Absolute (Filter) Rating

The absolute filtration efficiency, also known as absolute retention rate, describes a filter’s ability to completely retain particles of a defined size under standardized test conditions. It is based on the concept of an absolute rating, where the stated pore size corresponds to the diameter of the largest spherical particle that can just pass through the medium in a laboratory test.

For example, if a filter is specified with an absolute rating of 10 µm, this means that particles with a diameter of 10 µm or larger are retained at a rate of 99.98%. The absolute filtration efficiency is typically determined using standardized test dusts or glass beads, with clearly defined filter media and test conditions to ensure comparable results.

In contrast, the nominal filtration efficiency only provides an approximate retention rate, which can vary depending on particle shape, test method, and operating conditions. Therefore, the absolute rating is especially important for safety-critical applications where strict limits on particle contamination must be met, such as in the pharmaceutical, food, and microelectronics industries.

In practice, a filter’s actual retention performance depends not only on pore size but also on factors such as differential pressure, fluid viscosity, particle shape, media aging, and fouling.

GKD uses absolute ratings to characterize the performance of process belts, cartridge filters, and plate filters. This information helps system designers select the appropriate filter media to achieve a defined cleanliness class or protect products from contamination. Combined with metrics such as the Beta ratio—which indicates the ratio of particle count before and after filtration—the absolute filtration efficiency provides a reliable basis for designing, monitoring, and optimizing filtration processes.