The calculation of measuring radon exhalation rate in open-loop is simple, without considering the effects of leakage and back-diffusion. But the pump flow rate is a very important parameter in the measurement. If the exhalation rate of the medium is high, the time for the radon concentration in the chamber becomes constant decrease while the pump flow rate is high. The measurement efficiency will be improved. If the exhalation rate of the medium is low, the radon concentration in the chamber will be very low for measuring while the pump flow rate is high, and the statistical fluctuation is la...