End users of vacuum systems should know the basic capabilities of their system including an estimate of its likely best base pressure. The formula is simple:
Gas Load/Effective Pumping Speed = Ultimate Base Pressure
For example, in the case of a stainless steel vacuum chamber with internal dimensions of 100 x 100 x 100 cm, an outgassing rate for unbaked steel of 2 * 10-8 Torr.liters/cm2.sec, and an effective pumping speed of 500 Liters/sec we have the following math:
The internal surface area is 100 x 100 cm x 6 sides, or 60,000 cm2 x 2 * 10-8 Torr.liters/cm2.sec resulting in a gas load of 1.2 * 10-3 Torr.liters/sec. Dividing that gas load by the effective pumping speed of 500 Liters/sec yields an ultimate pressure for the system of 2.4 * 10-6 Torr.
In reality there are several contributors to gas load in addition to the steel walls of the vacuum chamber. To put a fine point on the calculation one needs to consider sources like permeation, diffusion, back streaming of hydrocarbon pump oil vapors, and others. But the calculation above provides a reasonable first pass at determining the ultimate base pressure of a basic vacuum system.
Category: Vacuum Systems