The downstream volume is closed and it is empty at the beginning of the simulation run. Thus the throwed gas molecules stay in the downstream volume and the downstream pressure increases. The permeation curve has got a saturation plateau. The permeability of the membrane and the diffusion coefficient can be determined by the measurement of the slope of the first part of the curve. There is an error on the determination of the diffusion coefficient and the permeability because of the downstream volume size, but usually permeation cells are dimensioned in order to give a sufficient accuracy.
Example 1, case of a permeation cell :