Measurement of Pressure for the Determination of the Speed of High-Vacuum Pumps

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Abstract
The results of the determination of the pumping speed employing conventional methods deviate from the product ACef based on concepts of the kinetic theory of gases, where ACe=Aυav/4 is the conductance of an orifice, if the capture probability f is not small. This deviation may be attributed to the technique of pressure measurement. In order to obtain proportionality between capture probability and speed with a unique constant of proportionality in the whole range 0≤f≤1 it is recommended that the pumping speed be defined using the pressure in a large vessel at a place where the isotropic state is maintained to a high degree. This speed is named intrinsic speed and it may be measured using Knudsen's large chamber fitted with a proper gas inlet system. The measurement of very large pumps according to this definition however may be carried out by applying a conversion for the incidence density in the pump mouth to an isotropic pressure. The considerations presented in this paper are intended as a contribution to an international standardization of the speed measurement of high-vacuum pumps. They are based on molecular flow in the test dome.

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