Example: Diffusion of Search Gas in Air

The diffusion of one gas into another at atmospheric pressure is relatively slow (and even slower at higher pressures). This requires special attention when test pieces are filled with search gas for a sniffer test. With long tubes with closed ends, the search gas cannot bypass the air which is in the test piece as far as the end of the tube. This air cushion can remain there for some time, before it is mixed with the other gases by diffusion. A leak, which happens to be at this end of the tube, may not be detected by the sensor, which is only sensitive for the search gas.

To avoid this problem, test objects should be evacuated, prior to filling with searchgas. Not a very good vacuum is needed. The diffusion speed increases with decreasing pressure. This can be explained by the expanding mean free path and therefore less collisions. If a filling of 100% search gas is wanted, the evacuation down to a pressure of 1 to 5 mbar is good enough (1/1000 - 5/1000 of atmospheric pressure).