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.
What gas ballast does
When a rotary vane pump compresses vapour-laden gas, the vapour can condense back into liquid inside the pump before it’s exhausted — ruining the oil and destroying the vacuum. Gas ballast adds a measured amount of air during compression so the mixture is exhausted as vapour before it can condense. It’s one of the most useful features on an oil-sealed pump.
When should you open the gas ballast?
- Whenever the process releases water or solvent vapour — drying, distillation, freeze-drying.
- During warm-up, to help clear moisture from the oil.
- If the oil has gone cloudy or milky (then change it — see how often to change vacuum pump oil).
The trade-off: ultimate vacuum
Because gas ballast deliberately lets a little air in, it raises the ultimate pressure the pump can reach. So once the vapour has cleared, close the gas ballast to pull down to the deepest vacuum. Leaving it open by mistake is a common reason a pump won’t reach its ultimate vacuum.
How to use it
Most pumps have a simple knob. The Edwards E2S, for example, has three positions — closed (0) and two open settings (I and II) — plus an optional solenoid valve for automatic control. Open it for wet running, close it for dry running.
Questions about running your pump?
Our engineers can advise on gas ballast, oils and accessories for any make of pump. Email [email protected] or request a quote.
Frequently asked questions
What does gas ballast do on a vacuum pump?
It admits a controlled amount of air during compression so water and solvent vapours are exhausted instead of condensing in the oil. This protects the oil and keeps the pump working when handling vapour.
When should I open the gas ballast?
Open it whenever the process releases water or solvent vapour – drying, distillation, freeze-drying – and during warm-up. Close it once the vapour has cleared to reach the deepest vacuum.
Does gas ballast affect ultimate vacuum?
Yes. Because it deliberately lets a little air in, gas ballast raises the ultimate pressure slightly. Close it for dry running to reach the pump’s deepest vacuum.
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.