Molecular flow

In molecular flow, the gas mean free path is large compared with the channel or vessel dimensions, so molecules encounter walls more frequently than one another. The Knudsen number $\mathrm{Kn}=\lambda /L$ characterizes this regime.

For a small aperture of area $A$ between equilibrium gas reservoirs, the vacuum conductance is $C=A\overline{v}/4$, where $\overline{v}$ is the mean molecular speed. At fixed temperature this scales as the inverse square root of molecular mass. Continuum viscous-flow formulas need not apply.^[ref]