Filter Mesh Rating: Why Nominal Micron Is Not What Passes Through

A woven stainless filter cloth is usually ordered on two numbers: mesh count and micron rating. Neither is a measurement of what the cloth actually stops. Two suppliers can deliver 100 mesh, both quote nominal 150 micron, and the two rolls will pass different particles on the same test rig. The difference comes from wire diameter, weave style, and what the word nominal is being used to mean.

This article explains how the opening in a woven mesh is actually set, how nominal and absolute ratings differ, and what to write on a specification so the cloth can be verified instead of described.

Mesh Count Is a Count, Not an Opening

Mesh count states how many wires cross one inch. It says nothing directly about the hole. The opening is the pitch minus the wire diameter: for 100 mesh, the pitch is 25.4 divided by 100, or 0.254 mm. With 0.10 mm wire the opening is 0.154 mm, about 154 micron. With 0.11 mm wire it is 0.144 mm, about 144 micron.

A change of 0.01 mm in wire diameter therefore moves the rating by 10 micron, and general mill tolerance on fine wire is wider than that. This is the single most common reason two 100 mesh cloths behave differently, and it is entirely predictable once the wire diameter is specified with a fixed band instead of a general tolerance.

Nominal and Absolute Mean Different Things

Nominal rating

A nominal figure is a description, not a guaranteed cut point. Depending on the supplier and the standard used, a nominal rating may mean that the majority of particles above that size are retained, or that the average opening corresponds to that size. It allows a measurable proportion of oversize particles through, and the proportion is often not stated.

Absolute rating

An absolute rating is tied to a test method. It typically means that essentially no particles above the stated size pass, verified by a challenge test rather than calculated from the weave geometry. Absolute ratings cost more because the cloth has to be inspected and the weave has to be held tighter, not because the material is different.

Practical consequence: nominal 150 micron from one source can be coarser than absolute 150 micron from another by a wide margin. When the filter protects a nozzle, a pump or a downstream membrane, the absolute figure is the one that belongs on the drawing.

Weave Style Changes the Pore Geometry

Plain weave

One wire over, one under. The openings are square and uniform, which makes plain weave the easiest to rate and the best choice when the micron figure matters. It is also the least stable at very fine mesh, because the wires have little support at the crossings.

Twill weave

Two wires over, two under. The steeper path gives better wire support and allows finer mesh to be woven with fewer defects. At the same mesh count a twill cloth has a slightly smaller effective opening than plain weave, because the wires sit at an angle to the flow rather than flat across it.

Dutch weave

Warp and weft use different wire diameters. The large warp wires carry the load and the fine weft wires set the pore size. Openings are triangular rather than square, so the rating cannot be calculated from mesh count at all. Plain dutch and twill dutch differ in pressure capacity and in the ratio of pore length to pore width, which affects both dirt holding capacity and cleanability. Dutch weave is the normal choice for high differential pressure and for fine filtration in the 5 to 50 micron range.

Wire Properties That Decide Mesh Quality

Diameter tolerance

As shown above, the opening follows the wire diameter directly. For cloth below about 200 micron rating, a fixed diameter band agreed with the mill is more useful than any grade name. State the band, not just the nominal diameter.

Straightness, cast and helix

Wire that leaves the spool with residual curvature feeds unevenly. Warp tension varies across the loom, the cloth comes out with visible bands, and warp wires break at the reed. Straightness matters more on fine mesh than on coarse, because the wire has less stiffness to correct itself.

Surface and coating

Soap coated wire runs more cleanly through the loom and reduces die and reed wear. The coating has to be removed before the cloth goes into a filtration application, so the cleaning step must be agreed in advance rather than discovered at the customer's plant.

Grade

304 covers general filtration and screening. 316L is the normal choice for chloride bearing liquids, for cloth that will be welded or sintered into a pack, and for food or pharmaceutical contact where low carbon helps at weld seams.

Specification Window

Applications

• Liquid and hydraulic filter elements
• Polymer and fibre spinning packs
• Pharmaceutical and food grade screening
• Water treatment and desalination pre-filters
• Catalyst support screens and demister pads

What to Put on the Enquiry

State the weave type, the mesh count or the micron figure, whether that figure is nominal or absolute, the wire diameter with its tolerance band, the test method used for verification, the cloth width and the service fluid. With those seven items the cloth can be woven and verified against a number rather than matched by appearance.

TOPONE has produced stainless steel wire since 1993 in diameters from 0.15 mm to 12 mm, and supplies weaving grade wire with the diameter band, straightness and surface condition agreed against the mesh specification rather than against a general mill standard.