Gas evolving from inside the system itself: water and other molecules desorbing from surfaces, gas diffusing out of the bulk (hydrogen from stainless steel), and vapour from plastics, adhesives and lubricants. Rates are quoted per unit area in mbar·L/(s·cm²); unbaked stainless steel gives off around 10⁻⁹ after about ten hours of pumping, falling roughly in proportion to one over time. Outgassing, not leaks, sets the base pressure of most high vacuum systems.
Pressure isn't a leak, it's outgassing from that new cable bundle, it'll come down if we bake it.
Wall outgassing falls roughly as one over pumping time, so doubling the pump time roughly halves the gas coming off unbaked surfaces.
Also heard as
- offgassing
Related terms
Bake-out
Heating a vacuum system while pumping to drive off water and other adsorbed gas and lower the base pressure.
Wet
Said of a chamber whose walls still hold adsorbed water, so it pumps down slowly and stalls high.
Permeation
Gas passing through the solid walls or seals of a vacuum system by dissolving into and diffusing through them.
Virtual leak
Gas trapped in a hidden pocket inside the vacuum that bleeds out slowly and behaves like a small real leak.
Gas load
Total gas flow a pumping system must remove, summed from process gas, leaks, outgassing and permeation.