Transport of gas through a solid: it dissolves at the outer surface, diffuses through the material and escapes on the vacuum side. Elastomer O-rings pass helium, water and air, one reason elastomer-sealed systems rarely get far below 10⁻⁸ mbar; glass passes helium, and hydrogen permeates steel. Permeation climbs steeply with temperature and does not fade with pumping time, which separates it from outgassing.
Helium signal creeps up slowly then sits there, that's permeation through the door O-ring, not a leak.
Also heard as
- gas permeation
Related terms
Outgassing
Slow release of gas from the surfaces and bulk of materials inside a vacuum system.
O-ring
Elastomer ring with a round cross-section that seals demountable vacuum joints by being squeezed into place.
Hang-up
Helium that lingers in a leak detector or system after a big hit, holding the signal high and masking small leaks.
UHV
Ultra-high vacuum: pressures below about 10⁻⁷ mbar, reached only with baked, metal-sealed systems.