Permeation
Permeation is the passage of a fluid into, through and out of a solid barrier having no holes. The process involves diffusion through a solid and may involve many phenomena such as adsorption, dissociation, migration and desorption.
Permeation can have an adverse effect on helium leaktest when the leakrate specification is low and the test time long. One material needing special attention is Teflon. Helium has a high permeability through Teflon and helium leaktests on test pieces with Teflon seals are almost impossible.
Permeation does not have to be taken into consideration during routine leaktest, when the measurement occurs in too short a time to permit the influence of permeation.
It can be seen, that with Neoprene it takes about 2 hours to reach saturation, but already after 30 minutes there is a leakrate of 10-9 Pa x m3/s. The gasket dimensions used in determining the graph are:
sealing length = 25 mm
thickness = 5 mm
For a larger container with gaskets of a total length of 1 metre, these permeation rates are 2 x 10-7 Pa m3/s (2 x 10-6 mbar l/s). With natural rubber or silicon rubber the time to saturation is shorter.
Thus we learn from this that, for a vacuum leaktest helium should be sprayed on these gaskets only for a very short time.