Vacuum flow – how is it measured?
In order to obtain pressure lower than atmospheric pressure in a container, some of the air mass must be removed by a vacuum pump. For example, half the air mass must be removed to obtain a vacuum level of 50 -kPa. The air evacuated by the pump per unit of time is called the vacuum flow and is a measure of how quickly the pump can perform this function.
Many manufacturers of mechanical vacuum pumps state vacuum flow in terms of the pump’s displacement volume. This flow is called “displacement flow” or “volume flow”. Displacement flow equals the chamber volume times the number of revolutions per unit time. In mechanical pumps, this value is constant and can lead the observer to think, incorrectly, that the vacuum flow is constant during the entire evacuation process.
In the evacuation process the air actually becomes thinner and thinner for every stroke of the cylinder until the pump reaches the maximum vacuum level which is that point where the vacuum flow would then be zero. The pump is still pumping the same volume flow but the air mass is so thin that compared to air at normal atmospheric pressure it is as if there was no air.
To account for the change in air mass during the evacuation process Piab provides flow data in terms of normal litre per second (Nl/s). Also called free air flow, this method normalizes the flow to standard atmospheric conditions. As the vacuum becomes deeper and the air is thinner, a higher actual volume must be displaced to evacuate each normal litre. The table below lists one pump’s performance in terms of displacement flow (l/s) and free air flow (Nl/s). At zero vacuum, the flows are equal. This is because the actual conditions are in fact standard conditions. But as the vacuum level increases, the values diverge. At 50 kPa (50%) vacuum, the displacement flow figure is twice the free air flow figure. At deeper vacuum levels, the difference is even greater.