Migration of oil vapour, droplets or cracked hydrocarbons from a pump upstream into the foreline or the chamber. Diffusion pumps backstream when the cold cap is not cooled or the foreline pressure rises past the critical backing pressure, and rotary vane pumps backstream oil into an idle line held below about 0.1 mbar for long periods. The defences are cold caps, baffles, cold traps, foreline traps and never leaving a mechanical pump pulling on a dead-ended line.
Somebody left the mech pump sitting on the closed foreline all weekend, it backstreamed and the line's coated in oil.
Also heard as
- backstreaming
- oil backstreaming
Related terms
Diffusion pump
High vacuum pump with no moving parts that drives gas toward the exhaust with downward jets of hot oil vapour.
Cold trap
Chilled surface in a pumping line that freezes out water and oil vapour before it reaches the chamber or the pump.
Rotary vane pump
Oil-sealed positive displacement pump in which sliding vanes sweep gas around an off-centre rotor.
Oil creep
Slow spread of a thin oil film along metal surfaces, carrying pump fluid far from where it started.
Dry pump
Vacuum pump with no oil or liquid in its pumping chamber, used where oil vapour would contaminate the process.