Mesh to micron conversion chart and how mesh relates to slot size

Mesh to micron conversion and wedge wire slot size

Many screen specifications still give a mesh number, taken from an earlier woven screen, a laboratory sieve test or a supplier's data sheet. Wedge wire is not woven and has no mesh count; it is specified by slot width in microns or millimetres, so converting a specification means finding the opening size behind the mesh number.

What mesh means

For woven wire cloth, mesh count is the number of openings per linear inch (25.4 mm). The clear opening depends on the wire diameter as well:

Opening (mm) = 25.4 / mesh count - wire diameter (mm)

For example, a 100 mesh cloth woven from 0.10 mm wire has an opening of 0.254 - 0.10 = 0.154 mm, while the same mesh in 0.11 mm wire has a 0.144 mm opening. Mesh count alone therefore does not fix the opening.

Test sieves remove this ambiguity. In the US standard sieve series to ASTM E11, each sieve number is tied to a fixed nominal opening, and the same nominal openings are used in the ISO 3310-1 test sieve series. The Tyler series uses its own mesh numbers, which differ from the US numbers at some sizes, so the series should always be stated.

Mesh to micron conversion chart (US standard sieve series, ASTM E11)

Swipe sideways to see the whole table.

Openings are nominal. ASTM E11 also sets permitted variation and wire diameters for each sieve, which this chart does not reproduce. For sizes above No. 4, the series uses millimetre or inch designations, up to 25.0 mm (1 inch) and beyond.

SIMO wedge wire slots run from 25 microns to 25 mm, which spans the whole of this chart, from the No. 500 opening to the 25.0 mm (1 inch) sieve.

Units: micron, millimetre and inch

• 1 mm = 1,000 microns
• 1 micron = 0.001 mm
• 1 inch = 25.4 mm = 25,400 microns

A slot written as 0.25 mm, 250 microns or 250 µm is the same slot. A drawing should state one unit and use it throughout.

How slot relates to mesh

A mesh opening is a square, while a wedge wire slot is a long, narrow gap, so the two retain particles in slightly different ways.

• Round or near-round particles. Sand grains, resin beads and many crystals behave much the same on both, so a slot equal to the mesh opening retains roughly the same particles.
• Flaky or elongated particles. A square opening limits two dimensions of a particle and a slot limits only one, so fibres, flakes and needles can pass a slot lengthwise that a square opening of the same size would hold.
• Near-size particles. On mesh, particles close to the opening size jam in the weave and blind it. On wedge wire, a particle that passes the face clears the widening slot, and one that does not rests on two points and lifts off.

The mesh opening is therefore a sound starting slot for round particles. For flaky or fibrous solids a smaller slot is selected, or the choice is confirmed by testing a sample.

Worked examples

Replacing a 60 mesh basket

An outlet basket in a process line has a 60 mesh woven liner that blinds frequently, and the line carries sand-like solids.

1. No. 60 sieve opening = 250 microns.
2. Starting slot = 0.25 mm.
3. The solids are near round, so 0.25 mm is suitable without reduction.

The wedge wire basket also provides a rigid surface, so it carries the pressure without a separate support cage.

Holding ion exchange resin

A standard ion exchange resin is often supplied in a 16 to 50 mesh size range.

1. No. 16 = 1,180 microns and No. 50 = 300 microns.
2. The smallest beads are therefore about 300 microns.
3. The slot must be below 300 microns with a margin, for example 0.2 mm (200 microns).

Resin traps and laterals for this resin are specified with that slot. Fine-mesh and uniform particle size resins need a separate check against the resin supplier's data.

Converting a slot to an equivalent mesh

A data sheet asks for "100 mesh or finer", which corresponds to an opening of 150 microns. A 0.15 mm slot gives similar retention for round particles, and a 0.10 mm slot gives a margin for flaky solids.

Soil and sand fractions by sieve

Water well and filter design uses the same sieve series to describe sand and gravel, with the boundaries set by the Unified Soil Classification System (ASTM D2487).

Swipe sideways to see the whole table.

These fractions explain why well screen slots for sandy aquifers usually fall below 1 mm. Fines below 75 microns are removed during well development rather than retained by the screen.

Reading a sieve analysis

A sieve analysis reports the percentage of a sample passing each sieve, plotted as a cumulative curve against opening size on a log scale. Two points on the curve are widely used:

• d10: the size that 10 percent of the sample passes.
• d60: the size that 60 percent of the sample passes.

The uniformity coefficient is d60 / d10. A low value indicates a uniform sand, and a high value a well-graded mix of sizes.

For example, if d60 = 0.60 mm and d10 = 0.20 mm, the uniformity coefficient is 3.0. The slot for a well screen or filter is selected from points on this curve according to the design method in use, such as IS 8110:2019 for water wells in India.

Mesh and open area

Converting mesh to a slot preserves the opening, not the open area. Woven cloth open area is (opening / (opening + wire diameter))² x 100, and wedge wire open area is Slot x 100 / (Slot + Wire width).

At fine sizes, a thin woven cloth can show a higher nominal open area than wedge wire, although in service wedge wire usually retains more of its open area because it does not blind in the same way.

Stating mesh and slot on an enquiry

On a wedge wire enquiry the slot is the binding requirement. Where the original specification was written in mesh, the mesh number and series are added in brackets, for example "slot 0.25 mm (equivalent to No. 60 US standard sieve)", so that there is no doubt about which number governs.

Related products, tools and engineering support

• Products: candle filters and resin traps are the most common wedge wire replacements for fine mesh.
• Tool: the mesh to micron converter converts sieve numbers, microns and millimetres both ways.
• Engineering: where the replacement slot for a mesh is uncertain, see custom engineering; a sample of the solids allows SIMO engineering to confirm it.
• Further reading: slot size and open area and wedge wire vs perforated plate vs wire mesh.