radiation monitor 79 is completed the radiation monitor 79 is connected to a conversion unit 95 which can include a cpu that is programmed to make calculations from the signals received from the radioactivity monitor to determine the amount of hto collected during a given period of time for a known flow of methane gas from such readings and the knowledge of the amount of liters of gas flow and the length of time during which the test was carried out the unit 95 is programmed to provide a readout in the form of the number of grams of water i e hto which permeate through the sample 23 being tested per square meter per day or other desired unit of time under ambient conditions as an example of the overall operation an appropriate sized sample 23 of a barrier film to be tested is carefully installed in the mounting device 11 so that it rests upon the upper surface of the protruding square cross section o ring 29 in the lower half 19 of the device as best seen in fig2 a the upper half 17 is then carefully set in place and the carriage bolts and nuts 41 installed so as to clamp the film 23 securely between the mating o rings 29 and to seal the central receptacle about its periphery by the o ring 39 the ball valve 49 is then opened so as to allow the upstream subchamber 31 to be filled with an hto partial pressure at.