Understanding Wire Mesh Retention Ratings: Nominal vs. Absolute Micron Retention
Wire mesh is a versatile material consisting of parallel rows and intersecting columns of wire. It comes in many different shapes, sizes, weave designs, materials and metal alloys for a number of industrial and manufacturing applications.
Among the specifications used to describe wire mesh are the terms Nominal and Absolute retention rating, but what exactly do these terms mean?
What Are Wire Mesh Retention Ratings?
- Nominal retention rating: Describes the average size particle that will not pass through the filter.
- Absolute retention rating: Describes the percentile of particles larger than a specified micron rating that will be trapped on or within the filter.
The ratings are sometimes referred to as pore sizes, describing the opening of that size as opposed to the particle of that size. Both the nominal and absolute mesh ratings are attempts to quantify the mesh’s performance as a filter.
Wire Mesh Nominal Pore Size
The nominal pore size describes the average pore size.
Due to the nature of weaving, even though individual pore sizes may vary, any pores that skew oversize will be offset by adjacent pores that skew undersize, leaving the average the same regardless of how extreme that skew might be.
This number is straightforward and consistent across multiple samples of the same mesh.
Wire Mesh Absolute Pore Size
The absolute pore size describes the pore size greater than a certain percentile of all the pores in a given sample of mesh.
This is where some confusion lies. Average particle retention is a less-useful indicator of filter performance compared to knowing the maximum penetrating particle. But the maximum penetrating particle varies widely from sample to sample, so its predictive value (what can be expected from similar samples), is quite low.
As a compromise, a bit of certainty can be traded for a bit more predictive value. The 100% largest penetrating particle can vary for every sample. Step back just to the 99.5th percentile largest few particles and the variability between samples drops. This means that instead of assuming the single maximum, a better measure of the absolute opening should include a probability percentile in addition to the pore size.
Usually, this is the maximum 0.5%, or those greater than the other 99.5% of the pores. Another percentile commonly used in normal distribution statistics is 97%, also known as the Cut Point.
Any mesh data tables that list absolute micron ratings are suspect. Without a percentile, they are also incomplete.
So, what are the circumstances when each measure is appropriate?
Nominal vs. Absolute Micron Retention: Which Measure Is More Appropriate?
Nominal pore size is the most consistent measure of a mesh, especially when comparing different meshes from different manufacturers. If a table lists micron ratings without specifying which rating, it is probably the nominal rating.
The absolute rating is most desirable when assessing the largest particles that could reasonably be expected to penetrate a mesh. It has significantly more variability across samples than the nominal size.
Absolute micron ratings express the upper end of the nominal rating’s margin of error, so an absolute micron rating will always be a larger size than the nominal micron rating for a given sample.
If tasked with sourcing mesh to match a specified absolute micron rating and the mesh supplier only lists nominal ratings on their data tables, that is a good cue to speak with a mesh expert at that supplier. They are often hesitant to publish absolute ratings due to the ratings’ high variability.
Real-world filtration conditions include more irregular particle shapes and higher pressures than typical porometry testing, so any published tables of absolute retention ratings should be taken only as guidance, not as a guarantee.
A wire mesh retention rating is just one of many specifications for a mesh filter. Understanding how to interpret these values is important but shouldn’t be evaluated in a vacuum.
This is because having absolute and nominal particle retention in perfect agreement can steer buyers towards higher costs and longer lead times, and away from more cost-effective and functionally equal mesh products. A mesh expert can help you find the right mesh with the right pore size for your application, while balancing cost and lead times with performance.
Gerard Daniel Expertise
At Gerard Daniel, we’ve been sourcing, weaving, and distributing wire mesh for 70 years and stock the largest range of inventory in North America. Our application engineers use their deep expertise in woven mesh and components to develop the most effective solution, collaborating with your design, development, and production teams to ensure the lowest cost to manufacture. Please reach out if you would like to learn more about our capabilities.
Special Thanks to Keith Brocklehurst and Jon Miles of Whitehouse Scientific Ltd. whitehousescientific.com