The equilibrium pressure of a substance's vapour above its liquid or solid at a given temperature. Any material whose vapour pressure exceeds the target pressure keeps evaporating: water at 20 °C sits at about 23 mbar, so a single drop inside a chamber holds the pressure up until it is gone, and zinc, cadmium and brass are banned from heated UHV parts. Diffusion pump fluids, vacuum greases and epoxies are chosen for vapour pressures around 10⁻⁸ mbar or lower.
Those screws are cadmium plated, the vapor pressure's way too high for a bakeable chamber, pull them.
Also heard as
- vapour pressure
- saturation vapor pressure
Related terms
Outgassing
Slow release of gas from the surfaces and bulk of materials inside a vacuum system.
Fogging
Mist that forms inside a chamber during a fast pump-down when expanding air cools and its water condenses.
Wet
Said of a chamber whose walls still hold adsorbed water, so it pumps down slowly and stalls high.
Backstream
Of pump oil or vapour, to flow the wrong way, from the pump back toward the chamber.
Milky oil
Pump oil turned cloudy or creamy by an emulsion of water, a sign the pump has been swallowing wet loads.