As applied to particulate filtration this is the largest sized particle that will pass. However, it is not really applicable to fibre depth type filter elements. This is because the largest particle depends on the pressure drop, because not all particles are round and because the beta ratio is not a constant. In the past it had been stated that a particle size giving a beta ratio of 75 for the filter element was considered to be the absolute rating.

Particle size in micrometers removed at a given efficiency under a manufacturer's defined test condition. Implied is 100%, but more often defined as 98.67%, 99%, 99.9% and 99.99%, according to manufacturer.

The size of microns of the largest spherical particle that will pass through the element.