In the early hydraulic industry, "nominal rating" (in microns) was commonly used to indicate filter element performance, but this parameter lacks a unified test standard and only roughly suggests that particles above a certain size may be partially intercepted, offering limited reference value. Modern standards recommend "absolute rating," defined as the particle size corresponding to a beta ratio β_x measured by the ISO 16889 multi-pass test. Typically, the size corresponding to β₁₀₀₀ (99.9% efficiency) is considered the absolute rating. When selecting a filter, customers should request the beta curve from suppliers rather than relying solely on nominal ratings.
