EPA-AA-EOD-80-13
Technical Report
December, 1980
ASSESSMENT OF
TEST CELL
HUMIDITY MEASUREMENT
AND
CONTROL
by
Sherman D. Funk
NOTICE
Technical reports do not necessarily represent final EPA decisions or
positions. Their publication or distribution does not constitute any
endorsement of equipment or instrumentation that may have been evalu-
ated. They are intended to present technical analysis of issues using
data which are currently available. The purpose in the release of such
reports is to facilitate the exchange of technical information and to
inform the public of technical developments which may form the basis for
improvements in emissions measurement.
Engineering Staff
Engineering Operations Division
Mobile Source Air Pollution Control
Environmental Protection Agency •
2565 Plymouth Road
Ann Arbor, Michigan 48105
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ABSTRACT
The accuracy of measurement and degree of control of humidity during
light duty emissions and fuel economy tests have become matters of
increasing concern. Specific humidity values are used in the calculation
of NOx test results. It is also possible that in some vehicles humidity
may have an effect on the fuel economy test.
Comparison tests were made with the dew point method and the presently
used wet bulb psychrometer. It was found that the wet bulb method can
produce significant errors primarily due to factors in the wet bulb tem-
perature measurement. The dew point hygrometer proved to be accurate,
reliable, and easy to calibrate. It is the preferred method to measure
humidity in the vehicle test cells.
A series of humidity measurement location comparison tests were made at
three locations in Room 515. With proper seals maintained around the
overhead doors, the center of the room proved to be a suitable location
to measure humidity.
Tests were made on the room's air handling system. A few improvements
are suggested on the humidity control.
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I. INTRODUCTION
This paper is part of a continuum to earlier investigations on
this subject. It relates to and answers questions that were
raised on the earlier work. It not only supplements some of the
earlier data but also provides some additional information in a
more accurate manner under actual test conditions.
II. BACKGROUND
In late 1979, automotive manufacturers claimed that the EPA lab-
oratory's procedural, equipment, and environmental changes since
1975 had a detrimental effect on fuel economy measurements. One
of these changes involved humidity levels. In 1975, specific
humidity levels in our test cells averaged about 50 grains of
water per pound of air. In 1977 these levels were raised to 75
grains to control the NOx correction factor closer to 1.0.
Although the manufacturer claims were not conclusively proved, the
Administrator ruled that we would lower our humidity levels back
to the 1975 conditions, (50 grains). These claims also indicated
there were areas in our measurement procedures and equipment that
needed further investigation. Therefore, we designed a test plan
that would provide us more information on humidity measurement and
control.
This investigation was completed and a report written on April 17,
1980. The report included the comparisons of readings of humidity
measured with three wet bulb-dry bulb units, one in front of each
dynamometer and one in the center of the room. These tests showed
the following characteristics and observations:
A. A definite trend of lower humidity was seen at Dyno 1 at the
75 grain level in the range of a 19 grain average. At the 50
grain level the average was 9 grains lower.
B. Humidity levels always increased during highway fuel economy
tests.
C. Stability of room temperature and humidity improved at the 50
grain level.
D. Wick contamination and angle of air flow across the wick can
cause a 2-3 degree F error on wet bulb units. An additional
error of 1 degree or more can be realized from thermocouple
error (J thermocouple).
E. Humidity control for the air handler became a source of suspi-
cion when: a) outdoor conditions change, controls would have
to be re-adjusted; b) humidity would increase during high heat
load, such as the HWFET tests.
F. A 1.0°F change in wet bulb measurement can result in approxi-
mately a 2% change in the NOx correction factor.
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G. A number of other laboratories are using the more accurate dew
point meter to measure humidity.
III. TEST PLAN
After the first report was completed, it was determined that we
needed to investigate humidity further in our test cells. Surveys
were made with instrument manufacturer and automotive manufac-
turers to gather information on available instruments. The
General Eastern Model 1200 APS Dew Point Meter was selected
because of the experienced usage available, its claimed reli-
ability, accuracy and delivery schedule. Three instruments were
purchased and set up on the bench. A test plan was designed to
include check out and evaluation of the instruments, tests to
identify and characterize any differences in measurement between
the wet bulb psychrometer and the dew point meters, tests to find
best cell location for humidity measurement, further characteriza-
tion of the room air handling control and the development of an
interactive LCS computer programs that will calculate humidity
from inputs of barometer, dry bulb, wet bulb and/or dew point
temperatures.
Evaluation, calibration procedure, and check out of the General
Eastern dew point meters was done in a series of steps.
Information on calibration, maintenance and reliability was col-
lected from General Motors and General Eastern Corp. Bench
checking was done by using ambient and ice bottle temperatures to
check calibration as described in the instruction manual. An
operational check was devised by blending various levels of
humidity in air in a sample bag and passing it through the instru-
ment. Comparisons were made to other instruments in the lab and
to a certified thermometer accurate to _+ 0.2°F.
Tests were made to determine the difference in measurements
between the wet bulb and the dew point method. Sixteen point to
point comparisons were run. Since the dew point meter showed a
definite trend in the negative direction a set of controlled para-
meters tests was designed and run. This was an experimental test
set-up, devised and installed in a small room with a closely con-
trolled (humidity and temperature) environment. The set-up used
three GE dew point units measuring in series, one Sargent-Welch
wet bulb-dry bulb aspirated psychrometer, a calibrated mercury
thermometer, clean wicks, temperature controlled distilled water,
a heated wire anemometer, a variable speed blower, and a Honeywell
temperature recorder. Comparisons were made with the wet bulb to
the calibrated thermometer with and without the wet sock, with the
same velocity of air and with the higher recommended velocity,
with ambient temperature for wick moisture and with water cooled
to 60°F or just above the wet bulb reading. (Results are shown in
Appendix D and Dl).
Tests were made to find the best measurement location and to
characterize the room control system by installing three General
Eastern Model 1200 APS dew point meters in Room 515, one sensor in
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— -• •TTIW«WT(—• "
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front of the cooling fan for Dyno 1, one in front of Dyno 2, and
one in the center of the room. Also a Sargent-Welch (wet bulb)
unit measured humidity in the center of the room. Eleven certifi-
cation FTP's were measured along with six Highway Fuel Economy
Tests (HWFET).
Additional room control characterization was done by making a
series of various humidity settings in Room 515 on a day when no
vehicle tests were scheduled. This was done to determine response
of change and stability. A probe connected to the dew point sen-
sor was traversed across the vertical inlet duct and across all
room air inlets to determine humidity mix in the air. Also,
heated air from a heat gun was blown on the room's humidity sensor
to determine if any effect on room humidity.
An interactive humidity calculation was developed and implemented
on LCS that will provide specific and relative humidity, partial
vapor pressure and NOx correction factor by inputting, wet bulb,
dry bulb and/or dew point temperatures. This makes information
immediately available for experimental tests.
IV. SUMMARY OF RESULTS
The results of the major objectives of the test plan can be sum-
marized as follows:
A. The initial bench checks and calibration procedures of all
three dew point meters show that they measure temperature at
ice point and ambient to an accuracy of _+ 0.5°F and that they
correlate to each other to less than 0.5°F difference.
B. Four methods were found to be acceptable to check calibration
of the General Eastern unit. They are the ambient temperature
check, the ice point method, the blended sample bag method and
by maintaining an NBS traceable unit in the calibration
department.
C. Differences in the wet bulb psychrometer and the dew point
meter ranged as high as 15 grains but averaged 5.6 grains with
the dew point meter normally lower (See Appendix C and D).
The dew point meter method is considerably more accurate.
Error in measurement of wet bulb-dry bulb can be attributed to
thermocouple error, contamination of wick, angle and velocity
of air flow across wick, and temperature of water.
D. As long as there are no outside leaks to the room, humidity
levels at the vehicle cooling fans and at the center of room
locations prove to be within 1°F dew point (See Appendix B).
Specific humidity levels continued to rise during HWFET's.
E. The facility humidity control system appeared to have a dead-
band that is extremely wide. This results in no response to
routine external humidity changes. It was found that for any
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given humidity setting, the system sensor controls on a rela-
tive humidity rather a specific humidity. This causes
specific humidity to rise during the high heat load condition
of the Highway Fuel Economy Tests (HWFET).
F. There is no evidence of humidity stratification in the inlet
ducts.
V. DISCUSSION
The EOD study on humidity in the Light Duty test cells en which a
report was written in April 1980 led to the determination to pur-
chase three General Eastern Model 1200 APS dew point meters for
in-house evaluation.
These units use the basic principle of dew condensation at a tem-
perature relative to the specific amount of humidity per pound of
air. They use a platinum RTD thermometer accurate to ^0.2°C (See
Appendix A) to measure surface temperature of a mirror on which
the dew collects. The mirror is thermoelectrically cooled.
Cooling is controlled by a photodetector which senses amount of
light reflectance from the mirror. A light emitting diode pro-
vides the source of light that is directed on the mirror. The
units also have a self cleaning mirror feature which works auto-
matically or manually.
These units were extensively bench checked upon receipt by con-
necting the sensors in series, removing the cooler fuses, and
comparing ambient temperature readings to a certified mercury
thermometer. Units were also checked at the ice point by flowing
air through a coil in a bottle of ice and through the sensors.
Response to humidity changes and stability was also checked.
Units were run at three operational levels of 90, 75, and 50
grains of water as measured from a blended sample bag. This
method was developed in-house by calculating volume and weight of
different blends, injecting measured amounts of water into a
blended time measured bag of air. After a blending time period,
sample was drawn through the DP meter sensors and dew point tem-
perature measured. This value was Lhen used to calculate specific
humidity (grains of H2) per pound of air). Even though this
procedure needs further refinements, all tests were within accept-
able tolerances of advertised specifications.
After initial check out of the units comparison tests were made
with the presently used wet bulb psychrometers. Differences in
measurement were seen as much as 15 grains with the dew point
measuring the lower. Under closely controlled point to point
readings, the psychrometer read 4-5 grains higher than the dew-
point. After extensive comparisons confirmed this fact an
investigation was initiated to determine the cause of these dif-
ferences. This was done by a controlled parameters test as
described in the test plan and results tabulated in Appendix D.
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The difference is mostly attributed to wick water temperature
(should be just slightly above wet bulb temperautre ref. NBS Circ.
#512) and air velocity across the wick (should be a minimum of 900
feet per minute, ref. NBS Cinr. #512 and Humidity Measurements,
Instrument Technology). Wick contamination and angle of air flow
contribute in lesser degrees.
Once differences between our standard measurement devices
(Sargent-Welch) and the new dew point meters had been resolved, a
series of eleven FTP's and six HWFET comparison tests on Dynos 1
and 2 were run to determine a suitable location to measure
humidity and to observe room humidity control capabilities. The
tests were performed during actual certification tests. Humidity
levels in the cell were reasonably stable during the FTP compari-
sons. However, after a portion of the tests had been run, the
humidity traces for Dyno 1 became erratic and showed lower
readings than Dyno 2 and the center of the room. Finally it
became unacceptable and the problem was investigated. It was
finally traced to a damaged overhead door and a torn rubber seal.
The seal was repaired.
However, air current leaks then became apparent at the Dyno 2 door
but of a less magnitude. Even though both doors appeared to have
been damaged and are not 100% sealed, the leakages are now only
causing transients of less than 1°F dew point at the front of the
fan and was not considered to be a problem. (The test results
during the door leaking problems were not tabulated nor included
in this report since that was an abnormal condition). During this
investigation, a profile of humidity mixture of inlet air was
sampled traversing a probe across the inlet ciucts to check for
possible stratification. None was found. The leaking door prob-
lem could possibly explain the differences that were seen on Dyno
1 as documented in the April 1980 report. Results of all tests
(FTP + HWFET) showed close correlation (less than 1°F dew point
difference) from each dyno to the center of the room (See Appendix
B). Due to the fact that on the HWFET comparisons, humidity
traces from the three locations tracked on top of each other and
that on some tests (high heat load) overall specific humidity
increased as much as 10-15 grains. Hence, a table of estimated
average values for these tests would be meaningless and v.as not
included.
To further investigate this occurrence, a stream of heated air
from a heat gun was directed at the room humidity sensor. Within
a few seconds steam v-ss being injected into the loom st a very
high rate indicating humidity control relative to room tempera-
ture. Further investigation into building engineerng drawings
confirmed this to be true. Room humidity controls sense relative
humidity explaining why specific humidity rises during high heat
load tests. Further review of past work done on room air handling
systems revealed a memo written in October li<75 by Doug Berg based
on a study by Bene Engineering attesting to this fact. It also
agrees with this writer's tecommendation that we should study our
system to determine if we could modify our humidity sensing and
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control system to control on specific humidity levels rather rela-
tive. NOx factor is calculated from a specific humidity value.
Furthermore if a vehicle's carburetion system is calibrated such
that it would be sensitive to humidity levels, then it is specific
humidity values with which we are concerned, not relative (See
Conclusions/Recommendations)
VI. CONCLUSIONS/RECOMMENDATIONS
A. The General Eastern Model 1200 APS dew point meter is a more
accurate and reliable instrument for measuring humidity than
our presently used wet bulb psychrometers. It is recommended
that the three units be installed, one in each Light Duty test
cell mounted on the equipment rack with outputs connected an
analog (0-10 VDC) recorder and the LCS Computer. It would be
necessary to modify the computer program to accept dew point
input. Dew point temperature could then be monitored and
integrated over the period of the test and then calculated
directly to specific humidity and NOx factor. The L&N thermo-
couple should also be input to the recorder and the LCS for
measurement of ambient temperature.
B. Differences in measurement of the dew point meter and the wet
bulb units are the result of errors in the wet bulb units.
These differences are caused by lower air velocity than recom-
mended across the wick, using ambient temperature water for
wick moisture (should be just above wet bulb temp), and wick
contamination. The high degree of maintenance attention
required causes a potential error situation. It is recom-
mended the wet bulb units be phased out of service
completely. If the method of measuring and setting humidity
is changed to dew point it could be expected the actual test
cell humidity v/ould average 4-5 grains higher than is cur-
rently seen from the wet bulb control and measurement.
C. Air leakage from the soak area through the the overhead door
seal can cause humidity stratification at the front of the
vehicle. It is recommended that an additional sensor and
sampling kit be purchased for a routine measurement with the
sensor mounted on the cooling fan as part of the Repca diag-
nostics. This will minimize the possibility of any long term
problem existing undetected.
D. Relative humidity is the value reported on emissions test
results. Test cell humidity is controlled as relative to
ambient temperature. Given specific humidity (grains of water
per pound of air) is an engineering unit value used to calcu-
late NOx factor. It should be the value reported. Also,
since some test vehicles carburetion may be more sensitive to
specific humidity, it is recommended a study be initiated to
determine the feasibility of changing the air handling systems
in the test cell to control on specific humidity levels. This
would prevent the system from adding moisture where the inter-
nal heat load is high, such as the fuel economy tests. This
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recommendation agrees with the Bene Engineering study report
and Doug Berg memo of October 1975. Also it should be inves-
tigated to determine if after changing the control to specific
humidity that the deadband width settings could be adjusted to
be more realistic and applicable.
E. The use of the General Eastern dew point meter method for
measuring humidity can provide reliable and accurate data if
certain procedures of calibration checks and preventive main-
tenance are followed. It is recommended the following
procedures be used.
1. Mirrors in the sensors should be cleaned as prescribed in
the instruction manual once a month.
2. The thermometer should be checked every 90 days by
removing the cooler fuse to allow ambient air temperature
to be compared to a certified mercury thermometer accurate
to +0.2°F.
3. Condensation and operation checks should be made at dif-
ferent levels (90, 75 and 50 grains, . . .) by using the
blended bag method developed in-house. This procedure
needs to be documented.
4. Two more units should be purchased for accuracy trace-
ability and maintenance back up. One unit should be sent
to NBS for certification and maintained in the Electronics
Shop as a standard. The other unit would be used as a
spare or for additional studies.
VII. APPENDICES
A. General Eastern Model 1200 APS Dew Point Hygrometer Information
B. Humidity Measurement Location Comparisons
C. Wet Bulb-Dry Bulb vs. Dew Point Meter Humidity Measurement
Comparisons
D. Controlled Parameters Test WB-DB vs. Dew Point Method
Dl Wick H20 Temp and Air Velocity Effect on WB vs DP Method
E. Room 515 Humidity Sensor and Control
F. Dew Point/Wet Bulb Temp vs. Specific Humidity
VIII. REFERENCES
1. Reports and Critique on In-house Air Handling Systems. April
- Ocotber 1975, Bene Engineering and Doug Berg.
2. Humidity Measurements, Instrumentation Technology, P.R.
Weiderhold.
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3. EPA Memo, Configuration Standardization of the Wet and Dry
Bulb Psychrometers. Carl Ryan, April 18, 1980.
4. EPA Memo, Change in Procedures 1088,713 and 711, L. Hormes,
March 4. 1980.
5. EPA Memo, Monitoring Humidity control, C.D. Paulsell, March 6,
1980.
6. EPA Memo, Change of Test Cell Humidity, C.D. Paulsell, March
4, 1980.
7. Humidity Calculation and Report Program Documentation, Dave
Holland & Mary Saelzler, May 13, 1980.
8. NBS Circular #512, Methods of Measuring Humidity and Testing
Hygrometers, Arnold Wexler and W.G. Brombacher, Sept. 28, 1951.
9. EPA Memo, Comparison of Humidity Correction Factor Procedures,
E.P. Zellin, May 31, 1973.
10. GM Memo, Procedures for Calculating Test Area Humidity, David
Horchler, May 31, 1972.
11. Project Subtask Summary, Dew Point Hygrometer Evaluation, S.D.
Funk, Sept. 1980.
12. NBS Paper, Vapor Pressure equation for Water in the Range of
0-100°C, Arnold Wexler and Louis Greenspan, Feb. 19, 1971.
13. EPA Paper, A Procedure for Calculating Humidity, Eric Zellin,
July 1975.
14. Discussion of Paragraph 85.1320-1 of Proposed Rule Making,
Eric Zellin, June 1975.
15. EPA Paper, Test Cell Humidity Investigation Report, Sherman D.
Funk, April 17, 1980.
16. Technical Literature on Humidity Measurements, General Eastern
Corp., John Harding and P.R. Widerhold, June 1978 and June
1979.
17. EPA Memo, Progress on humidity Investigation, April 4, 1980,
S.D. Funk.
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SECTS C
PRINCIPLE OF OPERATION
APPENDIX A
Page 1 of 3
2.0 PRINCIPLE OF OPERATION
The Series 1200 Dew Point Hygrometers ore thermo-
elcctrically-cooled, optically-detected, automatically-
controlled, condensation (or "dew point") hygrometers.
The principle of operation may be seen from the
accompanying figure.
O'llCAl OAtAMCE
The condensate detection mirror is illuminated with a
high intensity, solid state, light-emitting-diode (LED). A
photodetector is configured so as to monitor the specular
(direct) component of the light from the mirror. A separate
LED and photodetector combination are used to
compensate for any thermally-induced changes in the
optical components. T.he phoiodetectors are arranged
•in an electrical. pridge'circuit-such.that:the-speculc'r:
- detector is fully illuminated when the mirror is clear of
dow, and sees reduced light as dew forms on the mirror,
due to scattering losses.
An optical offset is designed into the bridge, such that a
large bridge output current is developed v/henever the
mirror is in the "dry" condition. The bridge output is
amplified and used to control the direct current to the
thermoelectric cooler, causing the mirror to cool toward
the dew point. As dew begins to form on the mirror, the
optical bridge isdriven toward its balance point, causing a
reduction in the specular light, causing the bridge
output to decrease, and the cooling current to reduce.
A rate feedback loop within the amplifier is employed to
insure critic?.! response, and the system quickly stabilises
at a condition wherein a thin dew or frost layer is
maintained on tho mirror surface, i.e., the dew, or frost,
point. A precision thermometer element is embedded
within the mirror which monitors this dew poin! temper-
ature directly.
Ttie condensation hygrometer is a fundamental method
for measuring water vapor, affording a degree of
accuracy not available in other methods. Additionally.
the rcpeatabilty of the instrument can be checked at
any lime by opening the control loop and allowing the
mirror to heat and the dew to evaporate, and then
reclosing the control loop. Long term accuracies in the
order of ±0.1'C arc common with the condensation
hygrometer, making it suitable as a laboratory reference
instrument.
AUTO-REFLECTANCE/PACEFT FEATURE*
The 1200 "S" Series instruments are equipped with
General Eastern's Programmable Automatic Contami-
nant Error Reduction circuitry (PACER") which permits
the instrument to check its own performance and make
any necessary optical adjustments on a periodic basis
(once per 2. G, 12. or 24 hours, or on command). This
circuit automatically places the instrument into tho
"optical adjust" mode, then cools trie mirror for 30
seconds causing soluble contaminants to dissolve into
the excess water. This circuit then automatically causes
the mirror to heat to a dry condition and, as the dew
layer evaporates, any remaining soluble contaminants
collect at isolated sites, leaving a mirror surface with
substantial surface area available for undisturbed
growth of condensate. When the mirror is heated to the
dry condition the PACER" circuit then automatically
adjusts the offset current to its correct value, regardless
of mirror contaminants. While this function is being
performed, the output sig nal is held at a constant value,
equal to the last dew/frost point value, allowing full
compatibility with process control loop equipment. The
autoreflectance circuit then returns the instrument to
normal operation. Logic signals are provided at the rear
. of the instrument to identify "normal" and/lautoreflec-
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2-1
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APPENDIX A
Page 2 of 3
2.1 SUMMARY OF SPECIFICATIONS
2.1.1 RANGE
The basic dew point range of the sensor Is dictated by
the heal pumping capacity of the thermoelectrically-
cooled mirror, and is typically from M-80F to -40F. That
is, with the sensor installed in a typical environment of
+POF, daw points can be measured from the saturation
temperature- (+80F), down to -40F. Below +32F, of
course, the sensor actually measures the frost point
temperature, since dew cannot exist below -f 32F on a
continuous basis. If the ambient temperature of the
sensor is low;:r than 80F, then the lowest measurable
dew point ia lowered. Conversely, if the sensor is
installed at a higher-than-ambient temperature, the
high end of the range Is increased, and the lowest
measurable dew point is raised. (See Range Graph)
»200
+ 100
Ul
$
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-100
-100 0 +100
DE.W POINT TEMPERATURE.
+200
Each sensor is equipped with an optional water cooling
jacket which, when supplied with water at approximately
0.5 gpm, will permit a 0.67CF improvement in the lowest
frost point attainable, for each 1 °F reduction in temper-.
ature of the senspr'prpvided by the coolant, For example,",
if 55'"F cooling water' Vs available, the sensor ''can'
measure frost points from +55F down to -56F.
2.1.2 HIGH DEW POINTS
The sensor may be operated at dew points up to
+212:F. as long as the sensor body is kept above the
highest dew point anticipated to prevent condensation
on the walls of the sensor. Usually, the gas (or oven,
kiln, dryer, etc.) has sufficient heat capacity to raise the
temperature of the sensor body, if properly located or if
equipped with a simple insulative jacket.
2.1.3 ACCURACY
The accuracy of the Series 1200 dew point sensor is
determined almost entirely by the accuracy of the
condensation mirror temperature sensor.
The mirror is provided with a precision platinum resis-
tance, thermometer with NBS-traceable coefficients.
The accuracy is typically, ±0.40'F, as shown in the
graph.
+ 1.0
u.
o
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i^foyte?-: ;.' >sx.v..w..i..'f ...^.-•...-., ........
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, ' 1
00 0 +100 +200
DEW POINT. OF
2.1.4 DEW POINT SENSITIVITY
The fundamental sensitivity of all Series 1200. dew
point sensors is ±0.05°F.
2.1.5 DEW POINT RESPONSE
The response of all Series 1200 dew point sensors is a
function of the cooling and heating rate of the mirror
(typically, 3"F/sec), the sample flow rate, and the abso-
lute value of the actual dew or frost point being
measured. At dew points above 32F, the response is
almost entirely a function of the mirror cooling rate and
gas flow rate. Below 32F, the response is slowed by the
increasingly reduced availability of water vapor in the
sample as the dew point is lowered, and by the out-
gassing characteristics of sampling lines. At -40F
frost point, the response is several seconds. At
—60F, several minutes are required for a 63%
response to a change, and the lag is-a function of the
outgassing characteristics of the sampling lines, sample
flow, or availability of water molecules and crystal
growth rate.
2.1.6 AMBIENT TEMPERATURE LIMITS
The Series 1200 sensor will operate over the ambient
temperature extrern'es-"of*l-212F to -75F.The electronic
control module is suitable for operation between +120F
to -H32F.
2.1.7 CIRCUIT DESCRIPTION
The 1200 "S-Series" instruments all have essentially
identical circuitry. The only fundamental difference
between systems is the omission of the digital readout
meter in those systems packaged in NEMA enclosures.
The System consists of several separate subsystems:
(see Functional Block Diagram)
1. The mirror temperature dew point control loop is
located on the 1201 CAS printed circuit card. This
circuit amplifies the signals from the photot ransis-'
tor detectors in the sensor to a level suitable to
drive the thermoelectric cooler power amplifier
transistor. The final power transistor is mounted
directly to the instrument chassis for heat sink
purposes.
2-2
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APPENDIX A
Page 3 of 3
Bopitnlo.d wlih porml»!on f'om AWS |R«f. A)
1000
-70
Frost poinl.'C Dew poinl.'C
-60' -50 -40 -30 -HO_-IQ__0 10 20 30 40 50
•g lOOr-
Figure 3. Uncertainties In typical
humidity sansors. under laboratory
conditions, are illustrated here.
The gravimetric train, an extremely
accurate sensor which weighs ac-
tual moisture content In known air
volumes. Is used by NBS to cali-
brate other sensors. In industrial
installations. - few devices attain
the accuracies Indicated.
Typicot dec. ..
RUCP2D-C
(Oumno'C, Pope)
Kolled. (reeled
hoif@>25°C
LSO». soil ll.iCI)
de« poinl(u>25*C
1
•":',. 0.001-
:.' O.I .•:...'. . .. .1,
Mixing'ratio, g/kg
10
100
+200
+ 150 h-
-too
-100 -50 0 +50 +100
Ambient lemperoIufe/F
+ 150
— 2000 PPM
-200PPM
20 PP.M
]?. PPM
+ 200
Figure 2. Useful operating ranges
of various humidity sensors. Psy-
chromotors. percent RH. nnd sat-
urated salt dew point sensors op-
orate in the mid-ranrjos of temper-
ature and humidity. Condensation
hygrometers aro useful ovar a
largo temperature span and to
humidities as low as one purcont.
Electrolytic hygrometers operate
at low clow points over the entire
range of humidity.
-------
APPENDIX B
Humidity Measurement
Location Comparisons & WB
Date
10-9-80
10-10-80
10-15-80
10-16-80
10-17-80
10-30-80
10-31-80
11-20-80
11-20-80
11-21-80
11-21-80
FTP
Test Pair
Numbers
80-6369
6370
CENTER
6863
6864
CENTER
6388
6375
CENTER
6371
6391
CENTER
6353
6429
CENTER
6586
6621
CENTER
6652
6576
CENTER
6820
6822
CENTER
6816
6823
CENTER
6869
6865
CENTER
6825
6833
CENTER
Barometer
28.86
28.86
29.20
29.05
28.70
29.25
29.98
29.23
29.13
29.14
29.18
Dew Point
Temp.
44°
44°
44°
44°
44°
44°
51°
51°
51°
47°
47°
47°
48°
48°
48°
44°
44°
44°
48°
48°
48°
46°
46°
46°
47°
47°
47°
43°
43°
43°
45°
45°
45°
Dew Point
Spec. Hum
44. OCR
44.0
56.8
49.1
51.6
43.4
49.4
46.9
48.9
41.9
45.2
Wet Bulb
Temp.
59. 5F
59.8
59.0
59.2
59.3
57.5
59.5
59.5
60.3
58.5
58.5
Dry Bulb
Temp.
75. OF
75.5
72.0
76.0
75.5
75.0
76.0
76.5
77.3
77.0
76.5
WB Spec.
Hum
54.4GR
54.1
54.6
49.2
51.4
43.0
48.1
49.3
52.0
44.3
45.0
Hum
Diff
-10.4
-10.1
2.2
-.1
.2
.1
1.3
-2.4
-3.1
-2.4
.2
AVG. -2.2
NOTE 1: WB-DB readings are results of operator eye averaging resulting in
a less accurate comparison of WB-DB vs dew point meter than by
point to point as referred to in the text report. Dew Point was
most always lower.
2: 1st Number of Pair is DOOl 2nd Number of Pair is D002. CENTER
is center of room.
-------
APPENDIX C
Wet Bulb-Dry Bulb vs. Dew Point Meter
Humidity Measurement Comparisons
TEST #
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
BAROMETER
IN HG
28.81
29.00
29.00
29.00
29.06
28.21
29.00
29.00
28.81
29.18
28.88
29.15
29.10
28.94
29.05
29.20
WET BULB
TEMP. °F
60.5
58.0
58.0
58.0
58.5
61.2
58.2
58.0
60.0
57.4
59.0
61.0
58.9
58.3
63.0
60.5
DRY BULB
TEMP. °F
76.0
73.0
72.5
72.0
76.0
76.0
72.2
74.5
75.0
74.0
75.0
76.0
76.0
74.8
76.0
76.7
WET BULB
SPEC. HUM.
55.7
49.1
49.9
50.8
46.1
60.8
51.3
46.6
55.1
44.4
50.5
57.1
47.8
47.6
66.8
53.5
DEW POINT
TEMP. °F
45.8
45.3
44.9
45.6
44.5
45.8
45.0
43.9
45.3
42.6
45.3
45.3
43.6
44.3
48.0
46.9
DEW POINT
SPEC. HUM.
47.3
46.1
45.4
46.6
44.6
48.3
45.5
43.6
46.3
41.2
46.3
45.8
43.0
44.4
51.0
48.6
DIFF. DP HUM
-WB HUM.
- 8.5 GR
- 3.0
- 4.5
- 4.2
- 1.5
-12.5
- 5.8
- 3.0
- 8.8
- 3.2
- 4.2
-11.3
- 4.8
- 3.2
-15.8
- 4.9
DIFF NOx
FACTOR
-.0323
-.0113
-.0169
-.0155
-.0058
-.0490
-.0215
-.0108
-.0332
-.0115
-.0157
-.0439
-.0175
-.0115
-.0643
-.0132
Ave,
-6.1 Ave.
-.0233
NOTE: iThis table lists a portion of over 50 comparisons.
Maximum and minimum differences are included. Most
differences were 3-4 grains, dew point being lower.
^Comparisons were point to point with no vehicles being
tested.
-------
APPENDIX D
Controlled Parameters Test
WB-DB vs Dew Point Meter
Test Wet Bulb WB Wick Air Vel. Bar Dry Bulb Wet Bulb Wet Bulb Dew point Dew point Diff Dp Hum.
# Thermometer Water Temp. Across Wick In. HG Temp." F Temp." F Spec. Hum. Temp.0 F Spec.Hum. -WB Hum.
01
02
03
04
05
06
07
08
09
10
J
THERMOCOUPLE 75.0
CERT
MERC +0.2F 75.0
CERT
MERC +0.2F 75.0
CERT
MERC +Q.2F 75.0
CERT
MERC +0.2F 65.0
CERT
MERC +0.2F 60.0
J
THERMOCOUPLE 60.0
CERT
MERC + 0.2F 75.0
CERT
MERC +0.2F 60.0
CERT
MERC +0.2F 65.0
650. FPM 29.00 76.0 60.5 55.2 GR 46.9 49.2 GR -8.3 GR
650. FPM 29.00 72.2 58.4 52.2 44.9 45.4 -6.8
850. FPM 29.00 75.0 58.4 47.5 44.9 45.4 -2.1
950. FPM 29.00 75.0 58.2 46.7 45.0 45.6 -1.1
950. FPM 29.00 75.3 58.0 45.3 44.7 45.0 -0.3
950. FPM 29.00 75.6 58.0 44.8 44.6 44.8 0.0
950. FPM 29.00 74.8 58.0 46.1 44.3 44.3 -1.8
750. FPM 29.00 75.5 58.2 45.8 43.9 43.6 -2.2
750. FPM 29.00 74.5 58.0 46.6 43.9 43.6 -3.0
700. FPM 29.00 73.00 58.0 49.1 44.3 44.3 -4.8
-------
APPENDIX Dl
WICK H20 TEMP 6. AIR VELOCITY EFFECT
ON WB VS. DP METHOD
AIR VELOCITY
feet per tnin
1-15-81
WB ^- 58°F
60
WET BULB
H20 TEMP°F 65
75
650
700
750
850
Decreasing
950
-3.0(M)
-4.8(M)
-6.8(M)
-8.3(J)
0
-1
-0
-2.2(M) -2. 1(M) 1
.000
.8(J)
.3(M)
.KM)
-------
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