2.2.4. Determination of the Number of Blades and the Angle *β2p*

The number of blades and the angle *β*2*<sup>p</sup>* were evaluated by following the procedure described by Lobnanoff [37], using the statistical diagram shown in Figure 6. The use of the diagram presupposes the knowledge of the specific speed of the machine and of the ratio between the vacuum head *Hmo* and the head at BEP, *Hm*, available in any case from the catalog. Figure 6 shows the points relating to the range of pumps under examination. The values of the parameters chosen are obtained as a function of the curve that comes closest to these points. Being a statistical diagram, obtained from many tests carried out on different models of machines, in some cases it may provide accurate results, in other cases it may not. For this reason, since the number of blades is a simple value to find, it is obviously preferable to use the known value.

**Figure 6.** Statistical diagram for the determination of *z* and *β2p*.
