Free molecular transfer and pumping speeds

Abstract

The net free molecular mass flow rate to surfaces or openings in low pressure vacuum systems cannot in general be calculated from “pressure” and “orific speed”, since completely random molecular motion only rarely exists. Mass flow rate under such conditions can however be calculated by using network analysis as has been done in radiant heat transfer. The author obtains an expression for the pumping speed in case of a non-uniform temperature distribution and shows that under certain conditions this equals the orific speed as measured by a nude gauge immediately adjacent to the orifice.