The principle of minimum entropy production appears as a new unifying concept for the description of condensation and evaporation. The Chapman-Jouquet condition for these phenomena is equivalent to this principle and gives new relations for accurately measurable quantities such as pressure, density, and amount of condensate. To compare the predictions of these new relations we carry out an experiment for the onset of condensation of an argon-steam mixture in the expansion fan in a shock tube. The density change is measured by a Mach-Zehnder interferometer simultaneously with pressure. A numerical simulation (finite volume method of second order accuracy) allows the comparison both with the classical Volmer-Becker-Döring-Zeldovich theory and with the new Dillmann-Meier and Delale-Meier theories which are more successful for very fast changes of flow parameters.