A vacuum sweeper probe of a type introduced by previous investigators was evaluated for use in the study of heat and mass transfer during evaporation. A simple theory of the probe behavior was formulated and checked by experimental measurements over a substantial range of operating conditions. The results revealed that in the interpretation of experimental data the validity of some assumptions usually made is strongly dependent upon seemingly minor features of probe construction and operation. Errors as large as 200% were encountered when the gas flow distribution in the probe was not symmetrical. Consequently, some results now in the literature are open to question.
To test the technique and theory as well as to introduce a subsequent study of the effect of monolayers on evaporation, the evaporation rate from a clean water surface was measured over a temperature range of 2° to 22°C. The role of natural convection was clearly reflected in the increase of Nusselt number as the water bulk temperature passed through the density inversion at 4°C. A relation between Nusselt and Rayleigh numbers was determined and found to be consistent with earlier results of others.