3.2.2. Influence of Compressibility

When performing tests on low-pressure or medium-pressure fans in standardized test benches, it can be assumed that the air velocity is so low that its influence on density can be neglected. Thus, for fan pressures less than 2000 Pa , it is permissible to disregard the change in density (compressibility) between the fan inlet and outlet. Furthermore, other compressibility effects at reference velocities not exceeding  65\frac{m}{s}  may also be neglected.

For tests of high-pressure fans, on the other hand, a distinction must be made between the standstill values and the static values of temperature, pressure and density.