDIX A
Experimental Error Considerations
A.I ACCURACY OF REFERENCE COEFFICIENTS
In Experiments 1 and 2, values of Cp(s), obtained at 3000 and 2200
ft/min, were used as points of reference from which to evaluate the
low-range data. It is, therefore, desirable to establish the reliability
of these reference coefficients.
The reference coefficients used in Experiment 1 were taken from a
previous calibration study.3 A detailed discussion of the accuracy of
these coefficients is presented in the Appendix of Reference 3; considering
random errors only (i.e., errors caused by sensitivity limitations of
the manometer, incorrect "sight-weighted" averaging of flow pulsations,
etc.), the uncertainty in each coefficient value is estimated at +_ 1
percent (0.01).
The reference coefficients used in Experiment 2 were based on
data taken during Experiment 2. With the test-section velocity in the
wind tunnel held constant at 2200 ft/min, AP signals from the standard
and Type-S pi tot tubes were monitored for several minutes each, in
order that accurate determinations of W could be made from the chart
recordings. The AP values taken from the chart recordings were readable
to the nearest 0.005 in. H20 (see Figure AT). To estimate the uncertainty
in each value of Cp (ref), calculated from two such AP" readings, consider
the following example:
Suppose that the values of AP"std and AP~s, taken from a chart recording,
were 0.300 in. H20 and 0.420 in. H20, respectively. The value of Cp (ref)
corresponding to these AP~ values would be, by Equation 1,
0.99 /0.300/0.420 = 0.837. Referring to Figure Al, it is evident that
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139
0.298 0.303 0.308 0.313
in.
i
j 0.300
, '«
1
| 1
I READ
H* AS*
[ 0.300
1 h.
in. in. in.
1 II
I j 0.310
1 , in.
1
1 1
I: READ I READ
JU* AS M4+ AS *.
2 i 0.305 j 0.310
1 i in. p in.
Figure A1. Reading of AP"to the nearest 0.005 in.
H2O, from Reference 3.
0.0295
in.
i
i
i
r*
i
0.030
in.
I
1
READ
- AS -
0.030
in.
0.0305
in.
1
1
|
1
0.031
in.
READ
AS
0.031
in.
0.0315
in.
I
-j
Figure A2. Reading of A P to the nearest 0.001 in. H2O;
600 fpm < Vs < 1150 fpm; Experiments 1 and 2.
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140
although AP . , was read as 0.300 in. h^O, it actually could have been
as high as 0.303 in. h^O, or as low as 0.298 in. 1^0. Similarly, the
value of &FS, read as 0.420 in. h^O, could have been as high as 0.423
in. H20 or as low as 0.418 in. H^O. Considering the extreme cases
only, this implies that the actual value of Cp(ref) could have been
as high as 0.99/0.303/0.418 = 0.843, or as low as 0.99/0.298/0.423
= 0.831. Therefore, the uncertainty in each value of Cp(ref) is
estimated at about (0.006/0.837) x 100, or 0.7 percent.
A.2 RANDOM ERRORS IN LOW-RANGE COEFFICIENTS
Figures A2 and A3 illustrate the way in which the average A? readings
from Experiments 1 and 2 were taken from the chart recordings. Figure
A2 shows that for velocities between 600 ft/min and 1150 ft/min, A~F
could be read to the nearest 0.001 in. H20, whereas Figure A3 'shows that
for velocities between 400 and 600 ft/min, AP was readable to the nearest
0.0005 in. H20. The reason for the difference is that for Vs < 600 ft/min,
the flow was "quieter" (i.e., the magnitude of the pulsations was less),
making AF easier to read.
To estimate the uncertainty in each value of Cp(s), calculated from
two AP~ values, consider the following typical examples:
Case 1: 600 ft/min <_ V < 1150 ft/min
1 ' "'~ S
Suppose that the values of ^P~stc4 and AP. taken from a typical
chart recording,were 0.030 in. h^O and 0.040 in. h^O, respectively.
The value of Cp(s), corresponding to these AP" values would be, by
equation 1, 0.99 /O.030/0.040 = 0.857. Referring to Figure A2, it is
evident that although AP . . was read as 0.030 in. hLO, it could have
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141
0.0098
in.
i
1
I
M
!
0.0100
in.
f_-
READ
U. AS ml
0.0100
in:
0.0102
in.
I
|
I
I
M-*
1
1
O.I
READ
AS_^.
0.0105
in.
1108
n.
0.0110
in.
READ
^ AS «J
0.0110
in.
0.0113
in.
|
1
-h"
j
n
1
Figure A3. Reading of AP to the nearest 0.0005 in
400 fpm < Vs < 600 fpm; Experiments 1 and 2. '
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142
been as high as 0.0305 or as low as 0.0295 in. H20. Similarly, the
value of AP~ could have been as high as 0.0405 in. H20 or as low as
0.0395 in. H20. Considering the extreme cases only, this means that
the actual value of Cp(s) could have been as high as 0.99/0.0305/0.0395
= 0.870, or as low as 0.99/0.0295/0.0405 = 0.845. Therefore, the
uncertainty in the value of Cp(s) is estimated at 0.013/0.857 x 100,
or 1.5 percent.
Case 2. 400 ft/min <_ Vg < 600 ft/min
Suppose that the values of AP"stcj and AFS> taken from a typical
chart recording, were 0.0100 in. H20 and 0.0140 in. H20, respectively.
The Cp(s) value corresponding to these values of AP" would be, by Equation
1, 0.99/0.0100/0.0140 = 0.836. Referring to Figure A3, it is clear
that although AP~std was read as 0.0100 in. H20, it actually could
have been as high as 0.0103 in. 1^0, or as low as 0.0098 in. H20.
Similarly, W could have been as high as 0.0143 in. H20, or as low
as 0.0138 in. H90. Considering the extreme cases only, this implies
that Cp(s) could have been as high as 0.99/0.0103/0.0137 = 0.858,
or as low as 0.99 /O.0098/0.0143 = 0.820. Therefore, the uncertainty
in the value of Cp(s) is estimated at 0.020/0.836 x 100, or 2.4 percent.
In the light of the above examples, it seems safe to conclude that,
because of random errors in interpreting the chart recordings, an un-
certainty of 2 percent in the low-range values of Cp(s) is introduced.
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143
APPENDIX B
Velocity and Isokinetic Error Equations
The following two equations, developed by Shigehara et a!.6, are expres-
sions for calculating the probable error (i.e., the error within 3 standard de-
viations (3 a) of the mean) when (1) stack gas velocity, V , is measured
with a Type-S pitot tube, and (2) isokinetic conditions, V /Vs> are set with a
sampling train, utilizing a pitot tube and orifice meter:
J(vs)
Ql.35)(d Bwsf|
L J
2dD
]d
V
/dp r / \;
+W +fe)
dB
ws
,1-B
ws
(Equation Bl)
+ dAm
where: V.
s
AP
(Equation B2)
n
Bws
Am
stack gas velocity, ft/sec
Type-S pitot tube coefficient
Absolute stack gas temperature, °R
velocity head of stack gas, in. H2
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144
atm
gm
= orifice meter coefficient
= atmospheric pressure, in. Hg
= gage pressure of stack gas, in. Hg
= meter gage pressure, in. Hg
In the above equations, the derivative, d_£L , of a particular quantity, q,
q
represents the percentage error associated with the measurement of that
quantity. Assuming maximum values of percentage error for all quantities ex-
cept C (see Table 81), equations Bl and B2 simplify to:
27'2
+ 32.4
1/2
1/2
(Equation B3)
(Equation B4)
In order to make equations B3 and M relevant to the present study, the
term dC will be rewritten as dC(s) , where C (s) refers to the value of
P P K
Cp Cp(ref)
the pi tot coefficient at a point in the velocity range from 400 to 1000 ft/min, and
C (ref) is the coefficient value, obtained by calibration at a higher "reference"
P
velocity. The term
p(ref)
refers, therefore, to the percentage error that
would be made by assuming (without proof ) that the value of Cp(ref) is valid in
the range from 400 to 1000 ft/min. Thus.
p(ref)
can be replaced by $ (Experiment 1)
or y (Experiment 2), and equations B3 and B4 can be rewritten as follows:
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145
Table B1. MAXIMUM ERROR IN MEASUREMENT OF VARIOUS SOURCE
SAMPLING PARAMETERS*
, Measurement Maximum error. %
Stack temperature, Ts 1.4
Meter temperature, Tm 1.0
Stack gage pressure, PgS 0.42
Meter gage pressure, P^m 0.42
Atmospheric pressure, Patm 0.21
Dry molecular weight, M^ 0.42
Moisture content, d B^ (absolute) 1.1
d B^/1 - B^ 1.0
Pressure head, Ap 10.0
Orifice pressure differential. Am 5.0
Orifice meter coefficient, Km 1.5
Diameter of probe nozzle, Dn 0.80
"Adapted from Reference 6.
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146
27>21/2 - y2 + 27>2
32.4) 1/2 or (y2 + 32.4) 1/2 (Equation B6)
/ ^
By substituting the appropriate values of 4 and Y into equations B5 and B6,
the probable errors in velocity measurement and isokinetic adjustments, resulting
from the assumption that c (ref) 1s valid in the velocity range from 400 to
1000 ft/min, can be determined.
-------
United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park NC 27711
EPA-450-2-78-042C
October 1978
Air
Stack Sampling G-R\fA
Technical Information
A Collection of
Monographs and Papers
Volume
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