Isobaric measurement of gas permeability of polymers

The principles and design of a gas permeability measuring instrument based on thermal conductivity measurement are described. Since the thermal conductivity of a gas mixture is dependent upon the partial pressure fraction rather than absolute partial pressure of sample gas, and the permeation rate of reference and sample gases through polymer films differe considerably, a pressure‐equalizing device is necessary for the accurate measurement of gas permeability. The three types of measurements—integral, differential (flow method), and decay rate measurements—can be used with the instrument. The results of permeability constants and diffusion constants obtained with the methods showed good agreement with the conventional vacuum‐type method. With proper selection of methods, the instrument can measure the gas flux through the range of 10−10 to 10−3 cm3 (STP)/cm2 sec cm Hg. Some advantages of the methods are discussed.