Abstract: A water vapor generator having electrodes in a vaporization vessel is disclosed. Current passed through the water between electrodes causes water in the vessel to heat and boil off as steam, make-up water being added when the current drops below a preset minimum value. The make-up water is cut off when a preset maximum value is reached. Thus, the generator operates through successive cycles each containing a descending "boil" leg and an ascending "fill" leg. As water is boiled, its mineral content becomes more and more concentrated, causing increasing its conductivity. To return the water conductivity to its design value the frequency of the boil/fill cycles is measured over several cycles and compared with a predetermined value. If the measured frequency is higher, this indicates a higher than desired water conductivity and a drain cycle is initiated by operating a drain valve. The amount of water drained out of the vessel is proportional to the deviation of frequency from the predetermined value.