A method for the measurement of radon concentration in water is presented based on flushing a water sample with air in a closed loop with the Lucas cell as alpha radiation detector. The main feature of the method is washing radon away from a large sample of water (0.75 l) to a small volume of air, approximately 0.5 l, thanks to which a high radon concentration in air and a considerable sensitivity of measurement is achieved. Basic relations and results of measurements of a model of the gauge are given. The estimated measuring sensitivity (S) is 8.5 (cpm)/(Bq/l). The random error due to the statistical fluctuations of count rate at radon concentrations 1, 10, 100, 1000, 10 000 Bq/l is 11, 3.6, 1.1, 0.4, 0.1%, correspondingly at a counting (measuring) time of 10 min. The minimum detectable radon concentration in water is 0.11 Bq/l.

(in Polish)