High-resolution differential pressure transducer senses very small positive or negative pressure variations in the cryogenic tank relative to absolute reference pressure. System is useful in calibrating instruments where working fluid must be maintained at closely controlled temperature, or in processes requiring very fine pressure control

Kirchmeier, W. E., Jr.

English

NASA Technical Reports Server

Vent 
0/ 
Transducer 
Readout
Set Point
May 1971	 CORRECTED COPY	 Brief 71-10118 
NASA TECH BRIEF 
Lewis Research Center 
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. 
System Accurately Controls Presure in Cryogenic Tanks 
The problem: 
One way of determining the effectiveness of in-
sulation on cryogenic-liquid tanks is to measure the 
quantity of vapor boiled off by the heat leaking 
into the tanks. To ensure that the boiloff (vented
vapor through a mercury column) are used for pres-
sure control. These small pressure variations intro-
duce relatively large errors in the heat leak cal-
culations. A much more accurate pressure control 
is required.
Controller 
Vent Line
Electrohydraul Ic 
Differential-Pressure Transducer	 Pressure-Regulating 
Valve 
Cryogenic 
Test Tank, 
Reference 
Pressure Gas 
Ice Bath 
vapor) is due only to the heat leak; the cryogenic 
fluid in the tank must be kept at a constant tem-
perature. This requirement can be met by main-
taining the tank pressure at a known constant value. 
Small variations in tank pressure commonly occur 
when conventional methods (e.g., bubbling the vented
The solution: 
A high-resolution differential pressure transducer 
is used to sense very small positive or negative 
pressure variations in the cryogenic tank relative 
to an absolute reference pressure. The smallest 
variation which can be sensed is approximately 
etnuli j ilir ed nn erlerni) 
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0.0013 N/rn 2 ( ± 1 x 10 5 m Hg). The electrical 
output of the transducer is fed to a pressule 
regulating valve in the vent line, thereby con-
trolling the pressure in the cryogenic tank. Pres-
sure in a typical system has been held at 1.17 
X iO ± 1.38 N/m 2 (17	 0.0002psia). 
How it's done: 
A schematic of a typical control system is shown 
in the figure. One side of the differential-pressure 
transducer is connected to a fixed volume of gas 
maintained at constant temperature by an ice bath 
in order to provide a constant reference pressure. 
The other side of the transducer is connected to the 
vent line of the cryogenic test tank. The electrical 
output signal from the transducer is fed to a 
control unit for an elect rohydraulic pressure regulat-
ing valve in the vent line. To achieve the desired 
fine control of the tank pressure, this valve must 
be carefully sized to handle the boiloff gas flow 
rates expected from the test tank. The valve con-
troller is given a set point, and this results in con-
trolling the tank pressure to a constant value with 
respect to the reference pressure. 
Notes: 
I. In addition to measuring insulation effectiveness, 
the system should be useful in calibrating in-
struments where the working fluid must be main-
tained at a closely controlled temperature, or in 
processes requiring very fine pressure control. 
2. No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B71-10118 
Patent status: 
No patent action is contemplated by NASA. 
Source: W. E. Kirchmeier, Jr.

Lewis Research Center

(LEW-1 1329)
. 
El 
. 
Brief 71-10118	
Category 03


System accurately controls pressure in cryogenic tanks

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