8 - Gross Leaks

Abstract

Gross leaks can be defined in three ways: by the leak rate of the package, by detection of a liquid or its byproducts in the package, or by the size of the leak channel. They are detected rather than measured quantitatively. A quantitative value of a gross leak rate is usually of no interest, as any package having a gross leak is unacceptable. An exception to this is a failure analysis investigation into the cause of the gross leak. If a gross leak test method is positive (there is a gross leak), the package is a gross leaker. A negative response to a particular gross leak method does not eliminate a possible gross leak, as different test methods have different sensitivities. Liquids enter a package by viscous flow. This can only occur when there is a difference in total pressure between the inside and outside of the package. Packages that have large holes, such as when a pin falls out, can have liquids enter the package by displacing some of the gas. The relationship between the viscous conductance of the detector liquid and the viscous conductance of the helium, for cylindrical leak channels, is a function of the differences in their viscosity (η), their mean free path (mfp), their average pressure and their correction factor (Y).