The two main methods of specifying removal ratings for filters are nominal rating, and absolute rating. Nominal rating typically means that 90% (or sometimes 95%) by weight of the contaminants above a specified size (e.g. 10 μm) has been removed. The 2% (or potentially 5%) of the contaminant passing through the filter is not defined by the test. Therefore, it is possible to have particle much larger than the nominal size (e.g. 30 μm to 100 μm).
Absolute rating can be defined by one of two standards. The National Fluid Power Association’s (NFPA’s) standard of absolute rating states that the diameter of the largest hard spherical particle that will pass through a filter under specified test conditions is an indication of the largest opening in the filter. The Beta (β) Rating System determined by the Oklahoma State University, “OSU F-2 Filter Performance Test” determines the ratio of the number of particles of a given size in the influent divided by the number of particles of the same given size in the effluent. This results in the following equation for relating the β value to removal efficiency:
% removal = (β – 1) / β x 100 Eq 7-15
Most “absolute” filters typically have a β of 10,000 (99.99% removal).