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The central part of our photoacoustic membrane permeation measuring system is a measurement cell into which the membrane is inserted in a way to divide the cell into two hermetically separated volumes, the sample and the measurement volumes. Into the sample volume the component for which the permeation rate is to be measured is injected either in a gaseous form having a well-controlled high concentration in a buffer gas or in a liquid form. The measurement volume is purged initially with a gas having known (low or even zero) concentration of the measured component. The sample volume is connected through gas tubing with a membrane pump and a photoacoustic cell, in a way, that a completely closed volume is produced. The pump is controlled to be switched on regularly and mixes the gas through the gas handling system letting the measured component to enter into the measuring photoacoustic cell. By plotting the photoacoustic signal as a function of time, and with the help of the known volume of the receiving volume the permeation rate can be calculated in units of e.g. gram/m2/day. Advantages of the photoacoustic system 1. Very flexible technique. Various measurement conditions can be implemented. 2. Selectivity and high sensitivity especially due to the small gas volume. 3. Very high dynamic range. Membranes with highly different permeation coefficients can be measured. 4. Measurements can be done on a totally automatic manner. 5. Very stable operation. The system has to be calibrated only yearly (or even less frequently). 6. The measurement time can be considerably reduced, when applying a numerical fitting algorithm to the measured data points.