Pumping Specific Gases in High Vacuum: There's more to planning and designing a vacuum process pumping system than just maximizing pumping speed

摘要

All practitioners of vacuum technology share a single requirement in that, at some point, a chamber must be pumped down from atmospheric pressure after being opened to ambient air. This requirement often lulls the unwary practitioner into assuming that the entire pumpdown process is composed of pumping air. This easy to make assumption can lull the soon to be benighted practitioner into assuming that a "big" pump will solve any pumping problem that might arise. It's just not that simple. Consider that the pumpdown process starts with a gas called air that's really a mixture of a number of gases with a fixed gas-to-gas ratio. Except, that is, for water vapor which varies widely with changes in relative humidity (rh). For example, 100 liters of air at room temperature and 55 % rh will contain 1.4 ml of water while the same volume at 100% rh will contain 2.47 ml. That's a considerable spread and it will have an effect on the pumpdown and the pumping system. Variations in the water vapor content is only a single, but obviously important, example of the need to target specific gas pumping in vacuum technology.