How often should vacuum pump oil be changed?

On clean, dry air duty: every 3,000 to 6,000 running hours. But that number is a starting point, not a rule — what actually sets the interval is what you pump, not how long you pump it.

The reason is mechanical. In an oil-sealed rotary vane pump the oil does three jobs at once: it seals the clearance between vane tip and cylinder wall, lubricates the sliding vanes, and carries heat out of the compression space. Anything that degrades the oil attacks all three at the same time. That is why a pump on solvent vapour can lose ultimate pressure in a few hundred hours while an identical pump backing a clean load runs a year without attention.

Judge the oil, not the calendar. Three checks, in order of how much they tell you: colour and clarity in the sight glass (fresh oil is straw-coloured and transparent; darkening means thermal load, milky means condensate, black with suspended solids means wear particles); ultimate pressure measured at the pump inlet with a blank flange (a pump that used to reach 5×10⁻³ mbar and now stops at 5×10⁻² mbar is usually telling you about its oil before it tells you about its vanes); and oil level, which falls if the exhaust mist filter is saturated and rises if process vapour is condensing into the sump.

Shorten the interval when you pump condensable vapour (water, solvents), when the process carries dust or particles, when the pump runs hot or near its ultimate pressure for long periods, or when the duty is intermittent enough that the pump never gets warm enough to boil off condensate. Lengthen it only with evidence — measured ultimate pressure plus clean oil.

One correction worth making: 'the pump smells hot' and 'the oil looks dark' are not the same signal. Dark oil that still reaches rated ultimate pressure is usually fine for the rest of its interval. Oil that is still light-coloured but the pump has lost a decade of ultimate pressure points at the vanes or at a leak on the inlet side, and changing the oil will not fix it.

A practical routine: change the oil, run the pump 20–30 minutes against a blank flange to warm it, then record the ultimate pressure. That number is your baseline. Every later reading is measured against it, and you will know when the oil is done without guessing.