EPA-600/4-77-030
June 1977
Environmental Monitoring Series
A STUDY ON THE ACCURACY
OF TYPE S PITOT TUBE
Environmental Monitoring and Support Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
-------
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL MONITORING series.
This series describes research conducted to develop new or improved methods
and instrumentation for the identification and quantification of environmental
pollutants at the lowest conceivably significant concentrations. It also includes
studies to determine the ambient concentrations of pollutants in the environment
and/or the variance of pollutants as a function of time or meteorological factors.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
-------
A STUDY ON THE ACCURACY OF TYPE-S PITOT TUBES
by
J. C. Williams, III
and
F. R. DeJamette
Mechanical and Aerospace Engineering Department
North Carolina State University
Raleigh, North Carolina 27607
EPA Grant No. 803168
ROAP No. 26 AAG
Program Element No. 1 HA 327
Project Officer
William J. Mitchell
Quality Assurance Branch
Environmental Monitoring and Support Laboratory
Research Triangle Park, North Carolina 27711
Environmental Monitoring and Support Laboratory
Office of Research and Development
U. S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
-------
DISCLAIMER
This report has been reviewed by the Environmental Monitoring and Support
Laboratory, U. S. Environmental Protection Agency, and approved for publica-
tion. Mention of trade names or commercial products does not constitute en-
dorsement or recommendation for use.
ii
-------
FOREWORD
In some testing the measurement of the velocity of the stack gas is usu-
ally obtained with the Staubscheibe ("S" type) pitot tube. The velocity mea-
surement is one of the most critical measurements in source testing, because
it is used to determine the rate at which gas is drawn into the sampling probe
and is also used in calculating the volumetric flow rate. Since the Environ-
mental Monitoring and Support Laboratory, Research Triangle Park, North Caro-
lina has responsibility to establish the reliability of stationary source test
methods, we funded the study reported herein. The objectives of this study
were threefold: 1) to determine the behavior of a family of pitot tubes under
the flow conditions normally encountered in source testing; 2) to determine
the design parameters that affect the "S" type pitot calibration coefficient
(C ); and 3) to select the type "S" pitot tube that will be least sensitive to
angular flow, to yaw, to pitch and to the presence of a sampling probe nozzle.
Overall, the results of this study determined that the distance between
the two pressure ports of the "S" type pitot tube does not affect the calibra-
tion coefficient, C , under conditions of laminar flow. However, the calibra-
tion coefficient is extremely sensitive to this port-to-port spacing under the
conditions of yaw, pitch and angular flow. Increasing this distance seemed to
make the C less sensitive to these parameters. But, unfortunately, the re-
sults of this study are not sufficient to design a pitot tube that is insensi-
tive to the effect of yaw and pitch and yet will fit through most existing
sampling ports. Obviously, further work is needed in this area.
One possibility that should be investigated further would be to insert
the pitot tube approximately 3 cm further into the stack than the sampling
probe. The results in this report indicate that under this condition most
"S" type pitot tubes should give acceptable results.
Thomas R. Hauser, Ph.D.
Acting Director
Environmental Monitoring
and Support Laboratory
-------
ABSTRACT
This research program was initiated with the overall objective of deter-
mining the effects of geometry, construction and use on the accuracy of S-type
pitot tubes.
Fourteen different S-type pitot tube configurations were tested in the
North Carolina State University subsonic wind tunnel at speeds from 4.52 m/s
(15 ft/sec) to 30.48 m/s (100 ft/sec) to determine the effects of pitot tube
geometry and speed on the pitot coefficient, Cp. In addition, certain of these
S-type pitot tubes were tested in conjunction with sampling probes to assess
the magnitude of the aerodynamic interference of the sampling probe on the pi-
tot tube. Tests were conducted in which the S-type pitot tube was pitched from
-20° to +20° or yawed from -30° to +30°, in increments of 5° in each case, to
determine the effects of misalignment on the pitot coefficient. Finally, the
effect of swirl on the pitot coefficient was determined by testing a S-type
pitot coefficient in a swirling flow field.
It was found that S-type pitot tube 3-04 could be yawed to ±30° or pitch-
ed from -14° to 20° and its pitot coefficient would change less than five per-
cent of the accepted value of 0.85. This same pitot tube was also found to be
the least sensitive to swirling flow, and therefore, it is recommended for
future applications.
This report was submitted in fulfillment of Grant No. 803168 by the De-
partment of Mechanical and Aerospace Engineering, North Carolina State Univer-
sity, under the sponsorship of the U. S. Environmental Protection Agency. This
report covers the period June 1, 1974 to December 31, 1976, and work was com-
pleted as of December 31, 1976.
iv
-------
CONTENTS
Foreword * iii
Abstract iv
Figures vi
Symbols ix
Acknowledgment . , x
1. Introduction 1
2. Conclusions and Recommendations 3
3. Apparatus , 5
4. Experimental Procedure , 10
Calibration of S-Type Pitot Tubes and Determination
of the Effects of Geometry 10
Interference Tests - Zero Sampling Rate 10
Interference Tests - 0.85 x Isokinetic Sampling Rate .... 11
Interference Tests - Isokinetic Sampling Rate 12
Pitch Tests . . , 12
Yaw Tests 16
Swirl Tests 19
5. Data Reduction 20
6. Results and Discussion 22
Effect of Geometry on S-Type Pitot Tube Calibration 22
Interference Effects - Zero Sampling Rate 25
Interference Effects - Sampling at 0.85 x Isokinetic
and Isokinetic Rates 30
Interference Effects - Misalignment of S-Type Pitot Tube,
Sampling Probe Combination 30
Effect of Yaw and Pitch in the Absence of a Sampling
Nozzle 35
Effect of Yaw with' Interference 46
Effect of Pitch with Interference 46
Effect of Swirl 63
References 68
Appendix
A. Velocity Profile of Wind Tunnel Test Section 69
-------
FIGURES
Kumber Page
1 Typical S-Type Pitot Tube and Nomenclature 6
2 Tunnel Set-Up of Assembly for an Interference Effects Test.
Distance Between S-Type Pitot Tube and Sampling Probe,
10.188 inches 9
3a Relative Positions of Sampling Probe and Pitot Tube with
Sampling Probe 0.05 m Above Pitot Tube 13
3b Relative Positions of Sampling Probe and Pitot Tube with
Sampling Probe 0.05 m Below Pitot Tube 14
4 Relative Positions of Sampling Probe and Pitot Tube with
Sampling Probe Port Slightly Ahead of Fore Port of
Pitot Tube , 15
5 Sign Convention , 17
6 S-Type Pitot Tubes 18
7 Pitot Coefficient Versus Velocity for S-Type Pitot
Number 3-01 23
8 Pitot Coefficient Versus Velocity for S-Type Pitot
Number 3-02 23
9 Pitot Coefficient Versus Velocity for S-Type Pitot
Number 3-03 24
10 Pitot Coefficient Versus Velocity for S-Type Pitot
Number 3-04 24
11 Effects of Port-to-Port Spacing and Inter Tube Spacing
on the Average Pitot Coefficient 26
12 Interference Effects on an S-Type Pitot Tube, Sampling
Rate = 0, Spacing = 0 27
13 Interference Effects on an S-Type Pitot Tube, Sampling
Rate - 0, Spacing = 0.01499 m 28
vi
-------
14 Interference Effects on an S-Type Pitot Tube, Sampling
Rate = 0, Spacing = 0.03018 m 29
15 Interference Effects on an S-Type Pitot Tube, Sampling
Rate = 0.85 x Isokinetic, Spacing = 0 31
16 Interference Effects on an S-Type Pitot Tube, Sampling
Rate = 0.85 x Isokinetic, Spacing = 0.01499 m 32
17 Interference Effects on an S-Type Pitot Tube, Sampling
Rate = Isokinetic, Spacing = 0 33
18 Effects of Misalignment of S-Type Pitot Tube - Sampling
Probe Combination, Sampling Rate = Isokinetic, Spacing
= 0 34
19 Effect of Yaw on S-Tube 4-10 36
20 Effect of Yaw on S-Tube 3-04 37
21 Effect of Yaw on S-Tube 3-01 39
22 Averaged Effect of Yaw 41
23 Effect of Pitch on S-Tube 4-10 42
24 Effect of Pitch on S-Tube 3-04 43
25 Effect of Pitch on S-Tube 3-01 44
26 Averaged Effect of Pitch . , 45
27 Effect of Yaw with Interference; Sampling Rate: 0.60 x
Isokinetic Flow Rate 47
28 Effect of Yaw with Interference; Sampling Rate: 0.85 x
Isokinetic Flow Rate 49
i
29 Effect of Yaw with Interference; Sampling Rate: 1.00 x
Isokinetic Flow Rate . . , 51
30 Effect of Yaw with Interference; Sampling Rate: 1.4 x
Isokinetic Flow Rate 53
31 Averaged Effect of Yaw with Interference on S-Tube 4-10 54
32 Effect of Pitch with Interference; Sampling Rate: 0.60 x
Isokinetic Flow Rate 55
33 Effect of Pitch with Interference; Sampling Rate: 0.60 x
Isokinetic Flow Rate 56
vii
-------
34 Effect of Pitch with Interference; Sampling Rate: 0.85 x
Isokinetic Flow Rate 57
35 Effect of Pitch with Interference; Sampling Rate: 0.85 x
Isokinetic Flow Rate 58
36 Effect of Pitch with Interference; Sampling Rate: 1.00 x
Isokinetic Flow Rate 59
37 Effect of Pitch with Interference; Sampling Rate: 1.00 x
Isokinetic Flow Rate 60
38 Effect of Pitch Interference; Sampling Rate: 1.40 x
Isokinetic Flow Rate 61
39 Averaged Effect of Pitch with Interference on S-Tube 4-10 .... 62
40 Effects of Swirl on S-Iube 4-10 64
41 Effects of Swirl on S-Tube 3-04 65
42 Effects of Swirl on S-Tube 3-01 66
APPENDIX A
Al Velocity Profile Across Test Section of NCSU Wind Tunnel .... 70
vlii
-------
LIST OF SYMBOLS AND TERMS
C pitot coefficient at a specific velocity
C average coefficient for"the velocity range 4.5 to 30.5 m/sec
C average pitot coefficient for S-type pitot without the sampling probe
Po° attached
p pressure
p pressure measured in the aft facing leg of S-type pitot tube
Si
p_ pressure measured in the forward facing leg of S-type pitot tube
p total (or stagnation) pressure
p^ pressure far ahead of pitot tube (static pressure)
R gas constant
T temperature
V velocity of the gas
V._ velocity at centerline of test section
Cii
p mass density of the gas
C-C centerline to centerline spacing between sampling probe and pitot tube
S distance between the two legs of the "S" type pitot tube (inter-tube
distance)
W outside diameter of pitot tube leg
D offset distance on tube, see Figure 1
Pitch rotation of the pitot tube about an axis which is normal to both the
flow and the centerline of the S-type pitot tube, see Figure 5 and
page 16.
Taw rotation of the pitot tube about the centerline of the tube, see
Figure 5 and page 16.
ix
-------
ACKNOWLEDGMENTS
The authors wish to thank the U. S. Environmental Protection Agency for
their financial support of this study. In addition, thanks are expressed to
Dr. William J. Mitchell of the Quality Assurance Branch of the U. S. Environ-
mental Protection Agency, Research Triangle Park, North Carolina, for his as-
sistance in obtaining the equipment necessary for this study.
The authors also wish to thank Mr, Herbert E. Moretz, Ms. Ellen W. Terry
and Mr. Benjamin F. Willis, graduate students in the Department of Mechanical
and Aerospace Engineering at North Carolina State University, for their assis-
tance in actually carrying out the tests reported herein and Mrs. Patricia T.
Hummel for preparing the manuscript for this final report.
-------
SECTION 1
INTRODUCTION
For an in-stack environment, high dust loadings that lead to solids build-
up may result in erroneous pressure measurements when using a standard pitot-
static tube. Consequently, the S-type (Stauscheibe or reverse type) pitot tube
is employed in source sampling. The tube is used to determine the stack gas
velocity and volumetric flow rate. Also in source sampling, the S-type pitot
tube is used in conjunction with a sampling probe in order to measure the com-
position of pollutant emissions. The local sampling velocities should equal
the actual local stack velocities when isokinetic sampling is required.
Normally, S-type pitot tubes are calibrated in terms of a pitot coeffi-
cient (Cp) which is defined as
C =
P
- V
A pitot coefficient of 0.85 is the generally accepted value for S-type tubes.
However, the S-type pitot tube is sometimes used in conjunction with a sampling
probe. Consequently, it has been suggested [1] that each S-type pitot tube be
calibrated with a sampling probe attached before use in sampling programs.
Unfortunately, there is very little data available to indicate how either
design parameters or use of S-type pitot tubes affect the pitot coefficient.
Gnyp, et al. [1] performed a detailed evaluation of a number of commercially
available S-type pitot tubes. Beyond this, however, the information available
in the literature seems to be scant and what data is available is not well doc-
umented. The distance between the upstream and downstream openings of the
pitot tube, the angle of the pitot tube faces and the body of the tube, the
angle between the pitot tube and the gas flow, and the range of gas velocities
may affect the pressure readings of the S-type pitot tube and will therefore
alter the pitot coefficient. In addition, when the S-type pitot tube is used
in conjunction with a sampling probe to determine particulate or acid mist
emissions, it is common practice to place the sampling probe close to the pitot
tube. With the pitot tube and the sampling probe in close proximity to one an-
other, the flow field about the sampling probe will alter the flow field about
the pitot tube and vice versa. This aerodynamic interference may certainly
alter the pressure indication of the pitot tube and lead to incorrect estimates
of the flow velocity. Gnyp, et al. [1] have also evaluated this interference
effect, but only for a number of commercially available S-type pitot tubes.
-------
Finally, when S-type pitot tubes are used in actual source sampling, the
flow in the source being sampled may be far from the ideal conditions of
smooth and uniform flow. There may be varying degrees of swirl in the flow
due to blowers, bends, and obstructions upstream of the sampling site. The
effects of such swirling flow on the indications of S-type pitot tubes have
apparently never been evaluated as a function of pitot tube design.
In view of the above, it was clear that there is a definite need for a
definitive study of the effects of pitot tube design, construction and use to
determine the accuracy with which measurements can be made using a S-type pi-
tot tube. In 1974 the Department of Mechanical and Aerospace Engineering at
North Carolina State University undertook such a study under the sponsorship
of the Environmental Protection Agency. In particular an investigation was
conducted to:
1. Calibrate a number of S-type pitot tubes.
2. Determine the effects of geometry changes, such as the
upstream and downstream port to port distance, on the accuracy of
S-type pitot tubes.
3. Determine the effects of misalignment of the S-type pi-
tot tube with respect to the air stream (pitch and yaw).
4. Determine the aerodynamic interference effects due to the
S-type pitot tube and the sampling probe being in close proximity
to one another.
5. Determine the effect of swirling type flow and also sam-
pling rate on the accuracy of S-type pitot tubes.
The present work reports on the results obtained in this study.
-------
SECTION 2
CONCLUSIONS AND RECOMMENDATIONS
The data obtained from this investigation leads to the following conclu-
sions :
1. The S-type pitot tubes, when tested independent of the sam-
pling probe, exhibit pitot coefficients within three percent of the
accepted value of 0.85 for speeds between 6.09 m/s (20 ft/sec) and
30.48 m/s (100 ft/sec). For speeds between 3.05 m/s (10 ft/sec) and
6.09 m/s (20 ft/sec), the variation of the pitot coefficient from
0.85 increases to five percent. Below 3.05 m/s (10 ft/sec), the
measured pitot coefficient is unreliable. The pressure reading fluc-
tuates as much as 50 percent for speeds lower than 3.05 m/s (10 ft/
sec) and this fluctuation shows on the micromanometer. The fluctua-
tion is so great that a "mean" pressure cannot be obtained.
2. In the range of flow speeds from 4.57 m/s (15 ft/sec) to
30.48 m/s (100 ft/sec) the pitot coefficient decreases slightly as
the velocity increases, but this decrease is sufficiently small for
the pitot coefficient to be represented by an average value in this
range of velocities.
3. When an EPA type sampling probe is used in conjunction with
an S-type pitot tube there may be an aerodynamic interference effect
that will alter the average pitot coefficient of the S-type pitot
tube. This interference is most noticeable when the centerline-to-
centerline distance between the sampling probe and the S-type pitot
tube is less than ten times the outside diameter of the pitot tube
legs. The interference effects are greatest with large size sampling
nozzles. However, sampling with the sampling probe tends to reduce
the interference effects. The interference effects on the pitot tube
were also reduced when the pitot tube was pushed 5 cm further into
the wind tunnel than the sampling nozzle.
4. With regard to yaw, S-type pitot tube 3-04 gives the best
results. This tube is capable of undergoing yaw angles from -30° to
30° while staying within five percent of the accepted value of 0.85
for the pitot coefficient. S-type pitot tube 3-01 requires a range
from -25° to 25° in order to preserve this accuracy. In the case of
S-type pitot tube 4-10, yaw angles from -9° to 9° are needed.
5. For the case of pitch, S-type pitot tube 3-04, again, proves
to be superior. It may sustain pitch deflections as large as ±20°
-------
without exceeding the prescribed five percent tolerance of Cp. Sim-
ilarly, as in the case of yaw, S-type pitot tube 3-01 gives inter-
mediate results. A pitch range spanning from -14° to 20° will in-
sure that the pitot coefficient is within five percent of the accept-
ed value of 0.85. Pitch angles ranging from -6° to 20° are required
to maintain a five percent tolerance for S-type pitot tube 4-10.
6. When interference effects from the sampling probe are cou-
pled with yaw, S-type pitot tube 4-10 gives values for the pitot co-
efficient which are outside of the desired five percent tolerance
for all angles of yaw between -30° and 30°. However, this error is
not more than 12 percent.
7. When interference is present during pitch, S-type pitot
tube 4-10 may be pitched from -15° to 2° and still maintain the five
percent tolerance. If the tube is pitched as much as ±20° the er-
rors encountered can be as large as ten percent.
8. While a swirling environment tends to increase the pitot
coefficient for all the tubes studied, S-type pitot tube 3-04 experi-
enced the smallest percent increase (2.7 percent).
-------
SECTION 3
APPARATUS
All of the tests reported in the present work were conducted in the North
Carolina State University Merrill Subsonic Wind Tunnel. This wind tunnel is a
continuous flow, single return tunnel with a vented test section. The wind
tunnel is capable of producing speeds from 0 to approximately 49.19 meters/sec
(110 mph) in a test section 1.143 m (45 inches) wide, 0.8128 m (32 inches)
high and 1.1684 m (46 inches) long (in the flow direction). The test section
is fitted with large plexaglass windows, in the sidewalls, through which the
test models may be observed, and a removable floor. Several test section
floors were constructed and fitted with supports for the pitot tubes which
were tested.
The actual velocity in the wind tunnel was measured in each test using an
elliptical nose, standard, pitot-static tube manufactured by Airflow Develop-
ments Limited, High Wycombe, England. This elliptical nose pitot-static tube
is regarded as the best available standard and is the only one recommended for
use without individual calibration [1]. This standard probe has a pitot coef-
ficient of unity. All S-type tubes were calibrated relative to this standard
tube.
The S-type pitot tubes to be tested were supplied by the Environmental
Protection Agency through the Grant Monitor. One tube (No. 4-10) was supplied
completely assembled (with the two legs of the probe welded together). Five
additional pairs of legs for S-type pitot tubes were supplied unassembled.
Each of the legs supplied was bent at one end to an angle of 45° and the probe
face was machined so that it was parallel to the main body of the tube. For
each pair of legs the offset (dimension D in Figure 1) was different. A num-
bering system was devised to identify each pair of legs. In this numbering
system the first digit indicates the length of the probe and the second and
third digits indicate the particular probe within the series. Three inter-
tube spacers were constructed at North Carolina State University. Each pair
of legs could be assembled either with no spacer between the legs or with one
of the constructed spacers between the legs. This combination of tube 4-10
plus five pairs of tube legs and four inter-tube spacings resulted in a total
of 21 different S-type pitot tube configurations which could be tested. A
total of 14 of these different configurations were tested. Table 1 lists the
various tubes tested, together with the important geometrical dimensions of
each tube.
Pressure indications at both the pitot and static ports of the standard
pitot-static tube and at both the fore and aft ports of the S-type pitot tubes
were measured on a micromanometer. With this instrument it is possible to
-------
PORT-TO-PORT
DIMENSIO
INTER
TUBE
SPACING
\
IM-
LEG
FIGURE 1. Typical S-Type Pitot Tube and Nomenclature
-------
TABLE 1. Geometry of S-Type Pitot Tubes Tested.
Tuue
Number
3-01
3-02
3-03
3-04
3-01
3-02
3-03
3-01
3-02
3-03
3-01
3-02
3-20
4-10
Port to Port
Dist. (P-P)
3.175
6.0325
8.2753
11.1176
3.3909
6.2357
8.509
4.6888
7.5463
9.7892
6.4567
9.050
3.9116
2.1742
Inter Tube
Spacing (S)
0.000
0.000
0.000
0.000
0.254
0.254
0.254
1.5138
1.5138
1.5138
3.0175
3.0175
0.9144
0.000
Offset Dis-
tance (D)
1.5875
2.9921
4.1148
5.588
1.5875
2.9921
4.1148
1.5875
2.9921
4.1148
1.5875
2.9921
1.4986
1.0871
O.D. of pitot
tube leg (W)
0.9525
0.9525
0.9525
0.9525
0.9525
0.9525
0.9525
0.9525
0.9525
0.9525
0.9525
0.9525
0.632
0.9525
NOTES: 1. All dimensions are in cm.
2. Tube numbers are determined as follows:
a. The leading digit indicates the approximate tube
length in feet, rounded to the next smaller even
footage.
b. The trailing two digits give the number of this
tube in the given length series.
For some tubes P-P
by the clamps.
S + 2D due to distortions caused
-------
measure pressure differences with an accuracy of ±0.00254 cm (±0.001 in) of
water. All pressures were measured relative to atmospheric pressure. The at-
mospheric pressure was measured using a standard mercury filled barometer,
which has an accuracy of ±0.0127 cm (±0.005 in) of mercury.
The gas temperature, used in calculating correction factors for the gas
velocity, was measured using a standard chromel-alumel thermocouple, mounted
inside the wind tunnel test section, in conjunction with a galvanometer read-
out . Specific humidity of the atmospheric air was determined from the wet and
dry bulb temperature measured using a sling-type psychrometer.
In the tests to determine the interference effects between the pitot tube
and the sampling probe, three sampling probe nozzles with inside diameters of
0.635 cm (0.25 in), 0.953 cm (0.375 in), and 1.27 cm (0.500 in) were used.
(The outside diameters of these three nozzles were 0.953 cm (0.375 in), 1.27 cm
(0.500 in), and 1.59 cm (0.625 in), respectively.) Figure 2 shows the S-type
pitot tube, the sampling probe and the standard pitot static tube in the wind
tunnel test section. The sampling probe was connected to a commercially pro-
duced Staksamplr*. Tests were conducted with the Staksamplr turned off (zero
sampling rate) and at various sampling rates through the nozzle-probe-Staksamplr
system. The sampling probe, sampling-probe nozzles, and Staksamplr used in
these tests were supplied by the Environmental Protection Agency through the
Grant Monitor. Additional, specific details regarding each piece of equipment
used in the present investigation are presented in References 2 and 3.
*Trademark
8
-------
FIGURE 2. Tunnel Set-Up of Assembly for an Interference Effects
Test. Distance Between S-Type Pitot Tube and Sampling
Probe, 10.188 inches
-------
SECTION 4
EXPERIMENTAL PROCEDURE
CALIBRATION OF S-TYPE PITOT TUBES AND DETERMINATION OF THE EFFECTS OF GEOMETRY
Each of the S-type pitot tubes used was calibrated against a standard el-
liptical nosed pitot-static tube. To accomplish this, the wind tunnel speed
was first set at a given (approximate) velocity and the standard pitot-static
tube was inserted into the wind tunnel so that the total pressure port was lo-
cated at the center of the tunnel test section. Total and static pressures
were then measured. The wind tunnel test section velocity is determined accu-
rately from these measured pressures. The standard pitot-static tube was then
withdrawn from the wind tunnel test section and the S-type pitot-tube immedi-
ately inserted at the same location in the wind tunnel (i.e. the upstream port
of the S-type pitot tube was located at exactly the same location previously
occupied by the ellipsodal nose of the standard pitot-static tube). The pres-
sures at the forward and aft ports of the S-type pitot tube were then measured.
The S-type pitot tube was withdrawn from the tunnel. The test-section velocity
was then increased and the above procedure was repeated at this new velocity.
Initially this process was repeated in velocity increments of 1.52 m/s (5 ft/
sec) for the velocity range from 1.52 m/s (5 ft/sec) to 30.48 m/s (100 ft/sec).
Upon completion of this sequence of tests, the S-type pitot tube was rotated
180 degrees so that the pressure port which had been facing forward now faced
aft and the pressure port which had been facing aft, now faced forward. The
entire procedure outlined above was then repeated for the tube in this new ori-
entation.
Preliminary testing showed that the measurements made at low velocities
(below approximately 4.572 m/s) were inaccurate because it was difficult to
accurately read the micromanometer at very low pressure differentials. For
this reason, the measurements made at velocities below 4.522 m/s (15 ft/sec)
were dropped and subsequent tests were made in the velocity range from 6.096
m/s (20 ft/sec) to 30.48 m/s (100 ft/sec) with increments of 3.048 m/s (10 ft/
sec).
Since all the tubes were calibrated in this manner, the effects of vary-
ing geometry could be determined by comparing the calibrations of different
pitot tubes.
INTERFERENCE TESTS - ZERO SAMPLING RATE
To determine the effects of aerodynamic interference of the sampling
probe on the measurements of the S-type pitot tube, the sampling probe and
10
-------
S-type pitot tube were tested together in the wind tunnel. To accomplish this
the S-type pitot-tube and the sampling probe were mounted in the wind tunnel
test section, through the tunnel floor, with a given centerline to centerline
separation between the S-type tube and the sampling probe. The standard pitot
static tube was inserted into the wind tunnel so that it was approximately
0.1524 m (6 in) from the S-type tube-sampling probe combination. (Figure 2
shows a view, looking downstream, of the S-type tube, the sampling probe and
the standard pitot static tube in the wind tunnel.) The variation in velocity
between the locating of the S-type pitot tube and the standard pitot tube was
always less than two percent (See Appendix A).
With the combination of probes in position the wind tunnel speed was ad-
justed to approximately the desired value. The pitot and static pressures
were measured, using the standard pitot-static tube, and then the standard pi-
tot static tube was retracted to the tunnel wall. The pressures at each of
the ports on the S-type tube were then measured for a number of different cen-
terline to centerline spacings between the sampling probe and the S-type pitot
tube. By moving the sampling probe the spacing between the S-type pitot tube
and the sampling probe was varied from 0.03016 m (1 3/16 in) to 0.2588 m (10
3/16 in) in increments of 0.0254 m (1 in). With the clamping arrangement used
in the present tests, 3.02 cm was the closest centerline to centerline dis-
tance at which the S-type pitot tube and the sampling probe could be mounted.
A spacing code was assigned to each of the centerline to centerline spac-
ing in the following manner: The spacing 0.0316 m (1 3/16 in) was assigned
the code number 0, the spacing 0.05556 m (2 3/16 in) was assigned the code
number 1, the spacing 0.08096 m (3 3/16 in) was assigned the code number 2,
etc. The highest code number used was 9, corresponding to a centerline to
centerline spacing of 0.25877 m (10 3/16 in).
After some preliminary testing, it was found that the greatest variation
in the pitot coefficient occurred at small centerline to centerline spacings.
For later tests, therefore, measurements were only obtained at the spacings
corresponding to code numbers 0, 1, 2, 4, and 9.
The entire procedure, outlined above, was repeated in the velocity range
from 6.096 m/s (20 ft/sec) to 30.48 m/s (100 ft/sec) in increments of 6.096
m/s (20 ft/sec). These tests were conducted with S-type pitot tube number 3-01
with inter-tube spacings of zero, 0.0151 m and 0.03018 m, and with sampling
probe nozzles of 0.00635 m (0.25 in), 0.009525 m (0.375 in), and 0.01275 m
(0.5 in) diameter. For these tests the sampling rate was zero for the sam-
pling probe.
INTERFERENCE TESTS - 0.85 x ISOKINETIC SAMPLING RATE
In cases where interference effects were determined with the sampling
probe in operation, the standard pitot static measurements were made without
the S-type pitot tube and the sampling probe in the tunnel. Instead, the stan-
dard pitot-static tube was inserted in the tunnel first so that the pitot pres-
sure port was in the same location as in the zero sampling rate test. The pi-
tot and static pressures were measured at a given tunnel setting. Once these
pressures were measured, the standard pitot-static tube was retracted to the
11
-------
wall of the tunnel and then the S-type pitot tube-sampling probe combination
was inserted into the tunnel in the same location as was used in the zero sam-
pling rate case. The Staksamplr was then turned on and adjusted so that the
flow through the sampling probe was 85 percent of the isokinetic value. The
pressures at the fore (upstream) and aft (downstream) ports of the S-type pi-
tot tube were then measured as the centerline to centerline distance between
the sampling probe and the S-type pitot tube was changed. Measurements were
made for centerline to centerline spacings corresponding to code numbers 0, 1,
2, 4, and 9. Once these measurements were obtained, the Staksamplr was turned
off. The S-type pitot tube-sampling probe combination was withdrawn from the
tunnel and the standard pitot-static tube was once more inserted into the wind
tunnel. The tunnel speed was adjusted to a new value and the above procedure
was repeated. Tests were made at 4.572 m/s (15 ft/sec), and from 6.096 m/s
(20 ft/sec) to 30.48 m/s (100 ft/sec) in increments of 6.096 m/s (20 ft/sec).
These tests were conducted using pitot-tube number 3-01 with inter-tube spac-
ings of zero, 0.0151 m (0.596 in) and 0.03018 m (1.188 in), and with sampling
probe nozzle diameters of 0.00635 m (0.25 in), 0.009525 m (0.375 in), and
0.0127 m (0.5 in).
INTERFERENCE TESTS - ISOKINETIC SAMPLING RATE
Tests to determine the interference effects with the sampling probe sam-
pling at the isokinetic flow rate were conducted in the same manner as the
tests in which sampling was at 85 percent of the isokinetic rate. Again mea-
surements were made at velocities of 4.572 m/s (15 ft/sec) and from 6.096 m/s
(20 ft/sec) to 30.48 m/s (100 ft/sec) in increments of 6.096 m/s (20 ft/sec),
with probes 3-01 and 3-02 with zero inter-tube spacing. Tests in this series
were made at centerline to centerline distances corresponding to spacing code
numbers 0, 1, and 9.
In addition, some of these tests were conducted with the sampling probe
0.05 m (2 in) beyond and 0.05 m (2 in) behind the S-type probe (Figure 3). Al-
so several additional tests were conducted with the nozzle of the sampling
probe extending slightly upstream of the upstream port of the S-type pitot
tube, but with the tube axes in the same plane (Figure 4).
PITCH TESTS
A series of tests were conducted to determine the effects of pitch on the
pitot coefficient for the S-type pitot tube. In these tests, and in subsequent
tests to determine the effects of yaw and swirl, the standard pitot-static tube
was not used to determine the speed at each test point. Instead, the wind tun-
nel speed was determined from the difference in two pressures measured by two
wall pressure-taps in the wind tunnel and indicated on a micromanometer mounted
on the outside of the wind tunnel. The wind tunnel speed was calibrated using
the standard pitot static tube to relate the indications on the micromanometer
to the pressure difference across the standard pitot static tube. Thereafter,
the pressure indications on the micromanometer were used to determine the pres-
sure difference which would exist across the standard pitot-static tube if it
had been placed in the tunnel.
12
-------
FLOW
0.05 m
•*- 2.5 cm ->
FIGURE 3a. Relative Positions of Sampling Probe and Pitot
Tube with Sampling Probe 0.05 m Beyond the
Pitot Tube.
13
-------
FLOW
T
0.05 m
I
FIGURE 3b. Relative Positions of Sampling Probe and
Pitot Tube with Sampling Probe 0.05 m
Behind the Pitot Tube.
14
-------
FLOW
FIGURE 4. Relative Positions of Sampling Probe and
Pitot Tube with Sampling Probe Port Slightly
Upstream of the Upstream Port of Pitot Tube.
15
-------
In the tests to determine the effects of pitch, the S-type pitot tube was
inserted in the wind tunnel test section and the test section velocity was set
at the desired (approximate) value. The pitch angle of the S-type tube was
then set and the pressure difference between the fore and aft ports of the
tube were measured. These measurements were made with the pitch angle varied
from +20° to -20° in 5° increments. The pitching mechanism employed in the
present tests was designed so that the center of rotation of the S-type pitot
tube was at the forward port of the tube. The sign convention used here con-
siders positive pitch to be the case where the probe is rotated so that the
rear, or aft, port moves up (Figure 5). Thus, as the S-type pitot tube is
pitched, the forward port of the tube remains at essentially the same location
in the tunnel. This minimizes the effects of even small variations in flow
velocity across the tunnel test section.
Once the S-type pitot tube had been tested at all the pitch angles noted
above, the test section velocity was increased and the procedure outlined
above was repeated. Pitch tests were conducted in the velocity range from ap-
proximately 6.096 m/s (20 ft/sec) to approximately 27.43 m/s (90 ft/sec) in
increments of approximately 6.096 m/s (20 ft/sec). Pitch tests were conducted
using pitot tubes 3-01, 3-04, and 4-10 (Figure 6).
Additional tests were conducted to measure the effect of pitch on the com-
bination S-type pitot tube and sampling probe. To conduct these tests, the
sampling probe was attached to the left side of the S-type pitot tube and the
assembly pitched as a unit. In this configuration the distance between the
centerline of the S-type tube and the sampling probe was 0.03175 m (1 1/4 in).
Tests were conducted at sampling rates of 0.6 x isokinetic, 0.85 x isokinetic,
1 x isokinetic and 1.4 x isokinetic using pitot tube 4-10 and the appropriate
nozzle size.
YAW TESTS
A series of tests were also conducted to determine the effects of yaw on
the pitot coefficient for the S-type pitot tube. In conducting these tests,
the S-type pitot tube was inserted in the wind tunnel and the test section
speed was adjusted approximately to the desired value. The S-type pitot tube
was then yawed to a prescribed value and the pressure difference between the
fore and aft ports of the S-type pitot tube were measured. Tests were conduct-
ed, at a given tunnel speed, with the yaw angle varied from -30° to +30° in in-
crements of 5°. Positive yaw angles correspond to counter clockwise rotation
of the tube, as seen from the pressure sensing end of the tube (Figure 5).
Once measurements were made for all the yaw angles considered, the tunnel
speed was increased to a new value and the yaw tests, outlined above, were re-
peated. In this manner, measurements were made over the velocity range from
approximately 6.096 m/s (20 ft/sec) to approximately 27.43 m/s (90 ft/sec) in
increments of approximately 6.096 m/s (20 ft/sec). Yaw tests were conducted
using pitot tubes 3-01, 3-04, and 4-10.
Additional tests were conducted to measure the effects of yaw on a com-
bined S-type pitot tube and sampling probe. For these tests the S-type pitot
tube and sampling probe were clamped together and yawed in combination. In
16
-------
Yaw
I I I
Flow Direction
Top View
Pitch
Flow Direction *•
o°
Side View
FIGURE 5. Sign Convention.
Upstream Port
Pitot Tube
Downstream Port
17
-------
3-01
FIGURE 6. S-Type Pitot Tubes
18
-------
this configuration the distance between the sampling probe centerline and the
S-type pitot tube centerline was 0.03175 m (1 1/4 in). Tests were conducted
with this combined probe at sampling rates of 0.6 x isokinetic, 0.85 x isoki-
netic, 1 x isokinetic and 1.4 x isokinetic using pitot tube 4-10 and the ap-
propriate nozzle size.
SWIRL TESTS
A series of tests were conducted to determine the effect of swirl in the
flow on the pitot coefficient of S-type pitot tubes. Swirl was produced in
the wind tunnel by inserting a small wing section into the wind tunnel just
upstream of the tunnel test section. The wing section was attached to the
tunnel ceiling and had a length such that the tip of the wing section was lo-
cated at the centerline of the tunnel. With this wing section positioned at
an angle of attack, a wing-tip vortex is produced along the centerline of the
tunnel. The existence of a swirling flow in the wind tunnel was verified by
introducing a tuft grid into the ^est section and observing the movement of
the tufts.
To determine the effect of swirl, the S-type pitot tube was first cali-
brated in the wind tunnel with the wing section removed from the tunnel. The
same S-type pitot tube was again tested in the wind tunnel over the same speed
range, but with the wing now mounted upstream of the test section. In the
present tests, S-type pitot tubes 3-01, 3-04, and 4-10 were tested in this
manner.
Additional details regarding the procedure used in each of the above tests
are presented in References 2 and 3.
19
-------
SECTION 5
DATA REDUCTION
The basic measurements in the bulk of the experiments reported herein
consisted of the pressures (relative to atmospheric) measured individually at
the pitot port and the static port of the standard elliptical nosed pitot-
static tube and the pressures (relative to atmospheric) measured individually
at the fore and aft ports of the S-type tube being tested. The wind tunnel
test section velocity is determined from the measured pitot and static pres-
sures on the elliptical nosed pitot static tube. Since this pitot-static tube
has a pitot coefficient of unity the velocity is related to the measured pres-
sures and the air density by:
V =
The velocity, determined from the measured pressures on the fore and aft legs
of the S-type pitot tube, is therefore given by:
where Cp is the pitot coefficient for the probe, p^ is the pressure measured
in the forward facing leg of the S-type pitot tube, pa is the pressure mea-
sured in the aft facing leg of the S-type pitot tube, and p is the density of
the gas (air in this case). Since the time between measurements made with the
pitot-static tube in place and the measurements made with the S-type pitot
tube in place is small, it is reasonable to assume that neither the velocity
nor the density in the tunnel changes during the test. Therefore, combining
Equations (1) and (2) yields:
C
i — if * •
P / (Pf - Pa)
The velocity in the wind tunnel is obtained directly from Equation (1).
The air density, p, is not measured directly but may be determined from the
equation of state for the air in the wind tunnel, i.e.
p = p_
where pm is the barometric pressure (which is the same as the static pressure
in the wind tunnel since the tunnel test section is vented) and T is the
20
-------
temperature (absolute) of the gas (air) in the wind tunnel. The gas constant
R is calculated quite accurately with corrections made for the moisture con-
tent in the air. Corrections were also made for the variation of the density
of the manometer and barometer fluids (water and mercury, respectively) with
temperature. These corrections, however, are quite small. Additional details
regarding the methods of data reduction may be found in References 2 and 3.
21
-------
SECTION 6
RESULTS AND DISCUSSION
EFFECT OF GEOMETRY ON S-TYPE PITOT TUBE CALIBRATION
The variation of the pitot coefficient versus the velocity for the S-type
pitot tubes studied can be seen in Figures 7, 8, 9 and 10. One curve in each
figure represents the results obtained with the "A" side facing into the flow
stream, the other represents the results obtained with the "B" side facing in-
to the flow stream. (The dashed line in each figure is the average coefficient
for that tube for the velocity range above 4.57 m/s when inter-tube spacers
were not used.) As can be seen in these figures, the pitot tubes gave readings
at speeds greater than 6.09 m/s that proved the tubes to be symmetrical.
At wind tunnel speeds less than 3.05 m/s, the accuracy of the calculated
pitot coefficient was found to be unacceptable since the pressure readings for
the two ports of the S-type pitot tube are almost equal. The maximum differ-
ence in the two port readings was 0.005 inches of water, and the micromano-
meter is not sensitive enough to make the pressure readings reliable. There
is a relatively large variation of the pitot coefficient in the range of 3.05
m/s to 6.09 m/s. However, above 6.09 m/s the pitot coefficient curves level
out.
At speeds greater than approximately 15.25 m/s, small unsteady fluctua-
tions were observed in the rearward facing port pressure readings although the
test section air speed was held steady. These unsteady fluctuations are attri-
buted to unstable turbulence patterns shedding from the forward facing port and
passing over the rearward facing port. When unsteady fluctuations arose, a
"mean" pressure reading was obtained by selecting the value in the center of
the fluctuation range.
Since the four pitot tubes represented in Figures 7, 8, 9, and 10 belong
to the family of pitot tubes where the inter-tube spacing is constant, but the
port-to-port dimension varies from 3.18 cm (1.25 in) to 11.1 cm (4.38 in), it
is possible to estimate the effect of port-to-port spacing on the pitot coef-
ficient for a family of pitot tubes. An inspection of Figures 7, 8, 9, and' 10
shows that as the port-to-port distance increases the average coefficient in-
creases slightly.
To determine the effect of inter-tube spacing on the pitot coefficient,
several spacers were placed between the two legs of the S-type pitot tubes be-
ing tested. The inter-tube spacers used were 0.25 cm, 1.5 cm, and 3.01 cm.
The results of these studies showed that the average pitot coefficients of
22
-------
0.90
C
P
0.85
0.80
C = 0.833
S
0 5 10 15 20 25
FIGURE 7. Pitot Coefficient Versus Velocity for S-Type Pitot Number 3-01
0.85
U»
P
0.80
0.75
(& <£
C = 0.834
P
LEGEND
A "A" - Side Forward
O "B" - Side Forward
30
10 15 20
Velocity, meters/second
25
30
FIGURE 8. Pitot Coefficient Versus Velocity for S-Type Pitot Number 3-02
-------
0.85
C
0.80
*
C = 0.838
P
o
4Ji
,0— g^a
0 5 10 15 20 25
FIGURE 9. Pltot Coefficient Versus Velocity for S-Type Pitot Number 3-03
0.95
ro
0.90
C
P
0.85
0.80
LEGEND
A
MH^H I
O
£
A
O
"A"
"B"
Side Forward
Side Forward
C = 0.847
P
0 5 10 15 20
Velocity, meters/second
FIGURE 10. Pitot Coefficient Versus Velocity for S-Type Pitot Number 3-04
25
30
-------
S-type pitot tubes with spacers were slightly lower than those for tubes that
had an identical port-to-port dimension but did not have a spacer.
Also, it was found that filling the gap produced by the spacers placed
between the two legs of the tubes being tested did affect the average pitot
coefficient. That is, when the gap region was filled by taping the two legs
of the tubes together the taped pitot gave an average pitot coefficient higher
than that for the same pitot without tape. However, the pitot coefficients
for the taped tubes were found to be approximately equal to those for S-type
pitot tubes having the same port-to-port dimension and no inter-tube spacing.
It is possible to determine the overall effect of changes in geometry by
comparing the average pitot coefficients for the 14 tubes tested. The average
pitot coefficient "Cp for the tubes tested, is presented in Figure 11 as a func-
tion of the nondimensional port-to-port spacing (P-P)/W where W is the outside
diameter of the pitot legs. The nondimensional inter-tube spacing S/W for
each test is denoted by the symbol used. (The two flagged symbols in Figure
11 represent the results of special tests in which the gap between the legs of
the S-type pitot tube was enclosed by wrapping tape around the legs of the
tubes.) From Figure 11 it is clear that there is no discernible trend of the
average pitot coefficient with variation of either port-to-port spacing or
inter-tube spacing. Any variation due to changes in these dimensions is so
small that it is masked by the data scatter. The data scatter is remarkably
small, being no more than 2.5 percent. One is forced to conclude that for the
range of port-to-port spacings and inter-tube spacings tested, the average pi-
tot coefficient is essentially independent of either of these geometric dimen-
sions .
INTERFERENCE EFFECTS - ZERO SAMPLING RATE
A number of tests were conducted to determine the effect on the S-type
pitot tube of locating a sampling probe with nozzle close to the S-type pitot
tube. These tests were conducted using tube 3-01. Figures 12, 13, and 14
show the variation of the average pitot coefficient with the centerline to
centerline distance between the S-type pitot tube and the sampling probe with
no flow through the sampling probe (zero sampling rate). In these and subse-
quent figures, the average pitot coefficient has been normalized by the aver-
age pitot coefficient for the S-type pitot tube alone, TTpoo, and the centerline
to centerline distance, C-C, has been normalized by the diameter of the legs
in the S-type pitot tube, Wi
Figure 12 shows the effects with three different sampling probe nozzle
diameters of centerline to centerline spacing on the average pitot coefficient,
for the case where the Inter-tube spacing is zero. In general, for the two
smaller nozzles the average pitot coefficient for small centerline to
centerline distances is below the average pitot coefficient for the S-type
tube alone, ~Cpm. As the centerline to centerline distance was increased (by
moving the sampling probe), the average pitot coefficient increased to some
one to two percent larger than the average coefficient for the S-type pitot
tube alone, Cpoo (at centerline to centerline distances greater than ten tube
diameters). For the largest sampling probe nozzle inside diameter (0.0127 m
[0.5 in]) the average pitot coefficient at all centerline to centerline
25
-------
S3
0.9
0.85
0.8
S/W
O 0
D .2667
A 1.5733
3.168
& cPO .
"<> A ^ A
6
(P-P)/W
10
12
FIGURE 11. Effects of Port-to-Port Spacing and Inter-Tube Spacing on the
Average Pitot Coefficient.
-------
10
1.05 -
1.0 -
0.95 -
0.90
SAMPLING
SYMBOL NOZZLE
I.D.
O 0.00635 m
D 0.009525 m
O 0.01270 m
0
n a a a
a a
o
a O
0
O
O
0 5 10 15 20
(C-C)
w
a
o
O
25
FIGURE 12. Interference Effects on an S-Type Pltot Tube, Sampling Rate = 0, Inter-tube Spacing
= 0.
-------
1.05
1.0
to
00
0.95
0.90
SAMPLING
SYMBOL NOZZLE
I.D.
O 0.00635 m
D 0.009525 m
0.0127 m
O
o
o
o o
D
O
D
O
O
10
15
(C-C)
w
20
FIGURE 13. Interference Effects on an S-Type Pltot Tube, Sampling Rate
= 0.01499 m.
25
0, Inter-tube Spacing
-------
N>
1.05 -
1.00 -
0.95 .
0.90
SAMPLING
SYMBOL NOZZLE
I.D.
O 0,00635 m
D 0.009525 m
O 0.0127 m
0 0
0 O
O
n
o o
O
1 1 1 1
0 5 10 15 20
(C-C)
w
O
O
b
1
25
FIGURE 14. Interference Effects on an S-Type Pitot Tube, Sampling Rate
= 0.03018 m.
0, Intet-tube Spacing
-------
distances is less than the average pitot coefficient for the S-type pitot tube
alone.
A comparison of Figures 12, 13, and 14 shows the effect of inter-tube
spacing on interference. For an inter-tube spacing of 0.01499 m (0.59 in) the
Interference effects seem to be minimized, but for a larger inter-tube spacing
0.03018 m (1.188 inches) the effects seem to be of the same order as those for
zero inter-tube spacing. It is difficult to reach any definite conclusion re-
garding the effect of inter-tube spacing since the effects seem to- be small
and of the same order as the data scatter, particularly for large centerline-
to-centerline distance. It is clear, however, that for centerline-to-center-
line distances of less than ten diameters of the pitot tube leg, the .average
pitot coefficient is less than the average pitot coefficient for the S-type
pitot tube alone. Further, this effect is largest for the larger sampling
probe diameters.
INTERFERENCE EFFECTS - SAMPLING AT 0.85 x ISOKINETIC AND ISOKINETIC RATES
Figures 15 and 16 show the interference effects obtained while sampling
at a rate of 0.85 x isokinetic and Figure 17 shows the interference effects
obtained while sampling at the isokinetic flow rate. Again these tests were
conducted using S-type pitot tube 3-01. Comparison of Figures 12 through 17
shows the total effect of sampling at various flow rates on the interference
between the sampling probe and the S-type pitot tube. It is apparent that
sampling tends to reduce the interference effects and that this reduction in
interference appears most drastic when sampling at a rate of 0.85 x isokinetic
with an S-type pitot tube with zero inter-tube spacing (Figure 15). However,
the effects in all cases seem quite small, on the order of three or four per-
cent.
INTERFERENCE EFFECTS - MISALIGNMENT OF S-TYPE PITOT TUBE, SAMPLING PROBE
COMBINATION
A few additional tests were conducted to determine the magnitude of the
interference when the S-type pitot tube and the sampling probe were not in-
serted the same distance into the wind tunnel. These tests were conducted us-
ing S-type pitot tubes 3-01 and 3-02. Typical results for three positions of
the sampling probe relative to pitot tube 3-01 are shown in Figure 18 for the
variation of the ratio 'CpfCnoa with (C-C)/W. In all three positions the nose
of the sampling nozzle (1.27 cm I.D.) was in the same plane as the nose of the
S-type pitot tube. The circular symbols represent the case where the sampling
probe is inserted into the wind tunnel the same distance as the S-type pitot
tube. The square symbols represent the case where the sampling probe is in-
serted into the wind tunnel 0.0508 m (2 in) behind the S-type pitot tube and
the diamond symbols represent the case where the S-type pitot tube is inserted
into the wind tunnel 0.0508 m (2 in) beyond the sampling probe. Clearly the
interference effects are smaller when the sampling probe is inserted into the
air stream less than the S-type probe. Additional studies were conducted us-
ing pitot tube 3-01 with 0.635 cm and 0.953 cm I.D. nozzles. The results from
this additional study and from analogous studies using pitot 3-02 were similar
to those shown in Figure 18. These additional studies on pitot/nozzle mis-
alignment are reported in Reference 2.
30
-------
1.05
1.0
0.95
0.90
SAMPLING
SYMBOL NOZZLE
I.D.
O 0.00635 m
C3 0.009525 m
0.0127 m
a
O
a
O
o
O
0 5 10 15 20 25
(C-C)
W
FIGURE 15. Interference Effects on an S-Type Pitot Tube, Sampling Rate = 0.85 x Isokinetic,
Inter-tube Spacing = 0.
-------
1.05
c
_J
(f
1.0
0,95
0,90
SAMPLING
SYMBOL NOZZLE
I.D.
O 0.00635 m
D 0.009525 m
O 0.01270 m
O
O
D
O
O
O
O
O
10
15
20
25
FIGURE 16. Interference Effects on an S-Type Pltot Tube, Sampling Rate
Inter-tube Spacing = 0.01499 m.
0.85 x Isoklnetlc,
-------
uo
1.05
1.0
0.95
0.90
FIGURE 17.
SAMPLING
SYMBOL NOZZLE
I.D.
O 0.00635 m
D 0.009525 m
0.01270 m
O
O
O
O
O
O
10
15
(C-C)
W
20
Interference Effects on an S-Type Pitot Tube, Sampling Rate
Inter-tube Spacing = 0.
25
Isokinetic,
-------
SYMBOL
1.05
1.00
LO
0.95
0.90
Sampling Probe Nozzle (1.27 cm I.D.) Aligned with Pitot Tube
Sampling Probe 5.08 cm Behind Pitot Tube (See Figure 3b)
Sampling Probe 5.08 cm Beyond Pitot Tube (See Figure 3a)
a
O
D
O
O
10
15
(C~C)
W
20
25
FIGURE 18. Effects of Misalignment of S-Type Pitot Tube (3-01 - Sampling Probe Combina-
tion, Sampling Rate = Isokinetic, Inter-tube Spacing = 0.
-------
EFFECT OF YAW AND PITCH IN THE ABSENCE OF A SAMPLING NOZZLE
The results of the yaw tests without interference are presented in Figures
19, 20, and 21. These figures show the relationship between pitot coefficient
and the angle of yaw for each particular S-type pitot tube tested. An analysis
of the figures show that, for each pitot tube, tunnel speed has no significant
affect on this relationship. This means that the four figures for each pitot
tube may be averaged together. The result is a single curve that is represen-
tative of all the curves for that tube. The averaged curves for the three dif-
ferent S-type pitot tubes are presented in Figure 22.
For S-type pitot tube 4-10, the pitot coefficient has a maximum value of
0.855 in the 0° yaw position. This is a difference of 0.6 percent from the
accepted value of 0.85. On the other hand, one finds that for S-type pitot
tube 3-04, Cp is at its minimum value (0.846) in the 0° yaw position. The
averaged curve for S-type pitot tube 3-01 is somewhat erratic and notably lack-
ing in symmetry, possibly due to dents and nicks caused by handling. It is
observed that Cp reaches a maximum value at a yaw angle of about 5° while its
value in the 0° yaw position is 0.829.
From these figures it is seen that the necessary range of yaw angles need-
ed to insure that C_ lie within five percent of 0.85 depends on the particular
S-type pitot tube. For pitot tube 4-10 this range extends from -9° to 9°.
Figure 22 shows that S-type pitot tube 3-04 is relatively insensitive to yaw.
From this figure it is found that S-type pitot tube 3-04 can undergo a full
±30° yaw and still easily maintain Cp within five percent of 0.85. More spe-
cifically, Cp never differs from 0.85 by more than 3.88 percent. With S-type
pitot tube 3-01, the acceptable yaw range is from -25° to 25
°
Figures 23, 24, and 25 represent the results of the pitch tests with no
interference. As in the case of yaw without interference, an analysis of the
data reveals that the performance of any specific pitot tube is nearly inde-
pendent of the tunnel velocity. Consequently the curves can be averaged to-
gether, allowing each S-type pitot to be represented by only one performance
curve. For purposes of comparison, these averaged graphs are all presented
together in Figure 26.
The basic shape of the curve representing pitot tube 4-10 is that of a
"check mark" with its vertex (the minimum point) occurring at about 6° pitch.
The value of Cp at 0° pitch is of particular interest. From Figure 26 it is
found that this value, 0.852, is within 0.24 percent of the assumed value of
0.85. It is also seen that within five percent accuracy, the pitot tube may
be pitched from -6° to 20°.
The characteristic curve for S-type pitot tube 3-04 shows that the pitot
coefficient is nearly insensitive to pitch angles from -10° to 20°. Also, the
pitot coefficient does not exceed the five percent tolerance from -20° to 20°.
Unlike the averaged curves for S-type pitot tubes 4-10 and 3-04, the
characteristic curve for S-type pitot tube 3-01 cannot be represented by two
straight line segments. In order to keep the pitot coefficient within a five
percent tolerance, this pitot tube can only be pitched in a range going from
35
-------
C 0.8&-
P
0.70
•»
(
0
©
1
©
0 I
1
1 ' 1
> , V = 30.61 m/sec f
® 1 | 1
©0, ,
1 ? I
1 '
1 °
V • ? \J
C 0.8&
P
0.70
-
1
,
| © C
?
©
© 1
)
1 ' 1
3 V = 20.$6 m/sec '
© | «
o © I |
1 ® 1
I , 0
* * *^
c o.sa
p
0.70
"
1
1
I
©
1
© * 1
|
• \
0 0
1
1
.
I
1
' o 1
©
1
I
!
V = 13. \2 m/sec
© |
© ©
1
|
I
1
1
0.90
cp o.sof-
0.70
|
|
I
1
0
i
1
-30
1 1
1
1 ' © ° 0 I
© ©
© -
© l '
1 1
i 1 i 1 i
i 1 ' I '
©
1
1
I I
1 ~
-20 -10 0 10
1
V = 6.88 m/sec
© 1
G
©
III?
1 I • 1
20 30
Yaw Angle [Deg.]
FIGURE 19. Effect of Yaw on S-Tube 4-10
36
-------
0.890
0.880
0.870
0.860
0.850
0.840
; i
- ' 1
© 1
1 .
" ' • 1
- 1 1
- 1 ] os
1 °
{,
• •*
V =
0 c
)
30.61 m/sec
© C
•>
c
0
)
>
0.880-
0.870-
0.860-
0.850-
0.840
1 |
© 1
" j 1 1
I © 1 1
_ | * I °
1 1 ^
1 t
1
V = .20.56 in/sec
• °
(
©
©
&
3 © °
,
1
-30
-20
20
-10 0 10
Yaw Angle [Deg.]
FIGURE 20. Effect of Yaw on S-Tube 3-04
30
37
-------
0.880 -
0.870-
0.860-
0.850 -
0.840
1
-
<•
-
1
,
0
1
II
1
° . 0 °
1
1
i
1
V - 13.72 m/sec
0
0
1
6 e e a
i 0
1 i
1,
1
1
V. O3U -
0.880-
0.870-
0.860-
0.850-
0.840
0
•
-
0
0
1
1 1
1 1
0
0
1
1 '
|(D
>S^
1 1
1 1
V = .6.88 m/sec
1 o «
1 1
Q 0
^» XX
0 1
0 1 |
I i
1 l
1 1
i t 1 1 I
1 1 I 1 1
)
:
1
1
1
-30 -20 -10 0 10 20 30
Yaw Angle [Deg.]
FIGURE 20. Continued
38
-------
0.85-r-
P
0.80
0.75._
-_ ©
o
0 O
0 0
00
V = 30.61 m/sec
-30
-20
-10 0 10
Yaw Angle [Deg.]
FIGURE 21. Effect of Yaw on S-Tube 3-01
20
0
c
P
0.80-
r\ -7«c -
© 0
0 &
0
0©°
- 0
1 1 1, 1 1 I i
O
0
© °
V = 20.56 m/sec
i 1 1 1 1 1 — 1
30
39
-------
2
0.80-
0.85-
C
P
0.80-
0:75
0 0 (
- 0
0 0
0
0
) ©
0 0 0
V = 13.72 m/sec
0
0
© © T ©
000
0 0
O
0
II 1 1 1 I
1 II 1 1 I
V - 6.88 m/sec °
! 1 1 1 1 1
11(111
-30 -20
-10 0 10
Yaw Angle [Deg.]
FIGURE 21. Continued
20
30
40
-------
LEGEND
O S-tube 4-10
D S-tube 3-04
A S-tube 3-01
0.900
0.880
0.860
0.840
0.820
0.800
0.780
0.760
0.740
0.720
0.700
H 1 1 • h
+
4-
+
*
-30 -20 -10 0 10
Yaw Angle [Deg.]
FIGURE 22. Averaged Effect of Yaw
20
30
41
-------
1.000
C_ 0.900
0.800
1.000
©
° 0 V = 30.61 m/sec
0
0
© Q ©
,1,1)7,
©
1
0.900-h
0.800
1.000
©
° 0 V = 20.56
Q
0
© ©
1 1 I i 1 1 1
1 ' 1 ' 1 ' 1
m/sec
0 ©
• i
1 1
C 0.900 -J-
0.800
1.000
O
O
V = 13.72 m/sec
O
O
O
H 1 1 • 1-
o
0 ©
H 1-
O
c 0.900 4-
0.800
0 V = 6.88 m/sec
0
0
0
0 0 0 ®
1 i 1 i 1 i 1 i
1 n 1 ' 1 ' 1 1
0
1
1
-20
-10 0 10
Pitch Angle [Deg.]
FIGURE 23. Effect of Pitch on S-Tube 4-10
42
20
-------
1.000
C 0.900--
0.800
1.000
V - 30.61 m/sec
O
o
© 0 © © © ©
H 1 I 1 • 1 H-
G
c 0.900
P
0.800
1.000
V = 20.56 m/sec
O
© © © © © 0
* 1 1 1 « 1 1-
©
0.900 --
0.800
1.000
0
0
V = 13.72 m/sec
0 © © © © ©
H 1 1 « 1 H
©
C 0.900
0.800
©
V =
6.88 m/sec
©
1
©
1 1
©
— 1 —
©
1
© ©
1 1
° ©
i 1
-20
20
-10 0 10
Pitch Angle [Deg.]
FIGURE 24. Effect of Pitch on S-Tube 3-04
43
-------
JL. * \f\J\J
0.900 -
Ofinn -
o
V = 30.61 m/sec
©
»
O
0
0
0 0 O O
i .1 . i r . i
1 000
0.900 -
0.800 .
0
V = 20.56 m/sec
0
©
0
0 © © 0 ©
i . i i i i i . i
I • i • i • I • i
1 000
0.900 -
0.800
0
V = 13.72 m/sec
©
0 0
© o © © ©
i i i . i . i . t
I i I i I • t • i
1.000
0.900 _
o.Ron
V = 6.88 m/sec
©
©
0 ® 0 © © 0 ©
i i i i i • I • i
J ,. 1 ' 1 • 1 • 1
-20 -10 0 10 20
FIGURE 25.
Pitch Angle [Deg.]
Effect of Pitch on S-Tube 3-01
44
-------
LEGEND
O S-tube 4-10
O S-tube 3-04
A S-tube 3-01
1.000
0.980
0.960
0.940
0.920
0.900
0.880
0.860
0.840
0.820
, 0.800
-e-
-20 -10 0 10
Pitch Angle [Deg.]
FIGURE 26. Averaged Effect of Pitch
20
-------
-14° to 20°. As mentioned earlier, this pitot tube was damaged slightly which
could account for the difference from the other pitot tubes.
EFFECT OF YAW WITH INTERFERENCE
Figures 27, 28, 29, and 30 show the relationship between pitot coefficient
and angle of yaw for the various speeds and flow rates tested. A comparative
analysis shows that for any given flow rate, the tunnel velocity has very lit-
tle effect on the performance of the S-type pitot tube. (That is, for a speci-
fic flow rate, changes in the tunnel velocity do not significantly alter the
relationship between C_ and the angle of yaw.) A similar analysis also reveals
that the functional relationship between pitot coefficient and yaw angle is
insensitive to changes in the sampling probe flow rate. This means, in effect,
that the 14 individual curves in Figures 27, 28, 29, and 30 are essentually one
and the same. Hence, an averaging process can be applied to these curves to
obtain a single curve that is representative of the entire set. For purposes
of comparison, this single curve is presented along with the averaged curve for
yaw without interference in Figure 31.
From Figure 31 it is observed that interference affects the yaw perfor-
mance of S-type pitot tube 4-10 in an unsymmetrical fashion. Also, it is ob-
served that the value for the pitot coefficient corresponding to 0° yaw is not
within five percent of the desired value of 0.85. More importantly, however,
the figures show that for the entire yawing range of ±30°, Cp varies up to 12
percent of the 0.85 value. In the neighborhood of ±20°, the presence of the
sampling probe serves to diminish the value of the pitot coefficient, whereas
at the higher yaw angles this condition is reversed.
EFFECT OF PITCH WITH INTERFERENCE
As in the case of yaw with interference, interference of the sampling
probe also had to be considered in the pitch study. Figures 32 through 38 por-
tray the relationship between pitot coefficient and angle of pitch for S-type
pitot tube 4-10 at various tunnel speeds and sampling probe flow rates. An
analysis of these figures shows that with respect to pitch the performance of
this pitot tube is nearly independent of tunnel speed and sampling probe flow
rate. Consequently, all the figures may be averaged to produce a single graph
which describes the dependence of pitot coefficient on pitch angle. This
graph is given in Figure 39 together with the averaged graph for pitch without
interference.
Figure 39 shows that for pitch angles between -15° and 2°, the pitot co-
efficient is within five percent of the accepted value of 0.85. It is also
observed that the pitot coefficient reaches a m1n1nuim value of 0.783 at the
10° pitch angle position. Hence, it is possible to pitch the pitot tube be-
tween -15° and 20° and never experience errors larger than 7.9 percent. For
the entire range tested (±20°), the error is within 9.7 percent for S-type
tube 4-10.
46
-------
0.810-
0.790-
C
P
0.770-
0.750-
0.730.
0
O
0 O
- 0 o e 0
O
V = 30.61 m/sec
O
0 °
Ofi^n
0.810-
0.790-
P
0.770-
0.750-
0.730-
0 °
0 ° ° 0 © 0
0
0 V = 20.56 m/sec
0
00
i.i. i.i. 1.1. t
-30 -20 -10 0 10 20 30
Yaw Angle [Deg.]
FIGURE 27. Effect of Yaw with Interference; Sampling Rate:
Isokinetic Flow Rate
0.60 x
47
-------
0.810-
0.790-
C
P
0.770-
0.750-
0.730
0
0 © 0 ° ©
0
0
© V = 13.72 m/sec
O
0 O
0 830
0.810-
0.790-
C
P
0.770-
0.750_
0.730-
0 0 © °
0
0 a
-00© ©
V = 6.88 m/sec
0
1 i 1 1_ 1 1 1 1 1 1 1 1 1 1
-30
-20 -10 0 10
Yaw Angle [Deg.]
FIGURE 27. Continued
20
30
48
-------
0.810-
0.790-
c
P
0.770-
0.750-
0.730.
O
O
0 0
° ° 0
0 ©
O V = 30.61 m/sec
0
0 ®
0 830
0.810-
C
P0.790-
0.770-
0.750-
n 7in_
0
Q
° 0 0 0 0
o *
0
O V = 20.56 m/sec
0 ©
i 1 1 1 1 1 1 1 1 1 1 1 1
-30 -20 -10 0 10
Yaw Angle [Deg.]
20
FIGURE 28. Effect of Yaw with Interference; Sampling Rate:
Isokinetic Flow Rate
30
0.85 x
49
-------
0.830.
0.810-L
0.790--
0.770--
P
0.750 —
0.730.
©
0
0
0 0 ©
0
0
V = 13.72 m/sec
0 O
0.810-
0.790-
0.770-
cp
0.750-
Q.?™
0
0
© 0 Q
' © 0
© ©
© 0
V = 6.88 m/sec
0
i . i . i © i . i . i . i
u • ' -"•' i i | i | i i • i i i • |
-30 -20 -10 0 10 20 30
Yaw Angle [Deg.]
FIGURE 28. Continued
50
-------
0 830
0.810-
0.790-
CP
0.770-
0.750-
0.730
0 °
0 0
0
e o o
° e e
O V = 30.61 m/sec
Ooon
.O JU
0.810-
c
p
0.790-
0.770-
0.750-
0.730
0 °
0 °
" ° 0
0 O
o
0 V = 20.56 m/sec
1 } 1 1 1 1 1 I - 4- 1 1 4 - 1
-30
-20 -10 0 10
Yaw Angle [Deg.J
20
30
FIGURE 29. Effect of Yaw with Interference; Sampling Rate: 1.00 x Iso-
kihetic Flow Rate
51
-------
0.810-
0.790-
C
P0.770-
0.750-
0.730
©
• e o
O
0
O O
©
° 0 0
© Q V = 13.72 m/sec
0.810-
0.790-
C
0.770-
0.750-
0.730
0
©
©
0 ®
© 0
0
0 V = 6.88 m/sec
0
1 i 1 i 1 1 1 1 1 1 —
0
o •
— 1 1 1 —
-30 -20 -10 0 10
Yaw Angle [Deg.]
FIGURE 29. Continued
20
30
52
-------
0.830
0.
0.
810—
0.
0.
0.730-
O
O O
790-- O
770--
750--
© ° O
©
©
0
V = 13.72 m/sec
O O
0.810-
0.790-
0.770-
0.750-
0.730
©
©
1 i
O
© © ©
O
© ©
0
V = 6.88 m/sec
©
0
1 . 1... .1 1 1 1 1 1 t—
©
—4—
-30
-20 -10 0 10
Yaw Angle [Deg.]
! 20
30
FIGURE 30. Effect of Yaw with Interference; Sampling Rate: 1.4 x Iso-
kinetic Flow Rate
53
-------
LEGEND
0.900 -t-
O Yaw
D Yaw with Interference
_ B- TZT
-30
-20
-10
Yaw Angle [Deg.]
FIGURE 31. Averaged Effect of Yaw with Interference on S-Tube 4-10
54
-------
0.930-
0.910-
0.890-
0.870-
C
P
0.850-
0.830-
0.810-
0.790-
0.770-
O
0 V = 13.72 m/sec
O
O
O
0 °
O
0
1 I 1 1 1 1 f 1 1
1 1 1 1 1 1 1 1 1
-20 -10 0 10 20
Pitch Angle [Deg.]
0.950n
0.930-
0.910 -
0.890-
0.870 -
C
P 0.850 -
0.830 -
0.810 -
0.790 -
0.770
•o
O
V = 6.88 m/sec
M
O
0 0
0 0 °
o
1 1 1 1 I 1 1 1 1
1 1 1 ' 1 1 1 ' 1
-20
-10 0 10
Pitch Angle [Deg.]
20
FIGURE 32, Effect of Pitch with Interference; Sampling
Rate: 0.60 x Isokinetic Flow Rate
55
-------
0.950-r
0.930-
0.910-
0.890-
C
p 0.870-
0.850-
0.830-
0.810-
0.790-
0.770
0.930-
0.910-
0.890-
C 0.870-
P
0.850-
0,830-
0.810-
0.790-
0.770.
O
0
V = 30.61 m/sec
O
O
©
©
© °
©
1 1 1 I 1 1 1 I 1
1 1 1 1 1 ' 1 ' | •
-20 -10 0 10 20
Pitch Angle [Deg.]
- ©
©
V = 20.56 m/sec
©
©
©
©
©
©
©
1 I I i I l 1 i 1
I I I ' 1 ' I ' i
-20 -10 0 10 20
Pitch Angle [Deg.]
FIGURE 33. Effect of Pitch with Interference; Sampling Rate:
; 0.60 x Isokinetic Flow Rate
56
-------
0.930-
0.910-
0.890-
C 0.870-
P
0.850-
0.830-
0.810-
0.790-
0.770
0.950 -
0.930 -
0.910 -
0.890 -
C 0.870 -
P
0.850 -
0.830 -
0.810 -
0.790 -
0.770
©
0 V = 13.72 m/sec
o
©
©
©
©
©
©
i t I i I i r i 1
1 > 1 ' 1 > 1 ' 1
-20 -10 0 10 20
Pitch Angle [Deg.]
©
0 V = 6.88 m/sec
©
©
0
©
© ©
©
1 i 1 i 1 1 1 i 1
-20
-10 0 10
Pitch Angle [Deg.]
20
FIGURE 34. Effect of Pitch with Interference; Sampling Rate:
0.85 x Isokinetic Flow Rate
57
-------
0.950 -,
0.930 -
0.910 -
0.890 -
C
P
0.870 -
0.850 -
0.830 -
0.810 -
0.790 -
0.770 _
0
0
V = 30.61 m/sec
©
0
0
©
Q ©
©
1 i ! i 1 i 1 i 1
i ' i ' i • I i i
-20 -10 0 10 20
Pitch Angle [Deg.]
0.930 -
0.910 -
0.890 -
C
p 0.870 -
0.850 -
0.830 -
0.810 -
0.790 -
0.770
_ ©
0
V = 20.56 m/sec
©
©
©
o e
©
©
1 1 1 1 1 ill 1
1 1 I 1 J 1 1 ' 1
-20 -10 0 10 20
FIGURE 35.
Pitch Angle [Deg.]
Effect of Pitch with Interference; Sampling Rate:
0.85 x Isokinetic Flow Rate
58
-------
0.930 -
0.910 -
0.890 -
0.870 -
C
P 0.850 -
0.830 -
0.810 -
0.790 -
0.770
©
0 V = 13.72 m/sec
0
0
©
©
0
©
0
1 I 1 1 1 l 1 I 1
I I 1 1 1 1 1 I i
-20 -10 0 10 20
Pitch Angle [Deg.]
0.950 -,
0.930 -
0.910 -
0.890 -
0.870 -
C
P
0.850 -
0.830 -
0.810 -
0.790 -
0.770 -
©
0 V = 6.88 m/sec
0
©
o 0
O
0 o
1 1 1 I I I 1 1 1
1 1 1 1 1 l 1 ' 1
-20 -10 0 10 20
Pitch Angle [Deg.]
FIGURE 36. Effect of Pitch with Interference; Sampling Rate:
1.00 x Isokinetic Flow Rate
59
-------
0.950__
V • tf ^\J » .
0.930-
0.910-
0.890-
C
P 0.870-
0.850-
0.830-
0.810-
0.790-
0.770
0
©
V = 30.61 m/sec
©
0
©
0
0 ©
0
t i 1 i 1 I I i 1
I ' 1 ' 1 ' I ' 1
-20 -10 0 10 20
Pitch Angle [Deg.]
0.930-
0.910-
0.890-
CP 0.870-
0.850-
0.830-
0.810-
0.790-
0.770
©
0
V = 20.56 m/sec
0
0
©
©
©
©
0
1 i L-, i • 1 - 1- - 1 I - I
-20 -10 0 10
Pitch Angle [Deg.]
20
FIGURE 37.
Effect of Pitch with Interference; Sampling Rate:
1.00 x Isokinetic Flow Rate
60
-------
0.930-
0.910-
0.890-
0.870-
0.850-
0.830-
0.810-
0.790-
0.770-
©
©
V = 13.72 m/sec
©
©
©
©
©
©
Q
1 l i 1 I I t 1
1 ' ' 1 ' 1 ' 1
-20 -10 0 10 20
Pitch Angle [Deg.]
0.950-,
0.930-
0.910-
0.890-
0.870-
0.850-
0.830-
0.810-
0.790-
0.770
0
©
V = 6.88 m/sec
©
©
oo 0 •
0
1 1 1 1 1 1 1 1 1
I ' 1 ' I ' I ' 1
-20 -10 0 10 20
Pitch Angle [Deg.]
FIGURE 38. Effect of Pitch Interference; Sampling Rate:
1.40 x Isokinetic Flow Rate
61
-------
1.000 -
0.980 •
0.960 -
0.940 -
0.920 -
0.900 -
0.880 -
0.860 -
0.840 •
0.820 -
0.800 -
0.780 -
LEGEND
Pitch
0
Pitch with Interference
G °
0
Q
©
B
0
0 0
0 ©
CD
Q
a B
1 1 1 I 1 1 ? 1 I 1
| •• 1 1 1 1 1 1 1 1 '
-20 -10 0 10 20
FIGURE 39.
Pitch Angle [Deg,] r
Averaged Effect of Pitch with Interference
on S-Tube 4-10
62
-------
EFFECT OF SWIRL
The results of the swirl tests are presented in Figures 40, 41, and 42.
In comparing the curves for swirling flow with the curves for unobstructed or
no swirl flow it is evident that the average value of the pitot coefficient
for any given pitot tube is higher in the presence of swirl (See Table 2 be-
low) .
S-Tube
4-10
3-04
3-01
C
P
UNOBSTRUCTED
0.857
0.848
0.830
SWIRL
0.889
0.871
0.868
% Diff.
3.7
2.7
4.6
TABLE 2. Effect of Swirl on Average Pitot Coeffi-
cient for Several S-type Pitot Tubes.
Since swirling flow is intended to simulate actual flow conditions in the
stacks while the unobstructed flow represents ideal flow conditions, the above
table indicates how the behavior of an S-type pitot tube may change in an ac-
tual stack from its behavior under ideal laboratory conditions. Hence, the
pitot tube whose overall percent difference is the smallest would be preferred.
From Table 2, this would be S-tube 3-04.
Considering only the curves that represent swirling flow (Figures 40, 41,
and 42), it is apparent that the degree of variation in pitot coefficients var-
ies from tube to tube. For example, the coefficient of pitot tube 4-10, Cp
does not change by more than 1.3 percent and the coefficient of pitot tube 3-04
does not change by more than 2.8 percent. In contrast, the coefficient for
tube 3-01 varies by as much as 4.7 percent. Similar comparisons in the case
of unobstructed flow show that pitot tube 4-10 allows changes in Cp as large
as 4.6 percent. S-type pitot tube 3-04 gives changes not larger than 1.4 per-
cent and pitot tube 3-01 keeps Cp within a range of 0.85 percent. Thus, the
vehavior of pitot tube 4-10 is quite insensitive to speed in swirling flow,
but very sensitive to speed in unswirled flow. The behavior of tube 3-01 is
opposite to that of 4-10. However, the behavior of pitot 3-04 is insensitive
to speed under- both flow conditions.
Another useful means for judging the effects of swirl on the pitot coef-
ficient is simply to compute the difference between Cp in swirling flow and
the Cp for unswirled flow for each of the speeds considered. This will show
the velocity dependence that is characteristic of a particular pitot tube.
For instance, with pitot tube 4-10 it is found that at the lower speeds (around
6 m/s) the difference is as much as 6.39 percent. While at the higher speeds
(around 12 m/s and higher) it diminishes to around three percent. In the case
of pitot tube 3-04 this difference never exceeds 4.5 percent. Finally for
63
-------
0.910-,-
0.900--
0,870--
0.860
0.850
0.840
0.830
0.820
L E GEN D
D Swirling Flow
O Unobstructed Flow
©-
^ - -o- - -©
---- e ----- ©----©-
--0'
488
Velocity2[m/sec]2
973
FIGURE 40. Effects of Swirl on S-Tube 4-10
64
-------
0.910
0.900--
0.890--
0.880 —
0.870
0.860
0.850
0.840
0.830
0.820
LEGEND
DSwirling Flow
OUnobstructed Flow
O-
488 973
Velocity2[m/sec]2
FIGURE 41. Effects of Swirl on S-Tube 3-04
65
-------
0.910 --
0.900--
0.890--
0.880--
0.870 _.
0.860--
0.850--
0.840--
0.830--
0.820--
LEGEND
D Swirling Flow
O Unobstructed Flow
ft 0-
-I »-
488 973
Velocity2 [m/sec]2
FIGURE 42. Effects of Swirl on S-Tube 3-01
66
-------
pitot tube 3-01, unlike tube 4-10, this marginal difference increases with tun-
nel speed. For speeds of 18.3 m/s (60 ft/sec) and greater, the difference is
on the order of 5.4 percent. Thus, once again, S-type pitot tube 3-04 displays
better characteristics in swirl than the others.
67
-------
REFERENCES
1. Gnyp, A. W., St. Pierre, C. C., Smith, D. S., Mozzon, D., and Steiner, J.
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 Probe. University of Windsor,
Canada, February 1975.
2. Terry, E. W. Calibration of Stauscheibe (S-Type) Pitot Tubes and The Ef-
fects of Geometry and Interference on Their Accuracy. M.S. Thesis, North
Carolina State University, Raleigh, North Carolina, 1977.
3. Willis, B. F. The Effects of Yaw, Pitch, Interference and Swirl on the
Accuracy of Stauscheibe (S-Type) Pitot Tubes. M.S. Thesis, North Carolina
State University, Raleigh, North Carolina, 1977.
68
-------
APPENDIX A
VELOCITY PROFILE OF WIND TUNNEL TEST SECTION
A dynamic pressure survey of the North Carolina State University Subsonic
Wind Tunnel was conducted to determine the variation of the dynamic pressure
(which is related to the velocity) across the width of the test section. This
survey was performed at the same vertical and longitudinal positions as those
used to test the S-type pitot tubes. Tests were performed for three different
speeds at the center of the test section: 6 m/s (20 ft/sec), 17 m/s (55 ft/
sec), and 27.5 m/s (90 ft/sec). The results are shown in Figure Al, and this
figure shows that the velocity changes less than two percent for distances
within 0.15 m (6 in) of the centerline.
69
-------
0.03
.02
0.01
•0.01
_ V.
i.
(
1
) ;
1
6
i i
{
VCL - 2?
f <|
5.*. / « /QA £*• /«ixsji\
m/s v,yu it/sec J
t- i
t •
c
f
;>
c
v-v
I
~CL
CL
K^HBII_^_^
V,
A -0.5 -0.4
L-Near Wall
-0.3 -0.2 -0.1 7 0.1
DISTANCE FROM CENTERLINE, METERS
0.2
U.UJ
0 02
n m
-0.01
0.04
Onq
. UJ
On?
001
0
/
Cj
^
i
]
c
i
\
c
i
i —
i
p
i
VCL ~ 1?
" J
\
V~ £ «
PT
r;
i
s (55 ft/sec)
i 1
t ^
• /« ^Ort p<-/rm«^ —
i/s t^u tt/sec^ —
1 .
1
i
_ .
1
K
,_.. n
r •
[
t
i
r
L
\
C
0.3 0.4
Far Wall
FIGURE Al. Velocity Profile Across Test Section of NCSU Wind Tunnel
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
5. REPORT DATE
A Study on the Accuracy of Type S Pi tot Tube
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
F. C. Winiams, F. R. DeOarnette
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Mechanical and Aerospace Engineering Department
North Carolina State University
Raleigh, North Carolina 27607
10. PROGRAM ELEMENT NO.
1HA327
11. CONTRACT/GRANT NO.
R 803168
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Monitoring and Support Laboratory
Office of Research and Development
U. S. Environmental Protection Agency
Research Triangle Park, N.Q. 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA 600/08
15. SUPPLEMENTARY NOTES
16. ABSTRACT
A study was done to identify and quantify the design parameters that affect
the performance of type S pitot tubes in field use. Fourteen different pitot tubes
were studied. In addition the effect of the sampling probe on the performance of
several of these pi tot tubes was determined as a function of distance, pitch, yaw
and swirl. The results showed that the coefficient of the type S pitot tube under
non-ideal flow conditions was very sensitive to the distance between the static and
wake pressureports of the pitot tube. Increasing the spacing between the two ports
decreased the sensitivity of the coefficient to yaw, pitch and swirl. Inserting
the pitot tube 5 cm. further into the stack than the sampling probe also seemed to
reduce the effects of non-ideal flow on the pitot coefficient.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Pitot tube, Velocity measurement
13B
18. DISTRIBUTION STATEMENT
Release to public
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
20. SECURITY CLASS (This page)
.Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
71
EPA-RTF LIBRARY
------- |