Means through which air or another non-condensing gas is admitted into a vacuum pump are referred to as gas ballast.
If the pump is displacing vapour that would condense in the pump at the corresponding temperatures without gas ballast, the gas ballast enables the outlet valve to open before the vapour condenses. The vapour is discharged together with the ballast gas. Both atmospheric air as well as selected inert or process gases are used as ballast gas. The use of gas ballast slightly increases the attainable base pressure of a vacuum pump. Thus, the base pressure is specified both with and without gas ballast for gas ballast vacuum pumps.