Thirty-one commercially designed platinum resistance thermometers were evaluated for applicability to stagnation temperature measurements between -190 C and +65 C in the Langley Research Center's National Transonic Facility. Evaluation tests included X-ray shadowgraphs, calibrations before and after aging, and time constant measurements. Two wire-wound low thermal mass probes of a conventional design were chosen as most suitable for this cryogenic wind tunnel application

Compton, E. C.

Germain, E. F.

English

NASA Technical Reports Server

NASA Technical Memorandum 85803
NASA-TM-85803 19840015797
EVALUATION TESTS OF PLATINUM RESISTANCE THERMOMETERS
FOR A CRYOGENIC WIND TUNNEL ApPLICATION
EDWARD F. GERMAIN AND E. CONRAD COMPTON
APRIL 1984
liBRARY COpy
I,·; /l'l n () 1984
' '. " (. t..J ,
NI\SI\
National Aeronautics and
Space Administration
Langley Research Center
Hampton, Virginia 23665
LANGLEY RESEARCH CENTER
LIBRARY, NASA
HAMPTON, VIRGINIA
https://ntrs.nasa.gov/search.jsp?R=19840015797 2020-03-20T22:31:41+00:00Z

•EVALUATION TESTS OF PLATINUM RESISTANCE THERMOMETERS
FOR A CRYOGENIC WIND TUNNEL APPLICATION
Edward F. Germain and E. Conrad Compton
SUHMARY
Thirty-one commercially designed platinum resistance thermometers were evaluated
for applicability to stagnation temperature measurements between -190°C and +6SoC in
the Langley Research Center's National Transonic Facility. Evaluation tests included
x-ray shadowgraphs, calibrations before and after ageing, and time constant
measurements. Two wire-wound low thermal mass probes of a conventional design were
chosen as most suitable for this cryogenic wind tunnel application.
INTRODUCTION
The stagnation temperature of the Langley Research Center (LaRC) National
Transonic Facility (NTF) (ref. 1) must be measured to within ±0.30 C in the range
-190oC to +6SoC for accurate Mach number and Reynolds number determination. Fast
response is not an explicit requirement but a thermometer with a time constant of
less than 10 seconds is desirable in order to verify a stable test condition and to
minimize test time. A platinum resistance thermometer (PRT) was selected for this
application.
The market was surveyed for a PRT probe that would meet these accuracy and
response time requirements. Manufacturers were reluctant to guarantee a ±0.30 C
accuracy for repeated temperature measurements, especially for their fast response
probes. It was therefore necessary to set up a program to test candidate sensors.
The purpose was to find the fastest responding PRT sensor consistent with
specifications in accuracy, calibration stability, and mechanical reliability.
Initially, 13 probes and duplicates from 7 manufacturers were purchased for
testing. An additional five probes became available later and were included in the
test program. The probes were typically wire-wound or thin-film platinum resistance
elements cemented to a mandrel 2.5 cm long and surrounded with a metallic sheath.
Four-wire leads were specified whenever possible. Each probe was assigned a simple
3-digit identification number. The first digit shows the source and the last digit
the number of the probe from that source. A non-zero second digit indicates an
additional model in that probe category.
SYMBOLS
R (273.1SK) resistance at the ice point 273.15K, ~~
Ro resistance at the ice point OoC, Q
Rt resistance at temperature, t, Q
1
absolute temperature on the International Practical Temperature Scale
of 1968 (IPTS-68), K (ref. 2)
absolute temperature at the ice point, K
temperature, °c
resistance ratio R(T68 )/R(273.15K)
•
WCCT-68 (T68) reference ~esistance ratio adopted by the Comite' Consultatifde Thermometrie (CCT) of the Comite~ International des Poids et
Measures (ref. 2)
a, 13, 0: constants in the Callendar-Van Dusen equation (t ) OOC, 8 = 0)
TESTS AND RESULTS
X-Ray Analysis
An x-ray shadowgraph was made of each probe to provide internal construction
details such as positions of lead attachments. This formed a baseline for
diagnostics in case of failure and a point of comparison for future procurements of
similar PRTs. Typical x-ray shadowgraphs are shown in figure 1.
Calibration
Key to a PRT evaluation is its calibration, i.e., its individual temperature-
resistance relationship (ref. 2). The Callendar-Van Dusen equation shows the
relationship for platinum to be,
Rt t t t t 3
"R= 1 + a [t - 0(100) (100 - 1) -l3( 100 - 1)(100) ]
o
This has been modified for thermometry in the International Practical
Temperature Scale of 1968 (IPTS-68) (ref. 3) where the ratio of resistance of a
thermometer is expressed as Wc -68. Below OOC (273.15K) in the IPTS-68, probe
calibrations are shown as a dirrerence ratio ~W from a reference ratio such that
W(T68) = WCCT-68 + 6W (T68). The problem is how to display the calibration data suchthat changes are shown in a clear, and meaningful way while minimizing the required
number of calibration points. Carr (ref. 4) chose to use a and time variations in
a. Corruccini (ref. 5) and Sinclair et al (ref. 6) used a ratio of resistance
difference, Cragoe's z functions, defined as,
For simplicity the data in the current work was compared with the Callendar-Van Dusen
equation with a = 0.00392. Equivalent temperature difference is shown rather than
the measured resis tance after an early attempt to standardize with Ro = 100 ~~ proved
too restrictive.
Operational probes at NTF are calibrated in accord with IPTS-68 procedures (ref.
2). The PRT evaluation tests conducted were somewhat less stringent. First, all
probes were calibrated at the ice point (OoC) to measure Ro ' as a measure of
interchangeability. The initial calibration Ro data tabulated in Table I with
2
..
equivalent temperature differences show the degree of interchangeability which may be
expected from a cross section of industrial PRT's.
Next, the resistance was measured in boiling liquid nitrogen (nominally at
-195.8°C) for a quick look at the span of the temperature-resistance curve.
"Initial" LN2 point data are also listed in Table I as temperature equivalentdifferences.
To further investigate the shape of the calibration curve in order to detect,
for example, any strain effects in the PRT, additional data points were taken at room
temperature (~200C), -400C, -800C and -1000C. These are shown in figure 2 for the
two probe types later selected for more extensive tests, (series no. 31 and 50). The
fastest responding probes, series no. 41 and 42 were rejected at this point because
of their extreme sensitivity to strain. The no. 50 probes matched the a = 0.00392
curve within ±0.20 C down to -1000C throughout the period of harsh testing. The no.
31 probes showed some instability during calibration. The scatter of ±0.40C is
apparently characteristic of the probe design rather than deterioration caused by the
tests.
The temperature range -1000 C to -1900C is critical to NTF so a special effort
was made to detail the temperature-resistance curve in this range. The probes under
test are not adaptable to a cryostat for calibration and liquid calibration baths
were not available below -1000C. Therefore, a comparison calibration was attempted
with a standard PRT and the candidate PRT each inserted into a thermal equalizer
block. In one case, the block (aluminum) was chilled to tN2 temperature, shrouded
with an insulator then allowed to drift back to room temperature. In another test
the block was placed in a chamber cooled by expanding nitrogen to a temperature below
-lOOoC. Neither approach showed the necessary precision because of poor sensor-to-
block thermal contact and the need to thermally temper the probe stem.
Pre-Ageing Tests
Prior to ageing the probes, each was tested for self-heating, electrical
insulation, and immersion effects.
To test for self-heating, the probes were held horizontally in still air. A
milliamp level current was then applied to the appropriate leads across the PRT as
the voltage drop was monitored. The current was gradually increased until the
resultipg internal joule heating began to affect the readings. This test setup is
conservative because heat is transferred away from the probe by free convection
only. The actual NTF application will involve forced convection with a typical
stream velocity of 15 m/sec at pressures up to 9 atmospheres. Probes 50-1 and 50-2
(Ro = 1,000 Q) showed no self-heating at 0.1 rnA dc (equivalent to 100 mV with the
sensor at OoC.) Probes 31-1 and 31-2 (Ro = 100 Q) were able to take 0.5 rnA without
self-heating.
Electrical insulation was measured with a 5 volt insulatLon tester at room and
LN2 temperatures. The resistances were usually 10
3 megohm.s or greater. Lower values
indicated defects in insulation which showed up as intermittent shorts under
vibration.
The probes were then immersed into LN2 to check the immersion depth required to
negate heat transfer down the stem and for any thermoelectric voltages which might
disturb the electrical resistance measurement. The immersion was done in increments
3
of 2~5 cm and the resistance of the sensor monitored at each depth until the
resistance value stabilized. This depth was then recorded as the minimum working
depth. The polarity of the current was reversed during this process to check for
thermoelectric effects. As an example of the results, probes 31-1 and 31-2 required
a 10-cm immersion and were without thermoelectric effects. Probes 50-1 and 50-2
required an immersion of only 2.5 cm to eliminate stem conduction but indicated
thermoelectric voltages equivalent to an error of ±O.loC for up to an 18 cm
immersion. The 31 and 50 type probes were of interest at this point because of their
demonstrated rapid response on immersion into LN2 •
Ageing Tests
The probes were thermally cycled 100 times by inserting the sensor tip into a
LN 2 bath long enough for the tip to reach -190
0 C and then returning it to ambient air
until it indicated a temperature above OOC. The setup for the ageing tests is shown
in figure 3. The tests were automated with a push-pull cyclic drive mechanism shown
in the figure. Dwell time in the LN 2 bath and warm-up time out of the bath were pre-
set on the interval timers according to the needs of the probe under test.
Most of the probes physically survived the full 100 cycles and were then
recalibrated. Calibrations before and after ageing for probes 31-1, 31-2, 50-1, and'
50-2 are plotted in figure 2 as a deviation from the Callendar-Van Dusen equation
with a = 0.00392. It appears from these data that individual calibrations will be
required to meet the accuracy goal of ±0.30 C. Calibration stability appears
excellent for probes 50-1 and 50-2. The integrity of the probes selected for the NTF
application was partially determined by the stability of the calibration effected by
this test.
Time Constant Measurement
The response time of each probe was demonstrated during the immersion and ageing
tests. Time constants (time required for the probe to respond to 63% of a
temperature change) were found on immersion into liquid nitrogen. These data have
limited value because nitrogen nucleation alters the hea,t transfer and is further
influenced by the sensor geometry.
More meaningful time constant measurements were made on some of the probes using
a small calibration wind tunnel as a test facility. The tunnel was operated at
ambient temperature, one atmosphere pressure and a velocity of 15 meters/sec. This
facility and the associated test equipment used is shown in figure 4.
Time constants were determined by two methods. In the first method, the entire
probe (sensing element, insulation, and sheath) was heated with a heat gun external
to the tunnel, then quickly inserted into the air stream to cool. The time constant
is then determined from the exponential decay of temperature with time. This method,
however, is not compatible with current cryogenic wind tunnel facilities because the
probe is inaccessible and the environment is hostile. Therefore, a method for an in
situ n~asurement was designed.
The second method used the temperature sensitive resistance element as a heat
source. An alternating current was superimposed on the element above the direct
current used for resistance measurements, to such a level as to cause obvious self-
heating. With the probe affixed to the tunnel wall and the sensor in the stream with
4
•
..
•
a measurable "self-heating" the ac power was switched off. Time constants were
obtained from the exponential return of the sensor temperature to stream
temperature. These data are tabulated in Table II.
Comparison of the data show the time constant to be· lower with the second
method. Further analysis is required to explain the difference. The data served as
a basis for the selection of more responsive probes for additional tests. probes 31-
1 and 50-1 were then tested in the 0.3 M Transonic Cryogenic Tunnel (TCT) using the
second (internal heat) method. The results show a comparable time constant for the
same pressure and velocity at a temperature of -170oC.
Operational Tests
Finally, the two fastest responding probes which matched best the Callendar-Van
Dusen platinum temperature-resistance equation without effects of strain and drift
were used operationally. Probes 31-1, 31-2, 50-1, and 50-2 are in continuous use in
the 0.3 M TCT and are periodically checked to determine any defects or calibration
changes which might occur because of actual tunnel conditions. These data are also
shown in figure 2.
CONCLUDING REMARKS
Probe type 50 most closely meets the requirements and was chosen as the primary
temperature sensor for the NTF application. It was fast in response (with a 10
second time constant), closely matched to the Callendar-Van Dusen equation and had a
calibration stability of ±0.2oC. Probe type 31 with approximately the same time
constant but inferior calibration stability (±0.40 C) was selected as a backup sensor.
The two fastest responding probe types (41 and 42, both of special design) were
rejected for not meeting the accuracy requirement. One had a thermoelectric output
equal to errors of up to 30 C until fully immersed in LN2 • The other was not strainfree and showed the effect of side loads caused by buffeting air flow. The other
probes showed low thermoelectric voltage and strain effects. Probe 20 was reasonably
fast but had a weak attachment point on the leads. Most of the remaining probes were
marginal in terms of their calibration stability with thermal cycling and match to
the Callendar-Van Dusen equation.
Future work will track the history of calibration constants to further evaluate
the sensors. Finally, a new generation probe, an adaptation of a laboratory standard
PRT will be used along with probe 50 and 31 •
5
References
1. McKinney, L. W. and Gloss, B. B., "Status of the National Transonic Facility,"
AlAA 82-0604 paper presented at the AlAA 12th Aerodynamic Testing Conference,
Williamsburg, VA, March 22-24, 1982.
2. The International Practical Temperature Scale of 1968, Metrologia, International
Journal of Scientific Metrology, Vol. 5, No.2, pp. 35-44, Springer-Verlag,
Berlin, April 1969.
3. Riddle, J. L., Furukawa, G. T., and Plumb, H. H., "Platinum Resistance
Thermometry," National Bureau of Standards, Monograph 126, April 1973.
4. Carr, K. R., "An Evaluation of Industrial Platinum Resistance Thermometers,"
Temperature Its Measurement and Control in Science and Industry, Vol. 4, Part 2,
p. 971-982, (Reinhold, 1971).
5. Corruccini, R. J., "Interpolation of Platinum Resistance Thermometers, 100 to
273 .l50 K," Temperature Its Measurements and Control in Science and Industry, Vol.
3, Part 1, p. 329-338, (Reinhold, 1962).
6. Sinclair, D. H., Terbeek, H. G., and Malone, J. H., "Calibration of Platinum
Resis tance Thermometers," Temperature Its Measurement and Control in Science and
Industry, Vol. 4, Part 2, p. 983-988, (Reinhold, 1971).
6
..
•
TABLE I.- INITIAL CALIBRATION DATA
Ice Point LN2 Point
..
Temperature Temperature
Equivalent Equivalent
Probe Resistance Ro Difference DifferenceNumber S2 °c °c
10-1 99.975 -.06 +.52
10-2 99.966 -.09 +.55
20-1 100.032 +.08 +.58
20-2 Note 1
30-1 99.994 -.02 +.59
30-2 100.002 +.01 +.61
31-1 99.892 +.27 +.52
31-2 100.059 +.15 +.55
40-1 99.981 -.05 +.48
40-2 100.026 +.07 +.56
41-1 99.926 -.18 +.25
41-2 100.018 +.05 +.59
42-1 99.468 -1.34 +.37
42-2 100.136 +.34 +.43
50-1 1000.18 +.04 +.38
50-2 1000.30 +.08 +.36
51-1 99.885 -.29 +.45
51-2 99.910 -.26 +.22
51-3 99.863 -.34 +.03
52-1 500.09 +.05 +.39
52-2 499.91 -.05 +.34
52-3 500.10 +.05 +.42
52-4 500.12 +.06 +.38
60-1 99.84 -.40 +.56
60-2 99.82 -.45 +.56
61-1 99.69 -.78 +.67
61-2 99.76 -.60 +.73
61-3 99.73 -.68 +.63
62-1 100.046 +.12
70-1 100.03 +.07 +2.74
70-2 99.92S -.19 +3.20
•
Note 1 - Leads broke during initial calibration.
7
8TABLE 11.- TIME CONSTANT DATA
Probe Number Method I Method II
(External Heat) (Internal Heat)
10-1 43 sec. 31 sec.
20-1 11 sec. 7.5 sec.
30-1 13 sec. 9.3 sec.
31-1 9.3 sec. 4.8 sec.
40-1 56 sec. 15 sec.
41-1 5 sec. 1.7 sec.
42-1 1.8 sec. 1.4 sec.
50-1 10 .2 sec. 7.3 sec.
61-1 24 sec. 11 sec.
•
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Fig.ure l - 'l'yplcol X-Ray Shlldowgrnph
9
o INITIAL CALIBRATION
X REPEAT INITIAL CALIBRATION
o POST AGEING TEST
OPOST OPERATIONAL TEST
-0.2
-0.4
FIGURE 20" PROBE 5Q-1 DEVIATION FROM a=.00392 CURVE.
..
w
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z
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W
LL
LL
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-0.4
-0.2
+0.2
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_ ......_.-,__.,.._-..,__...,....;;;;;-..,,..........-;;;;;.....,....,;;;;:;;................~_..__..=0~X~"IiiiI'....o!--......__
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FIGURE 2b-PROBE 50-2 DEVIATION FROM a= .00392 CURVE
.....
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X POST AGEING TEST
o POST OPERATIONAL TEST
FIGURE 2c- PROBE 31-1 DEVIATION FROM a= .00392 CURVE.
-200 -180 -160 -140 -120 -100 -80
TEMPERATURE- - - - - -~C
~+'4
+.2
-20
-.4
+20
•
"
•o INITIAL CALIBRATION
X POST AGEING TEST
o POST OPERATIONAL TEST
+.6
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TEMPERATURE - -- --c
-200 -180 -160 -140 -120 -100 -80 -20
-.2
I
lLI
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Z
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FIGURE 2d- PROBE 31-2 DEVIATION FROM ex= .00392 CURVE.
14
•
Figure 3 - PRT TelDp"nJtun' Cyclina, and Al:ep'inK Set-up.
•
•
•
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I~=-"";.....- , -Figure ..
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Tunnei_T_e_s_' ___
•
Conditlonin rSignal l System
and Readou
1. Report No.
NASA TM-85803
2. Government Accession No. 3. Recipient's Catalog No.
4. Title and Subtitle
EVALUATION TESTS OF PLATINUM RESISTANCE THERMOMETERS
FOR A CRYOGENIC WIND TUNNEL APPLICATION
7. Author(s)
5. Report Date
April 1984
6. Performing Organization Code
535-31-53
8. Performing Organization Report No. •
Edward F. Germain and E. Conrad Compton
10. Work Unit No.
9. Performing Organization Name and Address
NASA Langley Research Center
Hampton, VA 23665
11. Contract or Grant No.
14. Sponsoring Agency Code
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, DC 20546
1- -113. Type of Report and Period Covered
Technical Memorandum
15. Supplementary Notes
16. Abstract
Thirty-one commercially designed platinum resistance thermometers were evaluated
for applicability to stagnation temperature measurements between -190oC and +6SoC in
the Langley Research Center's National Transonic Facility. Evaluation tests included
x-ray shadowgraphs, calibrations before and after aging, and time constant
measurements. Two wire-wound low thermal mass probes of a conventional design were
chosen as most suitable for this cryogenic wind tunnel application.
17. Key Words (Suggested by Author(s})
Platinum Resistance Thermometer, Sensor
Time Response, Thermometry, Cryogenic
Temperature Measurements, Wind Tunnel
Instrumentation
18. Distribution Statement
Unclassified - Unlimited
Subject Category 35
19. Security Classif. (of this report)
Unclassified
20. Security Classif. (of this page)
Unclassified
21. No. of Pages
16
22. Price
A02
N-305 For sale by the National Technical Information Service. Springfield. Virginia 22161




Evaluation tests of platinum resistance thermometers for a cryogenic wind tunnel application

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