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The beta ratio is a rating system introduced with the object of giving both filter manufacturer and filter user an accurate and representative comparison among various filter media. It is the ratio between the number of particles per unit volume
above a specific size in the suspension upstream of the filter to the same parameter in the flow downstream of the filter, and it is determined in a test rig that enables accurate particle counting in the two flow regions. The beta ratio is then:
where: 
is the beta ratio for particles larger than x μm, Nu is the number of particles per unit volume larger then x μm upstream, Nd is the number of particles per unit volume larger then x μm downstream.
It follows that the higher the value of the beta ratio, the more particles of the specified size, or greater, are retained on the filter. The filter efficiency at this particle size can then be determined from the beta ratio:
The beta ratio and the corresponding efficiency are illustrated in Table 1.5 in a test where the filter was challenged with 1 million particles per unit volume (the numbers in Table 1.5 refer, of course, to the same particle size).
