Absolute vs. Nominal Micron Rating for Filter Elements

Micron rating for filter elements is expressed in two ways: absolute micron rating and nominal micron rating. Are you confused about the differences between the two rating methods? This guide clarifies both terms to help you choose the right filter.

By reading this blog, you’ll understand the differences between them. When selecting filter elements, you’ll grasp their meanings more clearly and make the right choice faster.

What Is Micron Rating?

The micron rating quantifies a filter element’s ability to capture particles and is expressed in microns. A 1-micron rating indicates that the filter is typically capable of capturing particles 1 micron or larger. The lower the micron rating, the greater the filter’s ability to capture finer particles.

Due to discrepancies between theoretical and actual filtration performance, micron ratings are generally classified into absolute micron rating and nominal micron rating.

▶ Nominal Micron Rating

Nominal micron rating is a manufacturer-supplied rating that denotes the approximate particle size at which a defined percentage of particles are retained.

However, actual performance can be influenced by a variety of factors, including pore size distribution, operating pressure, contaminant concentration, flow rate, and temperature. As a result, the claimed efficiency cannot reach 100% and typically ranges between 60% and 95%.

Nominal micron rating is expressed not only as micron values but also as interception efficiency. For example, “90% of 10 microns” means the filter element captures particles of 10 microns and larger, while 10% of such particles still pass through.

▶ Absolute Micron Rating

Absolute micron rating is a tested and verified value. Unlike nominal micron rating, it represents a verified high filtration efficiency for filter elements, usually at 98.7% or higher. This means only a very small share of targeted particles are able to pass through the filter.

For example, a filter element with an absolute rating 10 microns can remove ≥98.7% of particles 10 microns and larger. However, this value is measured under standardized laboratory test conditions specified by relevant industry standards.

Key Comparisons

⭕️ Numerical Representation

Absolute micron rating refers to the particle size at which a filter reaches a defined high removal efficiency under standardized test conditions. Nominal micron rating, on the other hand, indicates an approximate particle size that can be removed at a specific efficiency level.

⭕️ Testing Standard

Absolute micron rating is verified using the strict ISO 16889 standard, but there is no universal standard for nominal micron rating. For specific testing information and filtration efficiency, you should contact the manufacturer.

⭕️ Production Cost

Due to different requirements for filtration performance, filter elements with an absolute micron rating cost more than those with a nominal micron rating.

⭕️ Application Scenarios

Nominal micron rating is good for situations where accuracy isn’t as important, like coarse filtration or pre-filtration. On the contrary, absolute micron rating is typically used in final filtration stages where strict accuracy is required.

How to Test Micron Rating?

▶ β Ratio

Beta Ratio is a simple yet powerful metric that measures a filter element’s filtration capability. It represents the ratio of upstream to downstream concentrations of particles larger than a specific size. A higher Beta Ratio indicates stronger particle interception and superior filtration performance.

Multi-pass testing or Beta Ratio testing involves periodically introducing particles of a specific size upstream, then using automatic particle counters to regularly measure the quantity of particles of that size both upstream and downstream. This yields the beta ratio, from which the filtration efficiency can be calculated.

For example, if there are 10 particles larger than 5 microns upstream of the filter element and 1 particle larger than 5 microns downstream, then at the 5 micron rating level, the filtration ratio of the filter element is 10/1 = 10. This is denoted as β5 = 10.

▶ ISO 16889

Regarding Beta Ratio testing, the globally unified standard is ISO 16889. ISO 16889 specifies the use of automatic particle counters to count the number of particles of a certain size, known as the ISO particle counting method.

In industry practice, a Beta ratio of 75 or higher (measured per ISO 16889) is the baseline for defining an absolute micron rating. For context, Beta 75 = 98.67% efficiency.

Most manufacturers today produce elements with a minimum Beta Ratio of 200 (99.5% efficiency) – and our absolute-rated elements go even higher, ensuring maximum protection for your system.

How to Choose the Right Micron Rating?

▶ When to Use Nominal Micron Rating?

Nominal micron rating permits minor material variations, enabling greater cost-effectiveness and accommodating higher flow rates. It is suitable for handling larger particles while controlling overall contamination levels.

Therefore, filter elements with a nominal micron rating are ideal for industries prioritizing cost and efficiency. For example, they are commonly used in upstream or pre-filtration stages in food and beverage processing.

▶ When to Use Absolute Micron Rating?

Absolute-rated filters provide highly reliable particle retention, reducing the risk of downstream contamination. This makes them suitable for high-risk applications such as the medical and semiconductor industries.

For instance, the pharmaceutical industry selects filter elements with a fine absolute micron rating to ensure quality, even at a higher cost.

Conclusion

In summary, you should already be aware of the difference between absolute and nominal micron rating. You could also be better equipped to weigh the differences between nominal and absolute ratings based on specific operating conditions, enabling you to select the filter element that best suits your needs.

If you have any further questions, feel free to contact us at any time.

FAQs

▶ Nominal vs. Absolute: Which One is More “Real”?

Absolute micron rating refers to a more “real” value. It is a precise numerical value where the actual performance meets or exceeds the claimed specifications, achieving near-complete filtration (≥98.7%).

▶ Is There Much Difference Between Absolute 1 micron and Nominal 1 micron?

Yes, there is a difference. When a filter element is rated as absolute 1 micron, it means the filter achieves very high retention efficiency at 1 micron under defined laboratory test conditions.

However, when a filter element is rated as nominal 1 micron, this merely indicates that its target filtration accuracy is 1 micron. It will still allow a certain percentage of 1-micron particles to pass through, as the nominal rating is defined based on a specific efficiency level rather than complete retention.