Gas permeation.
Example 2, case of a continuous process :

It is the cas of an industrial process. The downstream volume is opened and the molecules can go out. Usually the downstream pressure is not nil. The permeation curve has got no saturation and the stationary regime of permeation is characterized by a constant flux of molecules with time. A time-lag is observed at the beginnig of the curve as same as in the permeation cell but it can be neglegted after a long time.