What is the Difference Between Absolute and Nominal Filtration?
Absolute-rated filters have been tested per ISO 16889 and achieve ≥99.5% efficiency (β ≥ 200) at their rated particle size. Nominal-rated filters capture an unspecified percentage of particles at their rated size with no standardized testing. The distinction is critical when selecting replacement elements for systems with specific cleanliness requirements.
Absolute Filtration
Absolute-rated elements use glass fiber (microglass) media that provides consistent, predictable pore sizes. They are tested and certified per ISO 16889 with documented beta ratios. When a filter is rated “10µm absolute” or “β10 ≥ 200,” you know it captures at least 99.5% of particles 10µm and larger. This makes absolute ratings directly comparable across manufacturers.
Nominal Filtration
Nominal-rated elements typically use cellulose or wire mesh media with inconsistent pore sizes. A “10µm nominal” filter might capture anywhere from 50% to 90% of particles at that size, depending on the manufacturer’s internal definition. There is no industry standard for what “nominal” means, making it impossible to compare nominal ratings between brands.
Impact on System Performance
Replacing an absolute-rated element with a nominal-rated element of the same micron rating causes significantly worse filtration. A 10µm absolute element captures 99.5% of particles ≥10µm. A 10µm nominal element might capture only 50–80%. This degrades system cleanliness, accelerates component wear, and may void equipment warranties.
Reference Table
| Feature | Absolute | Nominal |
|---|---|---|
| Test standard | ISO 16889 (multi-pass) | No standard |
| Typical efficiency | 99.5–99.9% | 50–90% (varies) |
| Beta ratio | β ≥ 200 | β ≥ 2 (often unstated) |
| Media type | Microglass | Cellulose or wire mesh |
| Cross-brand comparison | Directly comparable | Not comparable |
| Cost | Higher | Lower |
Frequently Asked Questions
Can I use a nominal filter in place of an absolute filter?
How do I know if my filter is absolute or nominal rated?
Related Specifications
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