6
5. Pump Performance
5.1 Pressure Achieved
The term “pressure achieved” as employed in the catalogue and in this manual is defined as
“the minimum pressure obtained by the pump without introduction of gas from the pump inlet
(i.e. the no-
load condition)”.
Note that the indicator values for pressure may differ between types of vacuum gauges.
The pressure achieved in practice is higher than that noted in the catalogue for the following
reasons.
(1) The fact that the vacuum gauge is mounted a distance from the pump, the steam
generated by water droplets and rust etc on the inside walls of the pump and piping, and a
variety of gases present in the system result in increased pressure.
(2) Leaks into the vacuum system introduce other gases, resulting in increased pressure.
5.2 Evacuation Rate
The rate of evacuation varies with the type of gas used, and its pressure. The maximum rate
of evacuation is reached when air is introduced, and slowly decreases as pressure is reduced.
The resistance of the piping system increases with small bore piping which extends over long
distances, and this reduces the rate of evacuation.
The declared rate of evacuation for this pump is the maximum value achieved with dry air.
5.3 Power Requirements
The power required to drive the pump is the total of the work required to overcome the
rotational resistance of the pump (mechanical work), and the work required to compress the
air (compression work), and is at a maximum at an inlet pressure of 2.7 x 10
4
- 6 x 10
4
Pa.
If pressure drops below this level, compression work is considerably reduced and power is
expended in mechanical work.
