Method provides opportunity to effect repairs without compromising integrity of enclosed circuitry or mechanism by loss of atmosphere or by ingestion of foreign matter or gas. It is possible to detect leaks in modules which are sealed while fully evacuated, partially evacuated, or containing some form of gas

Lawrence, F.

Lusby, T. K., Jr.

English

NASA Technical Reports Server

Sensitive Dial 
Indicator 
Module 
Under Test
Bell Jar 
Nondestructive Leak-Test System 
March 1974
	 B73-10464 
.
NASA TECH BRIEF 
Langley 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. 
Nondestructive Leak Testing 
A method has been devised to leak test hermetically-
sealed instrument modules, such as those used for 
electronic circuitry and delicate mechanical equipment. 
It provides an opportunity to effect repairs without 
compromising the integrity of the enclosed circuitry or 
mechanism by loss of atmosphere or by ingestion of 
foreign matter or gas. With this method, it is possible to 
detect leaks in, and determine the leak rate of, modules 
which are sealed while fully evacuated, partially evacu-
ated, or containing some form of gas. It has proven 
effective for nondestructive leak testing of relatively 
small sealed modules, ranging in structure from semi-
flexible to semirigid.
The illustration shows a typical test setup. A module 
is placed in the fixture. The dial indicator is zeroed 
while its probe is in contact with a centrally located 
point on the most flexible surface of the module. A bell 
jar is placed over the fixture and partially or completely 
evacuated by the vacuum pump. Complete evacuation 
will create an inside-to-outside module pressure differ-
ential equal to the pressure of the atmosphere within the 
module. This, in turn, will cause even a slightly flexible 
surface area to act as a diaphragm and produce a 
deflection of the gauge dial. No gauge deflection 
indicates a gross leak, initial gauge deflection that falls 
back toward zero indicates a fine leak, and gauge 
(continued overleaf) 
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deflection which remains steady indicates a sealed unit. 
Leak rate can be calculated from leak-down time. 
If the module has been filled with gas, it will be 
evacuated during the test if a leak is present. In such a 
case, the gas can be replaced by closing the valve to 
the vacuum pump and flooding the bell jar with the 
same gas through a separate supply inlet. This will 
provide an opportunity to repair the module under 
original conditions. 
The test setup shown is effective for gauge deflections 
of 0.00127 cm (0.0005 in.) or greater. For very small 
or more rigid modules, the apparatus is modified by 
removing the dial indicator and replacing it with a 
sensitive strain gauge connected to a digital readout. 
This modified apparatus can then be calibrated to detect 
smaller diaphragm movements.
Note: 
No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Langley Research Center 
Mail Stop 139-A 
Hampton, Virginia 23665 
Reference: B73-10464 
Patent status: 
NASA has decided not to apply for a patent. 
Source: T. K. Lusby, Jr., and Frank Lawrence

Langley Research Center
(LAR-1 1561)
S 
B73-10464	 I	 Category 02,06	 S


Nondestructive leak testing

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