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Sievability and separability are two major criteria which have been used in the past to appraise the performance of filters. Although both criteria have significance in rating filters, it has been demonstrated in practical applications that such ratings cannot reflect the entire spectrum of filter performance. Test data obtained from more than 1200 different filters tested in the past decade at the Fluid Power Research Center at Oklahoma State University indicate that two major intrinsic filtration parameters must be addressed to appropriately and accurately assess the characteristics of a filter. They are separability (power to capture) and retentivity (power to retain). The Epsilon Rating Method was developed from the Elutivity Theory, which considers the separability (Beta Filtration Ratio), the retentivity (Delta Ratio used to express the degree of particle desorption or caking) and system operating parameters.

The Standard Multipass (Beta) Filter Method (ISO 4572) has been a highly recognized and widely accepted test throughout industry since its introduction in the early 1970's. In the past decade, the Beta Method has indeed made a major contribution in assisting users in selecting filters to meet system design requirements. However, many complaints have been voiced by users that filters normally produce a lower particle removal efficiency under field applications than they do during laboratory tests. Research results from work carried out at the Fluid Power Research Center at Oklahoma State University indicate that the degradation of the filtration (Beta) ratio in service depends mainly on the filter's retentivity characteristic. This paper highlights the theoretical basis of the Epsilon Rating Method and the concept of retentivity. Most important, the paper uses these concepts to correlate filter performance between laboratory tests and field operation.