A particle-size label needs an efficiency and a stated test method to describe performance. Donaldson explains beta ratio as the upstream particle count divided by the downstream count at the specified particle size. Always keep the size attached to the ratio.
The following table is calculated from efficiency = (1 − 1/β) × 100:
| Beta ratio at specified size | Calculated efficiency |
|---|---|
| 2 | 50% |
| 10 | 90% |
| 75 | 98.67% |
| 200 | 99.5% |
| 1,000 | 99.9% |
Do not assign a universal efficiency to the words nominal or absolute. Ask the supplier for the claimed efficiency, particle size, and test conditions. Hydraulic multipass results should not automatically be treated as performance evidence for every process filter.
Donaldson: Understanding Beta Ratings in Liquid Filters
Source: Donaldson. Manufacturer page inspected October 4, 2026 UTC.
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