Outgassing rate , UNIT: mbar.l/(s.cm²), (referenced to surface area)
In order to estimate the amount of gas which will have to be extracted, knowledge of the size of the interior surface area, its material and the surface characteristics, their outgassing rate referenced to the surface area and their progress through time are important.
Outgassing is a release of gases from the internal surface of a chamber. It may be adsorbed or trapped gases or liquids or even degradation of the material itself (polymers). It is essentially an extra flow the vacuum pumps must handle and is especially important as an influence on pumpdown time and end pressure achievable at pressures < 0.001 mbar.