Liquid-level indicator, consisting of a diode heated by a resistor, allows simultaneous use of two or three of the liquids nitrogen, hydrogen, and helium. Operation depends on strong temperature-dependence of the forward resistance of a germanium diode and the difference between liquid and vapor in heat-transfer properties

Hyman, L. G.

Sheppard, J. F.

Spinka, H.

English

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TEMPERATURE •K 
July 1969	 Brief 69-10210 
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TES
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s 
Improved Liquid-Level Sensor for Cryogenics 
A	 B 
Fig 1 (A, top to bottom) Diode and resistor thermally connected with alu-
minum tape, housing, and assembled sensor. (B) Voltage drop across a 
1 N34A diode as a function of temperature: diode current was 1 00 ma 
A	 B 
Fig 2 Responses of voltage acioss diode in containers in vapor (A) and 
in liquid (B) as functions of heater power (0 10, 040, 0,63, 0 63, 0 78, 
0 90 w, 1 00 ma, 19 seconds, 5 seconds per division, 1 v per division)
The problem: 
To develop an ideal liquid-level indicator for a 
closed and inaccessible cryogenic system. These sys-
tems often require the simultaneous use of two or 
perhaps three of the liquids nitrogen, hydrogen, and 
helium. Moreover it is customar y to precool a helium 
system with nitrogen and/or hydrogen.
The solution: 
A new liquid-level indicator (Fig. IA), consisting of 
a diode heated by a resistor, meets all the require-
ments. It is rugged, simple, and inexpensive and re-
quires no preselection of components: no elaborate 
calibration is required, nor adjustment for change in 
liquid. It is a "es no" device that is eas y to operate 
This document was prepared under the sponsorship of the Atomic	 liability resulting from the use of the information contained in this 
Energy Commission and/or the National Aeronautics and Space	 document, or warrants that the use of any information, apparatus, 
Administration. Neither the United States Government nor any per-	 method, or process disclosed in this document may not infringe pri-
son acting on behalf of the United States Government assumes any vately owned rights.
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and interpret. It is insensitive to changes of circuit 
parameter, and independent of the number of thermal 
cycles and of environmental conditions such as pres-
sure, geometry, and splashing of liquid. The power 
dissipated by the indicator is negligible relative to 
other heat sources in the system. The indicator pre-
sents the effects of diode current, resistor power, 
heater-pulse duration, and liquid pressure. 
How it's done: 
Operation of the instrument depends on (i) the 
strong temperature-dependence of the forward re-
sistance of a germanium diode (Fig. 113), and (ii) the 
difference between liquid and vapor in heat-transfer 
properties. 
The basic sensor consists of a I N34A diode thermally 
connected to a conventional 0.25-w, 200-ohm carbon 
resistor serving as the source of heat; they are con-
nected with aluminum tape. When the sensor is im-
mersed in liquid helium and about 0.75 w is supplied 
to the heater, there is little or no change in the diode 
voltage; when it is in vapor, and power is applied to 
the resistor, the change in voltage across the diode 
is about 5 v. The effect is more pronounced when the 
sensor is placed in a housing (Fig. 2), which also 
makes the sensor less sensitive to splashing of liquid; 
stainless-steel tubing (length, 1.75 inches; outer 
diameter, 3/16 inch; walls, 0.010 inch) equippedwith 
two liquid-intake holes (0.040-inch diameter) and six 
gas-vent holes (0.055-inch diameter) was used.
A sensor was thermally cycled rapidly between room 
and liquid-nitrogen temperature about 100 times with 
no adverse effects on its behavior. Reproducibly it 
located the liquid level within less than 1/16 inch, 
with or without the housing. When the mass flow of 
gas per unit of time, with the sensor in vapor, was 
varied, the device proved to be relatively insensitive 
to the variation. 
Notes: 
I. This sensor may interest anyone concerned with 
cryogenics. 
2. Inquiries may be directed to: 
Office of Industrial Cooperation 
Argonne National Laboratory 
9700 South Cass Avenue 
Argonne, Illinois 60439 
Reference: B69-10210 
Source: L.G. Hyman, J.F. Sheppard, and H. Spinka 
High Energy Physics Division 
(ARG- 10162) 
Patent status: 
Inquiries concerning rights for commercial use of 
this innovation may be made to: 
Mr. George H. Lee, Chief 
Chicago Patent Group 
U.S. Atomic Energy Commission 
Chicago Operations Office 
9800 South Cass Avenue 
Argonne, Illinois 60439
. 
Brief 69-10210	 Category 02


Improved liquid-level sensor for cryogenics

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