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Theory Of Operation
3.1
Introduction: Leak Detection Theory
When designing and manufacturing systems that contain gases or fluids,
or systems that are normally evacuated, the manufacturer or tester must
take into account the degree of leakage that can occur. To qualify these
products, various methods may be selected to test these products for leaks.
The amount of actual leakage that can be tolerated is different for every
application and therefore must be determined from the conditions of the
application itself. From this data the manufacturer will be able to
establish the limits of maximum permissible leakage and will
also be able to determine the maximum permissible pressure levels within
a product or system under actual working conditions. Further, the
manufacturer will need the ability to discover and localize any leaks in the
product or system tested. In all cases, the maximum allowable leak rate,
as well as the testing methods and procedures, should be specified before
a product or system is manufactured.
Many commercial, industrial and military products and processes require
hermetic sealing. In general, these products include equipment exposed to
pressures above or below those of its immediate environment, vacuum
equipment used in the manufacture of semiconductors and related
research, and items produced for industrial and commercial applications,
most notably the refrigeration and automotive industries. Hermetic
tightness (sealing) is narrowly defined as the absence of leakage, however,
no manufactured object can be considered totally leak tight. Statements
such as "no detectable leaks" or "zero leakage" are not valid specifications
for acceptance testing. Even in the absence of defect within a product or
system, gas will always permeate through metal, crystals, polymers and
glasses. Therefore, "leak tightness" is only a relative term.
The degree of permissible leakage depends on how and where the product
or system is to be used, the amount of the object's content (liquid or gas)
that is permitted to escape over a certain time period, or the amount of
external atmosphere or liquid that is permitted to penetrate the object over
a certain time period. The size of the permissible leak is defined as part of
the performance requirements of the product or system. Leak standards
are set for the following reasons:
Vacuum Instrument Corp.
2099 Ninth Avenue
Ronkonkoma, NY 11779
631.737.0900
1109-186-00 Rev. C
3-1
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