The Permeability Minimum and the Viscosity of Gases at Low Pressure

An expression for the gas permeability of capillaries covering the entire range of the diffusion, transition, and Poiseuille regions is given. It makes use of an expression for the viscosity when the mean free path is comparable to, or greater than, the capillary radius. One parameter occurs which, when adjusted to fit Knudsen’s data, gives a minimum in the permeability at r/λ = 0.45.