Measure for the effectiveness of a filter. The value is a ratio that is defined by the number of particles before and after passing through the filter with a specific pressure drop (Figure B 13):
Therefore at β x = 1 no particles are removed; 50 % of the particles are removed at β x = 2. The minimum practical value aimed for today β x = 75 then means that 98.7 % of the particles of dirt of particle size x have been filtered out. This means that even higher β values have no practical significance.
Simply stating the β x value is insufficient. Instead all parameters related to defining the β value must be stated, i.e. the complete test report. In ISO 4572 only the test procedure for particle size x = 10 μm is standardised. For all other particle sizes the boundary conditions must be selected based on this standard.