Helium could diffuse through the steel and glass from the outside air, but this effect is usually negligible due to the low abundance of He in the atmosphere.
[4] UHV pressures are measured with an ion gauge, either of the hot filament or inverted magnetron type.
In any vacuum system, some gas will continue to escape into the chamber over time and slowly increase the pressure if it is not pumped out.
While some gas release is inevitable, if the leak rate is too high, it can slow down or even prevent the system from reaching low pressure.
For example, vessels lined with a highly gas-permeable material such as palladium (which is a high-capacity hydrogen sponge) create special outgassing problems.
Water evaporates from surfaces too slowly to be fully removed at room temperature, but just fast enough to present a continuous level of background contamination.
Removal of water and similar gases generally requires baking the UHV system at 200 to 400 °C (392 to 752 °F) while vacuum pumps are running.
In order to reach low pressures, it is often useful to heat the entire system above 100 °C (212 °F) for many hours (a process known as bake-out) to remove water and other trace gases which adsorb on the surfaces of the chamber.
To save time, energy, and integrity of the UHV volume an airlock or load-lock vacuum system[6] is often used.
After sample introduction and assuring that the door against atmosphere is closed, the airlock volume is typically pumped down to a medium-high vacuum.
While the initial workpiece is being processed under UHV, a subsequent sample can be introduced into the airlock volume, pre-cleaned, and so-on and so-forth, saving much time.
Metal seals, with knife edges on both sides cutting into a soft, copper gasket are employed.
Although generally considered single use, the skilled operator can obtain several uses through the use of feeler gauges of decreasing size with each iteration, as long as the knife edges are in perfect condition.
At 0.1 millipascals (7.5×10−7 Torr), it only takes 1 second to cover a surface with a contaminant, so much lower pressures are needed for long experiments.