text: In chemical engineering, a Stefan tube is a device that was devised by Josef Stefan in 1874.  It is often used for measuring diffusion coefficients.  It comprises a vertical tube, over the top of which a gas flows and at the bottom of which is a pool of volatile liquid that is maintained in a constant-temperature bath.  The liquid in the pool evaporates, diffuses through the gas above it in the tube, and is carried away by the gas flow over the tube mouth at the top.  One then measures the fall in the level of the liquid in the tube.
  The tube conventionally has a narrow diameter, in order to suppress convection.
  The way that a Stefan tube is modelled, mathematically, is very similar to how one can model the diffusion of perfume fragrance molecules from (say) a drop of perfume on skin or clothes, evaporating up through the air to a person's nose.  There are some differences between the models.  However, they turn out to have little effect on results at highly dilute vapour concentrations.