Filtration Efficiency describes a material's ability to capture particles of various sizes. Filtration efficiency ratings however, may often be misunderstood. For example:
Nominal rating tends to be an arbitrary micrometer value, often supplied by the filter manufacturer. These ratings have little value, as tests show that particles as large as 200µ will pass through a nominally-rated 10µ filter.
Absolute rating, which is a more common rating for filters, gives the size of the largest particle that will pass through the filter media. However, no standardized test method has been developed or agreed upon to determine its value. However, absolute ratings are better than nominal ratings to determine the effectiveness of a filter.
Beta rating is the best and most commonly used filtration efficiency rating in the filtration industry. Beta rating comes from the Multipass Method for Evaluating Filtration Performance of a Fine Filter Element (ISO 16889:1999).
To test a filter, particle counters are used to accurately measure the size and quantity of upstream particles per known volume of fluid, compared to the size and quantity of particles downstream of the filter. The ratio is defined as the upstream particle count divided by the downstream particle count, at the rated particle size. Using the beta rating, a 5µ filter with a beta 10 rating will have on average 10 particles larger than 5µ upstream of the filter for every 5µ or greater particles downstream.