A small valve that admits air or dry nitrogen into the compression stage of a rotary vane pump, so the exhaust valve opens before condensable vapour such as water reaches its saturation pressure. The vapour leaves with the ballast gas instead of condensing into the oil. Running with ballast open raises the ultimate pressure, often by about a factor of ten, and increases oil loss, so techs open it for wet loads and close it once the oil has cleared.
Open the gas ballast for an hour after the freeze-dryer run, the oil's picked up a lot of water.
An hour or two of running hot with the ballast open will often clear water-contaminated oil without an oil change.
Also heard as
- ballast
- GB
Related terms
Rotary vane pump
Oil-sealed positive displacement pump in which sliding vanes sweep gas around an off-centre rotor.
Milky oil
Pump oil turned cloudy or creamy by an emulsion of water, a sign the pump has been swallowing wet loads.
Vapor pressure
Pressure at which a liquid or solid is in balance with its own vapour, which decides what can go inside a chamber.
Mech pump
Lab slang for the oil-sealed rotary vane pump that sits under the bench or behind the tool.