In chemical engineering, a Stefan tube is a device that was devised by Josef Stefan in 1874.[1] It is often used for measuring diffusion coefficients.[1][2] 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.[1][3][4] 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.[1][3] One then measures the fall in the level of the liquid in the tube.[4]
The tube conventionally has a narrow diameter, in order to suppress convection.[4]
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.[5]