Three types of flow are mainly encountered in vacuum technology: viscous
or continuous flow, molecular flow and – at the transition between these two
– the so-called Knudsen flow.
Conductance describes the ability of pipes and orifices to allow gas to flow. Short, wide pipes are better than long narrow ones, and the diameter of the pipework should match the inlet flange size of the pump. At lower pressures into molecular flow, conductance of gas through a pipe becomes more important and more influential. It is one of the reasons a high vacuum pump is located right at the chamber.