Nominal vs Absolute Micron Ratings
Understanding nominal vs. absolute ratings, Beta ratios, and how to specify the correct micron grade for your process.
Nominal Rating
A nominal rating is a supplier-defined retention claim at a stated particle size. The efficiency and test method must be supplied: there is no single percentage that makes every nominal-rated filter equivalent. Particle passage through the medium is different from mechanical bypass around a failed seal.
Absolute Rating and Test Evidence
An absolute rating must also be tied to a stated retention efficiency and test method. Beta ratios quantify particle-count performance at a defined size, but the word absolute is not a universal Beta threshold. A membrane pore label alone does not prove microbial retention or sterile-service suitability.
What to Specify for Each Filtration Duty
Start with the downstream acceptance criterion. Industry names alone do not determine the required micron rating, filter construction or validation.
| Industry | Selection Input | Evidence Required | Candidate Family |
|---|---|---|---|
| Critical microbial retention | Target organism and process | Product-specific retention and integrity validation | Membrane / Capsule |
| Membrane protection | Upstream solids and final-filter requirement | Retention efficiency and pressure-drop curve | Depth / Pleated prefilter |
| Food & beverage clarification | Product clarity and sanitation requirements | Retention and applicable food-contact documents | Bags / Pleated cartridges |
| Chemical process | Medium, concentration and target particles | Compatibility and retention method | Compatible membrane / Metal |
| Water prefiltration | Flow, solids distribution and equipment limit | Rating definition and clean pressure drop | Depth / High-flow / Bags |
| Coarse screening | Target aperture and particle geometry | Mesh or slot dimensions and tolerances | Mesh bags / Wedge wire |
Reading a Beta Ratio
At particle size x, βx is the upstream count divided by the downstream count for particles at or above x under the specified test. Retention efficiency = (1 − 1 / βx) × 100%.
| Beta ratio | Calculated efficiency |
|---|---|
| 2 | 50% |
| 20 | 95% |
| 200 | 99.5% |
| 1000 | 99.9% |
Example: 10,000 upstream particles and 10 downstream particles at the same threshold give β = 1,000 and 99.9% efficiency. This does not establish microbial removal or chemical compatibility.
Before comparing two 5 μm filters
- Confirm the efficiency at 5 μm and the test method.
- Compare test fluid, flow, contaminant and loading conditions.
- Confirm pressure-drop data, housing fit and sealing.
- For critical service, obtain validation for the actual product and process.
Technical reference: Donaldson — Understanding Beta Ratings. The reference explains the measurement concept; it is not certification of JINLV products.