A nominal hydraulic filter rating is an approximate micron size the filter is expected to capture at a manufacturer-chosen efficiency, usually somewhere between 50% and 98%, with no standardized test behind it. An absolute rating identifies the largest particle that will pass through the media under controlled laboratory conditions, and it is verified today through the Beta ratio from the ISO 16889 multi-pass test. In short, absolute ratings are repeatable and comparable, while nominal ratings are loose estimates that differ from one brand to the next. If you have ever compared two "10-micron" filters and found that one kept your oil far cleaner than the other, you have already met this problem. The micron number on the box means very little unless you know how it was determined. This guide explains what nominal and absolute ratings really mean, how the Beta ratio fits in, and how to choose the right option for your hydraulic system.
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