Leakage Rate

A leak is a small hole in one or several parts of the system that allows the uncontrolled entry or exit of gas.

Leakage rate is the maximum rate at which a fluid is permitted or determined to leak through a seal. As for the leakage rate, this is dependent on several factors including: The type of fluid, the differential pressure across the seal (between the inside of the system and the outside), the direction of leakage, and the location of the seal. 

There are several reasons why a system may fail to maintain its vacuum-levels, including outgassing or contamination. Furthermore, different vacuum processes and applications call for different leak rate requirements i.e. what is acceptable at a lower vacuum would be considered utterly unacceptable (and possibly dangerous) at a higher vacuum level.
Reducing or eliminating leaks are important for a number of reasons, including:

●operator safety (i.e. egression of toxic gases/fluids)
●product safety (e.g. to stop air from entering a system where it may significantly contribute to the formation of an explosive mixture)
●to ensure and maintain the pressure/vacuum
●to ensure a long user-life for products
●for environmental and quality standards
●and for process efficiency

Despite such reasons, it must be accepted that no system can ever be absolutely vacuum-tight…and indeed it need not be—it just needs to be manageable or at least low-enough so that the operating pressure, gas balance and ability to reach and maintain a final/ultimate pressure, are not overly impacted.