Vacuum is measured as an absolute pressure below atmosphere, and the lower the number, the deeper the vacuum. The common units are the millibar (mbar), Torr, pascal (Pa) and, in the US, microns and PSI. As a quick guide: 1 mbar = 0.75 Torr = 100 Pa, and atmospheric pressure is about 1013 mbar (760 Torr).
Pumping speed is the volume of gas a vacuum pump removes per unit of time — quoted in m3/h for rough and medium-vacuum pumps, or litres per second (l/s) for high-vacuum pumps. It determines how quickly you can pump down a chamber. It is not the same as ultimate vacuum, which is the deepest pressure the pump can reach.
Vacuum is divided into four ranges: rough (atmosphere to ~1 mbar), medium (1 to 10-3 mbar), high (10-3 to 10-7 mbar) and ultra-high vacuum, UHV (below 10-7 mbar). Each range needs different pump technology, and high and ultra-high vacuum always need a backing pump beneath the main pump.
Gas ballast is a valve that admits a small, controlled amount of air into an oil-sealed rotary vane pump during compression, so that water and solvent vapours are carried out with the exhaust instead of condensing inside the pump and contaminating the oil. It slightly raises the ultimate pressure, so you open it when pumping vapour and close it to reach the deepest vacuum.
A vacuum pump that has become noisy is usually telling you something. The common causes are low or contaminated oil, worn bearings, worn or sticking vanes, a sticking anti-suck-back valve, cavitation from an air leak, or loose mountings. The type of noise — rattle, knock, whine or gurgle — points to the cause.
If your vacuum pump is leaking oil, the usual causes are a worn shaft seal, a failed gasket or O-ring, overfilling, back-pressure from a blocked exhaust, or oil mist from a missing or spent exhaust filter. The first step is to work out whether you have a genuine liquid leak or oil mist coming from the exhaust — they have different fixes.
A single-stage rotary vane pump has one compression stage and reaches an ultimate vacuum of around 10-1 mbar; a two-stage pump passes the gas through two stages in series and reaches around 10-3 mbar — a deeper, more stable vacuum. Choose single-stage for rough and medium duty at high throughput, and two-stage where you need a deeper, cleaner vacuum.
A dry scroll pump is oil-free and clean, reaching around 10-2 mbar; an oil-sealed rotary vane pump reaches a deeper vacuum (down to ~2×10-3 mbar for a two-stage) at a lower purchase cost, but needs oil changes and carries a small risk of oil back-migration. Choose a scroll pump for clean, oil-free duty, and a rotary vane pump for the deepest vacuum at the lowest cost.
For most laboratories, the right pump depends on the task: a diaphragm pump for chemistry, filtration and rotary evaporation (oil-free, ~1–20 mbar); a two-stage rotary vane or scroll pump for freeze-drying and general vacuum (~10-3 to 10-2 mbar); and a turbomolecular pump backed by a scroll pump for high-vacuum analytical instruments. Match the pump to the application, not the other way round.
Reconditioning a vacuum pump is usually worth it when the pump body and motor are sound and the fault is wear-related — worn vanes, seals or bearings. A professionally reconditioned pump costs far less than a new one, comes fully tested to its original ultimate vacuum, and keeps a proven pump in service. It’s usually better to replace when the pump is undersized, obsolete beyond spares, or the casing is damaged.