Detector and indicator system, utilizing commercial perforated germanium cryogenic thermometer as level sensor containing adjustable level discriminator with indicators, operates reliably over pressure range from 50 to 900 mm Hg without electronic adjustments

Krawczonek, W. M.

Stephenson, B.

English

NASA Technical Reports Server

+5V 
Common 
May . 1972	 B72-10145 
NASA TECH BRIEF 
Lewis Research Center 	
-4 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
A Reliable Liquid Helium Level Detector
Figure 1. Circuit schematic of the level 
detector and indicator system. 
A liquid helium level detector and indicator system 
has been developed which can operate reliably over a 
• pressure range from 50 to 900 mm Hg without elec-
tronic adjustments. The system should also be applicable 
for level detection in other liquified gases such as 
oxygen, hydrogen, and nitrogen. 
The problem of detecting the liquid helium level in a 
system where the pressure can vary anywhere from 50 
to 900 mm Hg required development of a level sensor 
operation and electronics to sense the variation of the 
level sensor output between liquid and cold helium gas, 
but which are not sensitive to the pressure variations on 
the liquid. In the system developed, a commercial perfo-
rated germanium cryogenic thermometer is used as the 
level sensor (but not as a temperature sensor) with an
adjustable level discriminator with indicators. The sensor 
acts as a variable resistance dependent on the power 
dissipated. The power dissipated varies greatly between 
immersion in the gas or in the liquid; hence, a large 
signal difference is available to discriminate between the 
two. Also, this sensor has the unique characteristic that, 
as the pressure drops, the signal when out of the liquid 
helium increases while the signal when in the liquid 
helium decreases. With the germanium sensor, it is not 
necessary to reset the circuit to differentiate between 
liquid and gas as the pressure decreases or increases, and 
accurate readings are obtained over a wide range of 
pressures. All that is necessary is that the sensor be 
calibrated at one atmosphere. 
The amount of power required for reliable sensor 
(continued overleaf) 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19720000145 2020-03-17T03:32:07+00:00Z
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 For Typical Sensor as
	
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Sensor Just Emerges 
From Liquid He into Gas
	 +
Sensor 
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10092 
0.900 
0.800
>
0.600 
I	 Upper 
0.500 f 
I200 mV	 Set Points for Level 
Differential	 Discriminator 
0.400--
Lower/ 
- 
0.300--
0
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W (0 
-J 
0 
> 
I-
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0.700 
0.200
operation over the stated pressure range was determined 
experimentally at a safe operating current and a mini-
mum dissipation in the helium system. Power dissipa-
tion of the sensor in the liquid helium is approximately 
4 mW at 1 atmosphere. After initial calibration, no other 
adjustments are necessary for repeated operation and 
cycling from room to liquid helium temperatures. 
Response time is less than 50 ms. 
Figure 1 is a circuit schematic of the level detection 
and indicator system. The germanium sensor and the 
input resistor of the voltage follower form a voltage 
divider. When the liquid level rises above or drops below 
the sensor, a voltage change is sensed across the 100-ohm 
resistor. The voltage follower is used for isolation. The 
SPAN and REFERENCE pots are adjusted so that the 
level discriminator will trip at the set points selected 
from the graph in Figure 2. These points were selected 
to be within the voltage levels at 900 mm Hg and will 
remain constant for this sensor at pressures down to 50 
mm Hg. The graph was plotted for a particular sensor 
which was cycled in and out of liquid helium at differ-
ent pressures. The state of the discriminator, either high 
or low, determines which lamp will be lit through the 
lamp drivers, indicating whether the sensor is in or out 
of liquid helium. 
Note: 
No further documentation is available. Specific ques-
tions, however, may be directed to: 
0.100
Technology Utilization Officer 
E For Typical Sensor. Lewis Research Center

	
Immersed in Liquid He At
	 21000 Brookpark Road Various Pressures	
Cleveland, Ohio 44135 
Reference: B72-10145 
Patent Status: I	 I	 I	 I	 I	 I	 —I 100 200 300 400 500 600 700 800 900 1000
	 No patent action is contemplated by NASA. 
PRESSURE - mm (Hg)
Source: W.M. Krawczonek and B. Stephenson of

	
Figure 2. Operational characteristics of the
	
Lewis Research Center level detector and indicator system.
	
(LEW-1 1487) 
B72-l0l45 Category 01, 03 


A reliable liquid helium detector

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