Absolute vs nominal micron rating: which filter specification should you trust?

If you've ever sought hydraulic filters or identified a filter for a system of fluid power You've probably come across two terms on a datasheet: nominal micron rating and absolute micron rating. Both describe the size of particles captured; they typically appear side-by-side and may appear at first glance similar. However, they're fundamentally different things, and using the wrong specification as a reference point could make your system more vulnerable than you thought.

This article will explain the meaning behind each rating, what each rating means, the reason why the distinction is important in real-world hydraulic applications, and the best way you can make the best choice when making comparisons between filters.

What does the micron rating actually indicate?

Prior to separating absolute from nominal, it is important to know what a micron's rating is intended to convey; that is, the amount of particles the filter is able to take in.

A micron (μm) is 1 millionth of a meter. The contamination of hydraulic systems is usually monitored and controlled within this range. The most common hydraulic parts—proportional valves, servo valves, and axial piston pumps—have clearances that are critical between 1 and 25 μm, which makes fine particulate pollution the primary reason for wear as well as stiction and failure.

The micron rating of a filter is supposed to reveal the point that particles are eliminated out of fluid. But the validity of this assertion is dependent on the method by which you came to the conclusion that the value was determined.

Nominal micron rating: An estimate, Not a guarantee

The nominal micron is a manufacturer-determined value determined by the average of performance in controlled testing conditions. A filter with a nominal rating of 10 um could capture 50-98 percent of particles that are greater than this size. The precise efficiency is determined by the manufacturer and is not always standardized.

This is the main issue with nominal ratings. There is no consensus among the industry on what "nominal" is in terms of the actual efficiency of capture. A 10um nominal filter could eliminate 50% of 10um particles. Another manufacturer's filter could remove up to 85%. If you don't know the method of testing and the efficiency number behind the score, you can't do an apples-to-apples analysis of filters made by different vendors.

Nominal ratings were a common standard prior to more stringent testing methods becoming accessible. They are easy to define and are easy to market and can make filters appear more efficient than a rigorous efficiency test could prove. For many general-purpose industrial uses such as return line filtration in systems with low sensitivity, for instance -- nominal ratings could be sufficient. However, in any system where the levels of cleanliness for fluids are defined (and in the majority of hydraulic equipment, there must be), nominal ratings provide inadequate security.

Absolute micron ratings: A clearly defined standard of performance

The micron count in absolute will reveal the most massive particles that are able to traverse a filter under the conditions of a standardized test. A filter that has an absolute value of 10um cannot allow particles greater than 10um to be able to pass through the filter medium. Not just often, however; it is a requirement under the test procedure used to determine the rating.

In the modern filtration process, the absolute rating is typically determined by using a beta ratio (ssx) that is derived through the multi-pass ISO 16889 testing. This test is a pumping of an unstandardized test fluid containing an established concentration of ISO Medium Test Dust particles through the filter and then counts particles downstream and upstream at specific intervals of size and calculates a percentage:

ssx = Upstream particle count/downstream particle count

A filter that has a beta ratio of ss10 = 200 is one that, for each 200 particles of 10 μm or above entering the filter, just one goes through. This gives an efficiency of 99.5 percent at the size of the particle.

Benchmarks for the common beta ratio that are used in hydraulic system specifications:

• ss10 = 2-50 percent efficiency (comparable to a rough nominal value)
• ss10 = 75-98.7% efficiency
• ss10 = 200 - 99.5% efficiency
• ss10 = 1000 - 99.9% efficiency

Absolute ratings supported with beta ratio data give you the ability to verify and test a performance claim, but not an estimation.

Why is this difference important for hydraulic system design?

The negative consequences of not understanding the specification of a filter aren't only a matter of. Take a high-performance servo system that requires ISO 4406 cleanliness code 16/14/11. If the designer uses the term "10 μm nominal" filter but does not know its true efficiency, the filter installed may only eliminate 60% of the particles in the target size. The system could technically have a 10 μm filter within the circuit; however, the level of cleanliness isn't going to reach the desired level, which can lead to increased wear of the components, erratic valve responses, and eventually failure.

However, the specification that a filter has the absolute value in the range of ss10(c) > 200 (using the ISO 11171 calibration standard with ISO MTD) provides the system designer with confidence that the filter can actually provide the required cleanliness under the operating conditions.

This is particularly relevant when it comes to the following situations:

• Electrohydraulic proportional and servo systems in which the clearances of valves are 1-5 um, and the sensitivity to contamination is extremely
• Axial piston motors and pumps in which tight barrel-port plate clearances are required to ensure constant fluid purity
• High-pressure systems with pressures above 250 bar, where the effect of particle erosion accelerates as pressure increases.
• Mobile hydraulic systems are used in extreme environments, where the rates of ingression are high and the load on the filter is increased

Absolute vs. nominal: A real-world contrast

One caveat to be aware of The "c" suffix

There are times when you will see ratings that read "ss10(c)" instead of simply "ss10." "(c)" is a reference to the fact that the "(c)" designation means the test was carried out using ISO 11171-calibrated particle counters as well as ISO Medium Test Dust, the most current international standard. Previous beta ratio tests used AC Fine Test Dust (ACFTD) that produced various particle count numbers. The ss10 = 200 results of ACFTD testing aren't directly similar in comparison to ss10(c) equal to 200 derived from ISO tests using MTD.

When comparing datasheets for filtering, be sure to verify that the rating is derived by using the calibration standard currently in use. A reputable filter manufacturer will state this in detail.

Which one of the ratings do you believe?

Use absolute micron ratings, particularly those that are backed through multiple-pass beta ratio data from ISO 16889 testing using ISO 11171-calibrated counters. They are the only rating options that provide you a clearly defined, comparable, and auditable performance claim.

Use nominal ratings as a rough guideline in applications with low sensitivity where system cleaning targets aren't officially established. Do not rely on a nominal rating as the sole basis for determining filtration requirements for proportional control valves, servo valves, or any other system in which ISO 4406 cleanliness codes form part of the maintenance or design specifications.

When you are sourcing filters, ask the seller for the full beta ratio curve, not just a headline number. A curve displaying the SSX values for a variety of dimensions of particles (e.g., SS6 and SS10, SS16 and SS25) provides a comprehensive overview of the performance of the filter and allows you to ensure that the specification is precisely aligned to the sensitivity of your system's contaminants.

A micron-sized rating displayed on a filter can only be dependent on the method of testing behind it. Nominal ratings have always been approximate, and absolute ratings backed by known beta ratio data are an engineering standard. Any hydraulic systems in which the longevity of components, their performance, and targets for fluid cleanliness are of importance—and that's the majority of them—absolutely rated ratings can be considered the sole requirement to build your filtration plan around.