In order to detect extremely low gas transmission rates through a sample, it is necessary to abandon direct flow measurement because flow meters are not very accurate. Diffusion Permeameters determine the low gas flow rates indirectly by measuring time rate of change of pressure on the outlet side.
The sample was loaded in the sample chamber which divides in two chambers. One chamber is on the inlet side of the sample and the other is on the outlet side of the chamber. The exact volume of outlet chamber is determined. The chambers on both sides of the sample are evacuated. Then a quantity of test gas is introduced to the chamber on the inlet side of the sample, its pressure is kept almost constant, and the pressure is monitored on both sides of the sample. The two pressures are recorded as functions of time. The data are used to compute transmission rate. In order to obtain the utmost accuracy, the system is maintained at a constant temperature.
