A pump in which a heater boils low-vapour-pressure oil, and the vapour shoots out of a stacked jet assembly as high-speed downward jets. Gas molecules that drift into the jets are pushed toward the foreline, where a mechanical pump removes them, while the oil condenses on water-cooled walls and drains back to the boiler. Diffusion pumps are cheap, rugged and fast for large furnaces and coaters, but they need a baffle or cold trap against backstreaming and must never run without backing.
Diff pump takes forty minutes to come up to temperature, so switch the heater on before you load the chamber.
Never let air into a hot diffusion pump: hot oil exposed to air oxidizes into tar, so let the boiler cool before the pump sees atmosphere.
Also heard as
- diff pump
- DP
- oil diffusion pump
Related terms
Backstream
Of pump oil or vapour, to flow the wrong way, from the pump back toward the chamber.
Cold trap
Chilled surface in a pumping line that freezes out water and oil vapour before it reaches the chamber or the pump.
Backing pump
Pump that keeps the exhaust of a turbo, diffusion or Roots pump below its maximum allowed pressure.
Crossover
Point in a pump-down where pumping switches from the roughing pump to the high vacuum pump.
Oil creep
Slow spread of a thin oil film along metal surfaces, carrying pump fluid far from where it started.