1.1 This part of ISO 1608 specifies a method of measuring the critical backing pressure of vapour vacuum Pumps. NOTE - The critical backing pressure is that backing pressure above which the operating conditions of the pump are affected in such a manner that its Performance ceases to be satisfactory. The dependence of the Performance of a vapour pump on the backing pressure tan only be completely described by means of a curve relating the inlet and backing pressures over the range of Operation. In many cases it is adequate to specify the critical backing pressure by a Single Parameter, which is defined in 2.1. In some cases, however, especially where Ultra-high vacuum performante is of interest, or where gases such as hydrogen and helium are concerned, the complete curve may be required. 1.2 The Pumps considered comprise the following three classes of oil and mercury vapour Pumps: - vapour jet vacuum pump; - diffusion Pumps; - diffusion-ejector Pumps.