Hatioaal Bureau of Standard;

Library, K.W. Bldg

OCT 2 9 1963

^ecknlcul <Vtote 92o. 200

CARBON RESISTORS FOR CRYOGENIC

LIQUID LEVEL MEASUREMENT

RICHARD C. MUHLENHAUPT AND PETER SMELSER

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS

technical ^©te 3.00

Issued October 9, 1963

CARBON RESISTORS FOR CRYOGENIC

LIQUID LEVEL MEASUREMENT

Richard C. Muhlenhaupt and Peter Smelser

Cryogenic Engineering Laboratory Division

National Bureau of Standards

Boulder, Colorado

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

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IMPORTANT NOTICE

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is subjected to additional evaluation and review. For this reason, the publication, reprinting, reproduction, or open-literature listing of this Report, either in whole or in part, is not authorized unless per- mission is obtained in writing from the Office of the Director, National Bureau of Standards, Washington

25, D. C. Such permission is not needed, however, by the Government agency for which the Report has been specifically prepared if that agency wishes to reproduce additional copies for its own us*.
CONTENTS

Page

1

.

Introduction 1

2. Description of Test Apparatus 2

3. Experimental Procedure . ; 3

4. Discussion of Results 4

5. Relative Precision and Reliability of Resistors .... 5

6. Conclusion 7

7. Appendix 7

CARBON RESISTORS FOR CRYOGENIC

LIQUID LEVEL MEASUREMENT

Richard Muhlenhaupt and Peter Smelser

Data are shown in graphical form. One set of plots

presents resistance ratio R /R as a function of "warming

up" time at various levels of constant power dissipation. A

second set of plots presents resistance ratio R /R~ as a function of nominal resistance at various levels of constant

power dissipation.

The use of the data and the design of a practical liquid

level inidcator are discussed in the appendix.

1. Introduction

In the past few years sufficient interest has been shown in the

use of ordinary carbon composition resistors for cryogenic liquid level

measurement to warrant a study of the various parameters involved

in the design of this type of liquid level indicator. Resistors are commonly used for the detection of liquid levels when a simple and inexpensive point sensor is desired and when precise indication and fast time

response are not critical requirements.

The principal of operation is based upon (1) the heat transfer

characteristics of the resistor in gas and liquid, and (2) the large

negative temperature coefficient of resistance which occurs when the

resistor is in a cryogenic environment.

The circuitry required for the detection of liquid levels by this

method includes a Wheatstone bridge (the liquid level resistor being one

of the arms), a power supply, an amplifier, and the desired readout

equipment (lightbulb, galvanometer, relay, etc. )•
When a liquid level indicator is to be designed, the following

three parameters should be known: the maximum allowable power

dissipation, the temperature of the liquid to be detected, and the desired

response time (determined by the vaporization rate and the allowable

liquid level drop). If data relating these parameters to design criteria

were available, the design of functional liquid level indicators would be

facilitated.

Accordingly, tests in liquid nitrogen, hydrogen, and helium

were conducted on a number of 0. 1 watt (manufactured by Ohmite) and

0. 5 watt (from general stock, manufacturer unknown) carbon composition resistors having nominal resistances ranging from 10 to 10,000

ohms. Carbon deposited film 1% resistors were also included in the

test program; these, however, were found to be relatively insensitive

to extreme temperature reductions (see figure 1) and are not recommended for liquid level measurement.

The test program is discussed in the following pages, and the

results are shown in the accompanying graphs. In addition, some observa^

tions regarding resistor precision and reliability are noted.

2. Description Of Test Apparatus

The test apparatus consisted of two major units: the probe

assembly and the associated electrical circuitry. Principal components

of the probe assembly are shown in figure 2. The guide tube houses a moveable plunger tube which in turn supports the test resistor. An

adjustable stop controls the positioning of the test resistor in relation

to the liquid-vapor interface, the location of which is determined by

the liquid level sensor fastened to the end of the guide tube. A microswitch, actuated when the resistor is passing through the liquid-vapor

interface, provides a zero time signal.

-2-
The electrical circuitry is shown in figure 3. A 60 watt, D. C. , constant voltage, power supply provides the necessary power. Resistors

R , R , R , and either R or R form a Wheatstone bridge. Decade

i <-. o 1 (_> box R is used to balance the bridge when the test resistor is in the

circuit, and decade box R (checking resistor) is used to determine the

resistance of R and to calibrate the recorder. A recorder charts the

bridge unbalance as the resistor passes through the liquid-vapor interface.

A potentiometer is used to measure the voltage drop across the bridge.

3. Experimental Procedure

Twelve 0. 1 watt and twelve 0. 5 watt carbon composition resistors

were tested. For each power rating, three resistors of each value

(10, 100, 1000, and 10,000 ohms) were chosen. After the selected

resistor was attached to the leads on the plunger tube, its room temperature resistance (R in figures 13 and 14) was measured with a Wheatstone bridge. Then the travel of the plunger tube was adjusted to

permit moving the test resistor from a submerged position to a point

slightly above the liquid-vapor interface.

With the sensor in the submerged position, the magnitude of

the resistance in liquid (R in figures 4 through 12) was determined by

balancing the bridge, substituting checking resistor R~, and adjusting

the resistance of R equal to that of R . A resistance calibration was

established by adjusting R (while in the circuit) to a number or pretty determined resistance levels and recording the resulting recorder

deflections.

The actual testing was begun with the test resistor submerged in

the liquid bath. The recorder chart drive was turned on, and the plunger

tube was quickly raised causing the micro switch to be actuated, which

-3.
momentarily placed a resistance in parallel with R for the zero time

indication. When a resistance change of about 5% had been reached,

the chart drive was turned off. During this process, the voltage across

the bridge was measured with the potentiometer.

This procedure was repeated three times at approximately the

same bridge supply voltage. The supply voltage was then changed to

provide a new power level and testing was resumed. When tests had

been conducted at three or four different voltage settings, a new resistor

was selected and the process repeated.

4. Discussion of Results

The data obtained from the test program were plotted in the form

of curves which are presented in figures 4 through 14. Each of the first

nine graphs (figures 4 through 12) show resistance ratio (R /R ) as a

function of "warming up" time at various levels of power dissipation.

The plots show that level indicating sensitivity improves with

increased power dissipation. They also show that sensitivity is increased

when the temperature of the resistor's environment is lowered (i. e. , sensitivity is best in helium, next best in hydrogen, and poorest in nitrogen).

It was expected that the ratio R /R in the time response curves G Li would begin at unity and continuously approach some lower value; however, for most plots this is not the case. Instead, the ratio becomes

larger than unity before dropping to lower values. This suggests that

the resistor, after passing through the liquid-vapor interface, senses a

lower temperature. At the present time no satisfactory explanation of

this behavior has been worked out.

In order to avoid this initial "negative" effect, it is suggested

that a resistance ratio of 0. 98 (an arbitrary figure that may be adjusted

to the designer's requirements) be used as the time response determining

-4-
figure.

It should be noted that: (1) the test conditions could not be identicallyreproduced for resistors of the same nominal rating, and (2) much of

the data was combined in order to reduce the number of final graphs.

The plots represent average data derived from three individual runs

for each test condition. In addition, the only 10 ohm resistors which

gave reasonable response times were those having 0. 1 watt nominal rating,

and furthermore these gave usable results only when used in helium. Although

deviations were observed, it was felt that they were insufficient to impair

the usability of the curves.

The final two graphs (figures 13 and 14) show resistance ratio

as a function of nominal resistance for several series of resistors

operated at various power levels. On these graphs the resistance ratio

is the measured resistance in liquid (R ) divided by the measured

Li resistance at room temperature (Rn)» while the abscissas of the graphs

represent the nominal resistance rating of the resistors. It will be noted

that as the nominal resistance increases or as the temperature of the

environment decreases, the resistance ratio becomes greater. In addition,

increased power dissipation tends to decrease the resistance ratio.

Within experimental error the semi-log plots appear as straight lines.

With the help of these graphs it becomes a simple matter to

determine the expected magnitude of the resistance in liquid once the

following are known: (1) the maximum allowable power dissipation,

(2) the nominal rating of the selected resistor, and (3) the liquid to be

detected.

5. Relative Precision And Reliability of Resistors

The precision of a number of liquid level sensors was determined

-5-
[1]

in separate tests conducted at the Cryogenic Engineering Laboratory

The sensors were lowered into a liquid hydrogen bath until a wet indication was noted and then raised until a dry indication was observed, the

distance traveled being measured to the nearest . 001 inch. From these

data a bandwidth denoting the maximum distance between the wet and dry

indications for any given sensor was produced. In order to determine

how pressurizing the liquid affects the magnitude of the bandwidth, a

series of runs were conducted on each sensor at pressures ranging from

two to 200 psig.

Among the commercial liquid level indicators evaluated in this

manner was a 0. 1 watt, 1000 ohm carbon resistor type, (manufactured by

Ohmite), which was positioned with its major axis parallel to the liquidvapor interface. The bandwidth was found to vary from 0. 015 to 0. 122 inch.

Bandwidths varying from 0. 020 to 0. 028 inch were found for a hot wire

sensor similarly tested. No simple correlation was found between pressure

and bandwidth.

Response time, determined in the same test program, was

found to vary from 0. 003 to 1. 128 seconds for the hot wire sensor, while

the response time of resistors in the present program (see Discussion

of Results and figures 4 through 12) ranges from about 0. 5 to 8. seconds.

Although reliability tests were not included in either program,

resistors have been frequently used for liquid level sensors at the

Cryogenic Engineering Laboratory, and it has been found that their

characteristics vary at times. Prolonged thermal cycling, for example,

l

' D. A. Burgeson, W. G. Pestalozzi, and R„ J. Richards, "The

Performance Of Point Level Sensors In Hydrogen," Advances in

Cryogenic Engineering 9, In Press.
has been known to cause resistance changes of as much as 10%, but

this can be corrected by a minor circuit adjustment.

In general, the resistor leads should be cut as short as possible

to reduce the effective mass, care should be taken when attaching the

lead-out wires because heating can increase the resistance by as much

as 10%, and the probe design should provide for liquid drainage away

from the resistor.

6. Conclusion

The results of the test program show that the time response of

resistors is somewhat slower than that of a hot wire sensor. Sensitivity

depends upon the allowable power dissipation and the temperature of the

liquid being detected, greater sensitivity being obtained when the power

is increased or when the temperature of the liquid is decreased. The

value of the resistance in liquid depends upon (1) the power dissipation,

(2) the ohmic rating of the resistor, and (3) the liquid. Once these

three parameters have been selected, the magnitude of the resistance

in liquid may be readily determined.

The bandwidth of 1/8 inch for a horizontally mounted resistor

is adequate for all but the most precise requirements. Resistors are

inexpensive and easy to mount and the associated electrical circuitry

can be simple.

It is apparent that ordinary carbon composition resistors perform

reasonably well as liquid level sensors, and within the indicated limits,

they are well suited for this purpose.

7. Appendix

The bridge components (see figure 15-a) are governed by the

following considerations: the power dissipation of the ratio arms should

7-
be a reasonable value, the resistance ratio R /R. should be approximatelyequal to R /R , and the resistances of R and R should be small in

J3 C B C

relation to the amplifier's input impedance in order that high sensitivitymay be obtained.

The design of a practical liquid level indicator will now be considered. Arbitrarily selected parameters are:

(1) maximum allowable power dissipation: 125 milliwatts or less

(2) liquid to be detected: hydrogen

(3) desired time response: two seconds or less.

Readily available components are:

(1) power supply: 26. 5 volt, D. C. , 600 ma

(2) indicator: 6 volt, 150 ma, #47 lightbulb

(3) amplifier: two-stage, transistorized, Z « 5000 ohms

(4) balancing potentiometer: 1000 ohm, 2 watt.

Figure 9 shows that a 1000 ohm, 0. 5 watt resistor dissipating

115m watts satisfies the above criteria, and figure 14 shows that a resistor of this nominal rating will have a resistance ratio R /R~ of

about 2. 50. The resistance of R is then found to be approximately

2500 ohms. When half the resistance of the potentiometer (design null)

is added to the resistance of R , the resistance between points "a" and

"d" on figure 15-a is 3000 ohms.

In order to find suitable resistance values for the other three

arms of the bridge, it is convenient to first find the current in the

branch adc. Since 115 milliwatts are dissipated in R , J_j

115 mw , __

= 6. 78 ma, and

adc \j 2500 ohm v

26 5 v R a = 1T^ = 390 ° ohm- adc o. 78 ma

-8-
This leaves 400 ohms for resistor R . A 0. 5 watt, 390 ohm resistor

(the nearest nominal resistance rating) is therefore selected. The

resistance ratio of the two adjacent arms then becomes

R / R = 3000/890

ad dc

= 3. 37.

Recalling that high sensitivity will be obtained when the resistance

of resistors R and R are small compared to the input impedance of

B C

the amplifier, a 300 ohm resistor is arbitrarily selected for R . The B

resistance of R is then calculated from the relation

RR /Rr= R . / R , B C ad dc

or R = 300/3. 37

\->

- 89 ohms.

A 91 ohm resistor is therefore selected for R .

It remains to be determined whether or not these resistors will

produce a reasonable power dissipation. The current in this portion of

the circuit will be

26. 5 volts

abc 391 ohms

= 68 ma.

With this current, R will dissipate 1. 39 watts, which is somewhat high. B

This indicates that larger resistors should be used to reduce I , . If abc

-9-
R is 1000 ohms, R will be about 300 ohms and the power dissipated in B C

the larger resistor will be 0. 4 watt. This is below the power rating for

the resistor and R , is still low when compared with the input impedance

abc

of the amplifier.

The resistors finally selected for the bridge are

R : 1000 ohm, 0. 5 watt

o R A

: 390 ohm, 0. 5 watt A

R_: 1000 ohm, 0. 5 watt B

R : 300 ohm, 0. 5 watt.

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GPO 836-534



U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A.'V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

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Radio Systems. Applied Electromagnetic Theory. High Frequency and Very High Frequency Research. Fre- quency Utilization. Modulation Research. Antenna Research. Radiodetermination. Upper Atmosphere and Space Physics. Upper Atmosphere and Plasma Physics. High Latitude Ionosphere

Physics. Ionosphere and Exosphere Scatter. Airglow and Aurora. Ionospheric Radio Astronomy.

RADIO STANDARDS LABORATORY

Radio Standards Priysics. Frequency and Time Disseminations. Radio and Microwave Materials. Atomic Fre- quency and Time-Interval Standards. Radio Plasma. Microwave Physics.

Radio Standards Engineering. High Frequency Electrical Standards. High Frequency Calibration Services. High

Frequency Impedance Standards. Microwave Calibration Services. Microwave Circuit Standards. Low Frequency

Calibration Services.

Joint Institute for Laboratory Astrophysics -NBS Group (Univ. of Colo.).
NBS