Nominal vs. Absolute Filter Ratings.
A micron rating tells you a particle size. To compare filters, you also need the removal efficiency and the conditions behind the rating.
By FiltruneOn this page
Nominal and absolute describe a filter’s retention rating, but the words alone do not tell you exactly how much it removes. A meaningful comparison needs three things together: particle size, removal efficiency and the test behind the claim.
What Does a Micron Rating Tell You?
A micron, written µm, is one thousandth of a millimetre. In a particle-retention specification, the micron value identifies the size being discussed. It needs an efficiency alongside it to describe the result.
When two cartridges both say “5 micron”, the label alone does not tell you whether they will provide comparable particle control. Start by asking how each rating was established and what fraction of particles is removed at the stated size.
Nominal and Absolute Ratings Compared
Nominal and absolute are shorthand descriptions used in filter literature. They are not a universal pair of efficiency thresholds. Check how the manufacturer defines the term for the particular element, rather than assigning a percentage from the word alone. [1]
| Label | How to read it | What to request |
|---|---|---|
| Nominal | A stated filtration grade with a manufacturer-defined performance basis. | Efficiency at the stated size, the measurement basis and test conditions. |
| Absolute | A specified retention claim that should be read against its declared quantitative basis. | The actual efficiency or beta value, particle size and supporting test method. |
A single filter can have different efficiencies at different particle sizes. Its complete retention information is more useful than treating nominal and absolute as two physical kinds of filter. An absolute label also should not be read as an unconditional promise of 100% removal.
Removal Efficiency and Beta Ratio
Beta ratio expresses the upstream particle concentration divided by the downstream concentration at the same particle-size threshold. Use comparable samples: both measurements must count particles at that size and larger, on the same volume basis. [2]
Keep the size threshold fixed and the relationship becomes clear. In the example below, changing beta changes the fraction passing through; it does not change the particle size. Select a value to compare the arithmetic.
9,990 retained from 10,000
at the same particle-size threshold.
| Beta ratio | Downstream count | Efficiency |
|---|---|---|
| 10 | 1,000 | 90% |
| 100 | 100 | 99% |
| 1,000 | 10 | 99.9% |
| 5,000 | 2 | 99.98% |
Higher beta means a smaller passing fraction at that threshold. These values describe the example’s mathematics; they do not assign nominal or absolute classifications, predict a cartridge’s life or represent measured Filtrune performance.
Check the Test Behind the Rating
Numbers are comparable only when their basis makes sense together. Different challenge particles, counting methods or test arrangements can produce results that should not be treated as equivalent. A percentage based on removed mass is not automatically the same measure as a percentage based on particle counts. [1]
For example, ISO 16889 describes a multi-pass method for hydraulic filter elements. Its scope is specific; the presence of a beta value does not establish that every water or process cartridge was tested to that method. Ask which method actually supports the selected product. [3]
When the datasheet gives a result without enough context, request the test information before treating it as a like-for-like replacement. A clear question is more useful than assuming that two similar labels were measured in the same way.
How to Read a Filter Specification
A useful retention line connects the size, efficiency and beta ratio. Read this original teaching excerpt from left to right, then look for the test information that belongs with it.
Particle retention
5 µm199.9%2β₅ = 1,0003
Check the applicable product data.
- 1Particle size
The threshold at which the stated efficiency is evaluated: particles 5 µm and larger in this example.
- 2Removal efficiency
The fraction removed on the stated counting basis. Read it together with the particle size.
- 3Beta ratio
The upstream-to-downstream concentration ratio at that threshold. Here it expresses the same result as 99.9%.
The retention line is only one part of selection. Also ask for pressure-drop information at the required flow, operating-temperature limits, compatible wetted materials and the correct dimensions and seals. A replacement needs both an appropriate retention specification and a suitable installed configuration.
A constructive request is: “Please provide the efficiency at our required particle size, the test basis for that value, and the flow/pressure-drop data for the proposed element length.” That gives the supplier a specific comparison to support.
Choose the Rating for the Filtration Task
Upstream particulate control
Start with the incoming solids and what the next stage needs. Define the target before comparing grades. Review the available retention information alongside flow, loading and the current replacement history; the smallest micron number is not a complete selection rule.
The melt-blown and string-wound categories explain construction and material directions. Neither construction name establishes the efficiency of every product within it.
Defined downstream protection
Where the next equipment or process has an explicit requirement, use that requirement to assess the filter’s stated performance and installation. Our RO pretreatment guide puts cartridge selection in the context of membrane-feed protection; pleated cartridge options provide another product direction to examine against the duty.
What to Ask Before Comparing Cartridges
Bring the rating back to a practical specification. Before approving a proposed element or an alternative, make these five questions part of the comparison:
- At which particle size is the efficiency stated?
- What removal efficiency or beta ratio is declared?
- Which test method, challenge and operating conditions support it?
- What flow, pressure-drop and loading information is available?
- Will the media, dimensions and seals suit the actual liquid and housing?
The useful outcome is a clear basis for choosing and comparing products: the required particle control, supported by data that fits the application and installation.
