How to choose a diffusion pump
A diffusion pump boils a low-vapour-pressure fluid, sends the vapour up a chimney and out through angled nozzles as a supersonic jet aimed downwards. Gas molecules that wander into the jet are driven towards the exhaust and swept away by the backing pump. There are no moving parts, so the pump is quiet, cheap to own, tolerant of particles and effectively immortal if the fluid and cooling are looked after.
Choose pumping speed and inlet diameter for the chamber, then set the supporting details: the backing pump must hold the foreline below the diffusion pump's critical backing pressure at all times, cooling water flow and temperature must meet spec, and the fluid — silicone or polyphenyl ether — has to suit the process and its bake-out temperature. A properly chosen cold trap or baffle is what keeps backstreaming out of the chamber, and it costs some speed.
Typical duties: vacuum furnaces and heat treatment, metallising and coating plants, electron-beam welding, freeze drying and large chambers where robustness beats cleanliness. Where budget and reliability matter more than an oil-free environment, a diffusion pump is still hard to beat.
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FAQ
Is a diffusion pump still worth using?
For large-volume, high-throughput high vacuum where the process tolerates hydrocarbon or silicone oil, yes — it has no moving parts, is cheap for its pumping speed and is very tolerant of process upsets. Where backstreaming matters, a turbo pump has replaced it.
How do I stop oil backstreaming into the chamber?
Fit a water-cooled baffle or a liquid-nitrogen trap between pump and chamber, never open the high-vacuum valve before the pump is hot and stable, and do not run the pump above its maximum inlet pressure. Most backstreaming problems come from operating sequence, not from the pump.
How often does the pump fluid need changing?
When it darkens or the pump no longer reaches its rated pressure. Fluid life is destroyed by air inrush at operating temperature — a single accidental vent while hot can crack the oil and require a full change and cleaning of the jet assembly.