Thermistor pressure gauges are characterized by large pressure range, good accuracy and stability, fast measurement, insensitivity to over-pressure, negligible out-gassing, ease in cleaning, and physical and electrical simplicity and ruggedness. A number of excellent papers have been published describing these gauges. However, a detailed account of design procedure and characteristics for a specific gauge would eliminate much of the trial and error encountered in designing a gauge having prescribed range, sensitivity, and stability

Ainsworth, J. E.

Flanick, A. P.

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*"P9 7 2 8 9 
Reprinted from THE REVIEW OF SCIENTIFIC INSTRUMENTS, Vol. 32, No. 3, 356-358, March, 1961
Printed in U. S. A.
CASE FILE 
GO Py 
Thermistor Pressure Gauge Design 
A. P. FLANICK AND J
.
 E. AINS WORTH
Goddard Spare Flight Center, Greenbelt, Maryland 
(Received March 3, 1960; and in final form, November 17, 1960) 
T
HERMISTOR pressure gauges' are characterized by 
large pressure range, good accuracy and stability, 
fast measurement, insensitivity to over-pressure, negligible 
out-gassing, ease in cleaning, and physical and electrical 
simplicity and ruggedness. A number of excellent papers 
have been published 1- 6 describing these gauges. However, 
a detailed account of design procedure and characteristics 
for a specific gauge would eliminate much of the trial and 
error encountered in designing a gauge having prescribed 
range, sensitivity, and stability. 
Maximum thermistor sensitivity is obtained when heat 
loss by gas conduction is large compared to loss by radia-
tion and through support wires. Fast response requires low 
heat capacity. Thus, the thermistor should have a high 
dissipation constant' and small, low thermal conductance 
support wires. Thermistor resistance-value range is selected 
for compatibility with available power supplies and voltage 
measuring instruments. Thermistors having high resistance 
give greater sensitivity, but useful sensitivity may remain 
the same because of increased sensitivity to ambient tem-
perature changes. Properly selected thermistors are cur-
L	
I 
33.00'C i:T 
E2,12
r 0.5 
	
E 1 ,1 1	 g 
0.1	 0.5	 1.0	 5	 10 I (MILLIAMPERES) 
FIG. 1. Characteristics of a thermistor as a pressure sensor and the 
circuit used to obtain these characteristics. The dashed curve is a 
typical operation curve.
(.124	 2h	 4000 109V 
'C SUPPLYC ULATOREA€V ]fd)B2 JA ±.I VO TS
T25ovTisov I	 I 
r--------
	
- -300 20OO 
L:6R0u: J DIG I TAL VOLT METER 001% ± I DIGIT 	 PRINTER O.IMV LEAST DIGIT 
FIG. 2. Thermistor electronic circuitry. 
rently available mounted in a Pyrex tube and surrounded 
by a water jacket.' The steps in system design' were as 
follows: (a) The thermistor was aged.' (b) Thermistor E, 
I, F, and R characteristics were determined as shown in 
Fig. 1. (c) Maximum operating temperature of the therm-
istor was selected to be 200°C and thermistor resistance 
R(200) was computed using the thermistor equation 
R(T)=R, exp P G — —To) 
where R, is the measured thermistor resistance at reference 
temperature To= 298'K, and j3=3900°K is the thermistor-
material constant. By using the computed value of R(200), 
the values of E,, I were determined from Fig. 1. (d) 
Arbitrary point E2, 12 was chosen on the 760 mm Hg 
characteristic line. (e) Specific values of Ej and R1 were 
computed using E,, Ii and E2, 12. (f) By using the com-
puted values of E. and Ri in the circuit of Fig. 1, a rough 
calibration of E vs F was obtained and plotted. By using 
the same data, the thermistor operation curve may be 
plotted on Fig. 1. The success with which point E2, 12 was 
chosen was then determined. Should the thermistor opera-
tion curve which was obtained approach tangency to any 
thermistor characteristic curve P= constant, the value of 
dE/dF suddenly becomes abnormally large. This condition 
is noticed as a step increase in E on the rough plot of E 
102 
1101
.e \,J
NOTES 
vs P and is remedied by increasing 12 until the step increase 
in E has been eliminated. 
The initial choice of 12 as less than I was made in order 
to obtain a thermistor 'operation curve which had an in-
creased sensitivity in the region from 2 to 10 mm Hg, e.g. 
compare the dashed thermistor operation curve with the 
operation curve I'-I. 
If a low value for maximum thermistor temperature is 
selected, greater sensitivity is obtained at low pressure but 
at the expense of reduced sensitivity at high pressures. At 
high thermistor bead temperatures long term drift in 
resistance is related to the ability of the glass coating to 
prevent oxidation of the thermistor material. At 300°C one 
may expect drift of as little as 0.1% per month from a glass 
covered bead thermistor. Use of thermistors in vacuum 
gauges is conducive to low drift since maximum tempera-
ture of the bead occurs in a vacuum while at atmospheric 
pressure bead temperature is somewhat reduced. 
With the thermistor operation curve defined, the values 
of E, R, E 1, I, E 2 , and 12 are sufficient to determine the 
design of the thermistor power and bridge circuitry (Fig. 
2). In order to avoid ground loops a floating input to the 
digital voltmeter must be used and the thermistor bridge 
must be grounded. 
Components located within the dotted lines of Fig. 2 
were placed in thermal contact with the water jacket and 
potted as shown in Fig. 3. Thermistor beads are sensitive 
to radiation and thus the water jacket must be coated with 
an opaque material. Water bath" temperature was set 
at 33°C. Higher water temperature may reduce thermistor 
sensitivity at high pressures while lower water temperature 
required cooling the water bath. Thermistor pressure gauge 
BRIDGE	 109V E 
WATEI 
IN
WATER 
JACKET 
FIG. 3. Thermistor block. Water leaving the water jacket circulates 
around the block before returning to the constant temperature bath.
TABLE I. Thermistor pressure gauge electrical and thermal 
characteristics. 
P=O mmllg	 P=760mmHg 
dE/dP	 2.0 mv/a Hg 
dE/dEa,	 -0.07 mv/v -0.2 mv/v 
dE;/dTroom	 -0.2 mv/°C -0.2 mv/°C 
dE/dTroom	 -0.6 mv/°C +0.0 mv/°C 
dE/dT th	 -5 mv/°C -5 mv/°C 
dE/dT th 	 -5 mv/°C -50 mv/°C 
dE/dE	 -0.4 -1.1 
A Ei for iP= 760	 0.161 v 
Noise (0.1 to 5 cps) at E and E1 less than 5 pv peak 
Water bath temperature regulation ±0.02°C 
Thermistor block temperature excursion ±0.01°C 
Water bath temperature error ±0.01°C 
dTw,,ter/dTroom ±0.00°/°C 
dTblook/dTroom ±0.01 °/°C
electrical and thermal characteristics are given in Table I. 
At pressures of 5 mm Hg and above, air convection cur-
rents assume importance in cooling the thermistor, and the 
thermistor should be used in the same orientation in which 
it was calibrated. At 5 mm Hg the gauge pressure reading 
was 0.5% higher when the thermistor tube was vertical 
than when it was horizontal. From 10 to 50 mm Hg the 
gauge pressure reading was about 1% higher for this same 
change in orientation. 
The time constant for the gauge is about 2.5 sec at 102 
mm Hg and 1.5 sec at 10 mm Hg. 
Long time repeatability is good. Three calibrations" 
over a period of 16 months were each followed by gauge 
disassembly and shipment. The calibrations showed a 
maximum pressure separation among themselves of 1% 
over the range from 102 to 101 mm Hg and indicated that, 
despite rough treatment, the gauge is capable of ±0.5% 
accuracy over this range. Short period repeatability of the 
gauge was about ±0.2% from 0.050 to 20 mm Hg. The 
thermistor gauge was designed and constructed hurriedly. 
We believe that with reasonable additional effort, signifi-
cant improvements in range and repeatability could be 
obtained without sacrificing simplicity. 
J . A. Becker, C. B. Green, and G. L. Pearson, Bell System Tech. 
J . 26, 170 (1947). 
2 A. J. Rosenberg, J
. 
Am. Chem. Soc. 78, 2929 (1956). 
'M. Varióak, Compt. rend. 243 893 (1956); J
. 
phys. radium 18, 
70A (1957). 
P. E. Seiden, Rev. Sci. Instr. 28, 657 (1957). 
R. E. Walker and A. A. Westenberg, Rev. Sci. Instr. 28, 789 
(1957). 
6 M. Vari5ak and B. Saftié, Rev. Sci. Instr. 30, 891 (1959). 
Thermistor time constant = heat capacity/power dissipation 
constant. 
9 Thermistor Assembly L- 252C, Thermistor Division, Gulton 
Industries, Metuchen, New Jersey. 
Considerable information concerning thermistors and their use 
was furnished by M. Sapoff of the Victory Engineering Corporation 
and by Dr. A. Rosenberg of the Lincoln Laboratory. 
10 Labline Constant Temperature Circulating System #3052, 
Labline, Inc., Chicago, Illinois. 
11 NASA TN D-504. 


Thermistor Pressure Gauge Design

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