The filter medium performs the filtering action: it separates the solid particles from the liquid. There are mainly three different filter media available:
This standard has been created to give both filter manufacturers and users an objective and accurate standard that can make a representative comparison between filter media. Determining this standard is done via the multipass test. Test fluid is constantly mixed with contaminant so that there is a constant contamination level. Then oil samples are taken upstream and downstream of the filter. The beta ratio is the ratio of the number of particles larger than a certain value of x (μm) before the filter to that after the filter. If a filter has a beta ratio greater than or equal to 75, this filter can be called an absolute filter. The example above is therefore an absolute 3 μm filter.
When the Beta ratio is proposed, it must always be related to a specific particle size (micronage or filter fineness), for example:
Incidentally, these are the efficiencies of the same filter, but measured differently.