In order to measure the diffusivity of a gas-vapour mixture, vapour transport must be set up in a configuration for which the governing equations admit an exact solution. Unfortunately, no such configurations are known. A popular configuration is the Stefan diffusion tube, in which a small amount of volatile liquid is placed in an open vertical cylinder and a gas stream of known vapour content is blown across the top. If the vapour transport is assumed to be isothermal, one-dimensional and quasistationary, and the gas stream is assumed not to disturb the gas-vapour mixture in the tube, the governing equations are soluble (Bird et al. 1960, pp. 522-7; §4.4).
Ampoules similar to Stefan diffusion tubes have also been used for the growth of crystals from their vapours (Markham & Rosenberger 1980).