Oil back-migration (or back-streaming) is when oil vapour from an oil-sealed vacuum pump travels back up the inlet into your chamber, contaminating the process. It matters most for clean, analytical and coating work. You prevent it with a foreline trap, correct operation, or by using an oil-free pump.
Why it happens
All oil-sealed rotary vane pumps produce some oil vapour. At low pressures, especially if the pump is left running against a closed or high-vacuum system, that vapour can migrate back toward the chamber.
How to prevent it
- Fit a foreline trap (molecular sieve or cold trap) between pump and chamber to catch oil vapour.
- Don’t leave the pump running at ultimate vacuum against a closed system longer than needed.
- Vent correctly and use an anti-suck-back valve so oil isn’t drawn back when the pump stops.
- Use an oil-free pump (scroll or diaphragm) where contamination cannot be tolerated – see scroll vs rotary vane.
When it matters
For general industrial vacuum, a little oil vapour is harmless. For analytical instruments, coating and sensitive lab work, back-migration can ruin results – which is why many labs choose oil-free pumps.
Worried about oil contamination?
We can supply foreline traps, or an oil-free pump if your process demands it – just ask. Email [email protected] or request a quote.
Frequently asked questions
What is oil back-migration in a vacuum pump?
It is when oil vapour from an oil-sealed pump travels back up the inlet into the chamber, contaminating the process. It is also called back-streaming.
How do I stop vacuum pump oil back-streaming?
Fit a foreline trap (molecular sieve or cold trap), avoid running the pump at ultimate vacuum against a closed system, use an anti-suck-back valve, or switch to an oil-free scroll or diaphragm pump.
Do oil-free vacuum pumps have back-migration?
No. Scroll and diaphragm pumps have no oil in the pumping chamber, so there is no oil back-migration – which is why they are preferred for clean and analytical work.
Written by the Girovac technical team. Girovac Ltd has supplied and serviced industrial and laboratory vacuum equipment from its North Walsham workshop since 1983. Last updated: July 2026.