TL;DR: Absolute filters remove ≥98.7% of particles at the rated size; nominal filters remove ~50–95%. Use absolute for precision, nominal for bulk removal or prefiltration.
Micron ratings are a critical part of specifying filtration equipment, but the terms nominal and absolute describe two very different performance standards. Both are valid engineering choices depending on the types of contamination as well as the cost constraints of the system. Understanding these distinctions guarantees that the selected filters will deliver the proper protection without unnecessary expense.
Micron Ratings and Their Role in Filtration
A micron (µm) is one-millionth of a meter. Micron ratings indicate the particle size that a filter is designed to target, such as 1, 5, 10, or 25 µm. However, the rating alone does not define efficiency. Nominal and absolute classifications specify how reliably particles at that size are removed, and under what test conditions. This distinction determines whether a filter is suited for polishing, bulk particulate removal, or staged filtration.
Absolute Micron Ratings
An absolute micron rating is verified through standardized multi-pass testing, such as ISO 16889, and is expressed with a Beta Ratio (βx). This ratio compares the number of particles of size “x” microns and larger upstream versus downstream of the filter.
Common Beta Ratios and corresponding efficiencies:
βx = 75 → 98.7% efficiency
βx = 200 → 99.5% efficiency
βx = 1000 → 99.9% efficiency
An absolute 10 µm (β10 ≥ 200) filter will remove 99.5% of particles 10 microns and larger under defined conditions. These filters are used in applications with strict cleanliness codes, tight-tolerance components, or OEM specifications that require documented filtration performance.
Nominal Micron Ratings
A nominal micron rating indicates partial removal of particles at the stated size — typically between 50% and 95% — but without a universal industry standard for measurement. The performance of two nominal filters with the same micron label can vary significantly between manufacturers.
Nominal-rated filters are often used for bulk contaminant removal, prefiltration ahead of absolute elements, or in non-critical systems where fine particle removal is not necessary for acceptable equipment life.
Key Differences
| Feature | Absolute Micron Rating | Nominal Micron Rating |
|---|---|---|
| Efficiency | ≥98.7% (βx ≥ 75) or higher | 50–95%, varies by manufacturer |
| Test Standard | ISO 16889, Beta Ratio testing | No universal standard |
| Application | Critical cleanliness requirements | Prefiltration, bulk removal |
| Consistency | Predictable, repeatable results | Variable performance |
When to Use Absolute Filters
Absolute-rated filters are the preferred choice when precision, repeatability, and documentation are required. Common scenarios include:
Meeting ISO 4406 cleanliness codes for hydraulic or lubrication systems.
Protecting servo valves, proportional valves, and other tight-clearance components.
Final polishing before fuel injection or turbine lubrication.
Commissioning or post-maintenance verification where cleanliness reports must be provided.
Absolute elements are typically pleated surface filters, designed for low initial pressure drop, high efficiency, and stable performance over their service life.
When to Use Nominal Filters
Nominal-rated filters are a practical and strategic choice in a wide variety of industrial applications:
Bulk Contamination Removal: Removing larger particles early in the process reduces loading on downstream filters.
Prefilter Duty: Extending the life of costly absolute elements in staged filtration systems.
High-Load or Variable Contamination: Depth media nominal filters have high dirt-holding capacity and handle shock loads well.
Non-Critical Equipment: When the cost of absolute filtration outweighs its performance benefit for the application.
Viscosity or Flow Constraints: Coarser nominal filters can help maintain acceptable pressure drop in high-viscosity or high-flow systems.
Staged Filtration Strategies
The most effective systems often combine both filter types:
1. Nominal Prefilter – captures bulk solids at a coarse micron rating, protecting the final stage
2. Absolute Final Filter – polishes the fluid to meet target cleanliness
Example configurations:
Fuel Transfer: 25 µm nominal bag → 10 µm absolute cartridge.
- Diesel Fuel Polishing: Up to three/foure stages of filtration.
Hydraulic System: 25 µm nominal element → 5 µm absolute polishing element.
Lube Oil Flushing: 50 µm nominal depth filter → 10 µm absolute pleated filter.
This approach lowers operating costs by extending the life of the final element while maintaining the required cleanliness standard.
Selection Considerations
In order to properly select a filter, one must consider the following:
Cleanliness Target: ISO 4406 code, ASTM fuel spec, or OEM requirement.
Contamination Profile: Particle size distribution, load, and variability.
Flow and Viscosity: Impact on pressure drop across the element.
Media Type: Pleated vs. depth, cellulose vs. synthetic.
Service Interval Goals: How often you can replace or clean elements.
Total Cost of Ownership: Media cost, labor, downtime, and potential component wear.
Evaluating these factors collectively ensures that the filter you choose is appropriate for your specific application. It is just as important to choose the proper filter as it is to align with the operational priorities of a specific application.
A filter that hits the cleanliness target but creates excessive pressure drop will ultimately compromise system reliability. Balancing efficiency and holding capacity against budget/maintenance constraints will result in a filtration strategy that supports both performance and long-term cost control.
How PFP Can Help You Choose the Right Filter
Our team of filtration experts can assess your contamination profile, cleanliness targets, and operational constraints to recommend the optimal filter configuration. With access to a broad range of filter media, ratings, and housings, PFP can deliver a data-driven solution tailored to your system’s needs.
We excel at delivering filtration solutions that meet performance requirements without adding unnecessary maintenance or downtime.
