In practice measurements during the design process or for final inspections often cannot be carried out on full scale machines. Mostly it is too costly to build an own test rig for each machine, so test rigs with standardized diameters and measuring equipment are used. Sometimes the diameters of the real scale machines are so large that a measurement is utterly impossible. Examples are water turbines or fans used for cooling in power plants – these machines may have diameters of several meters and require a brake respec- tively drive power in the MW range. On the opposite machines with diameters of only a few millimeters do not allow the operation of established invasive measuring techniques – such as Pitot-tubes, five hole probes or hot wire anemometry – because of the signifi- cant influence of the probes on the flow field. Hence measurements to determine the performance are carried out on models which are down or up scaled. As was shown by Spurk (1992) a full similarity between model and prototype machine is in the context of turbo- machines possible as long as the Mach number is small, i.e. the compressibility can be neglected