ABSTRACT
How does the flow of gas in vacuum systems compare with familiar examples of fluid flow in the world around us? At pressures sufficiently low that the molecular mean free path is comparable with or exceeds the size of the equipment, that is for Kn values >1, it is totally different, as we shall discover in detail in due course. But for vacua not so rarefied, in which mean free paths are such that gases still demonstrate fluid behavior (Kn < 0.01), the concepts and measures developed to describe fluid flow at atmospheric pressure remain appropriate.
