Measuring the pumping speed of rough vacuum pumps: conventional stationary method vs intermittent pump-down method (new standard DIN 28432)

Abstract

According to all testing procedures which have been standardized so far, the pumping speed of vacuum pumps is measured by injecting a stationary flow of gas into a test dome and measuring the equilibrium pressure. This stationary method is difficult to automatize due to the problem of automatizing the generation of gas flows over the required range of several decades. Alternatively, the pumping speed of rough vacuum pumps can be measured by evacuating a large vessel and monitoring the pump-down curve. However, one disturbance has to be considered: in the evacuation process, the temperature drops due to the gas expansion and then slowly rises towards ambient temperature due to heat transfer from the surroundings. Since changes of temperature cause changes of pressure, the derived pumping speed is affected. This disturbance can be overcome by interrupting the pumping process by means of an electrically controlled valve and subsequent waiting for thermal equilibrium. The setup required for the intermittent pump-down method is rather simple and the testing procedure can easily be automatized. In the present work, the various physical and technical aspects of pumping speed measurements by both methods are carefully examined. Theoretical considerations show that both methods in fact yield the correct pumping speed if properly applied to rough vacuum pumps. This result is confirmed by comparative measurements.