This paper presents the results of measurements to determine the average size and number of droplets which appear during spontaneous condensation of expanding undercooled steam. The measurements were carried out in a non-symmetric convergent–divergent nozzle for three expansion rates, , of 2000, 5000, and 8000 units per second, and pressure in the Wilson region of 0·4–1·5 bar. The method is based on the measurement of monochromatic light beam intensity attenuation when passing through a layer of wet steam of length l (width of the nozzle) and static pressure variation through the nozzle axis. The photometric method was verified by measuring the droplet sizes in sulphur emulsion. The average size and number of droplets during spontaneous condensation were found to be dependent on the expansion rate and pressure in the Wilson region, and the results of the measurement are in qualitative agreement with those obtained by Gyarmathy. The average droplet size measured by the photometric method is about 25–60 per cent greater than the corresponding values of Gyarmathy. The droplet size measuring error, determined by an approximate analysis, was found to be in the range 15–30 per cent.
