Procedures for measuring pumping speeds

Abstract

Vacuum pumps of the same type and of about the same size offered by different manufacturers frequently have minor or even larger differences in their specifications. Thus the question of the significance of these differences arises. Data by different manufacturers are generally obtained by different apparatuses and even worse-by applying different measuring procedures. The procedures for measuring pumping speeds recommended by ISO, DIN, PNEUROP, and AVS are, unfortunately, far from being identical. A particularly interesting case is the turbomolecular pump since its pumping speed is stable and since—depending on the pressure—two methods are employed for measuring the pumping speed, i.e. the flow-meter method and the orifice method. Experience in calibration of this pump reveals two basic problems: first, in practice the application of two methods is unnecessarily tedious, and second, the values obtained by both methods show unsatisfactorily large differences, when only the modest demands of the recommended accuracies are fulfilled. Apparently further work is required to define procedures for reliable measurements of pump specifications.