The tests behind a filter cloth specification
A filter cloth is specified by numbers that only mean something when the test behind them is stated. This page explains what each test measures, what it does not tell you, and exactly which fields a test report has to carry before you can use it.
Test methodology is public engineering knowledge, and it is useful to you whoever you buy from. Our own factory's methods, instruments and per-lot reporting format are being confirmed in writing and are not published yet — that status is at the bottom of this page, stated as it stands rather than filled in with plausible figures. The production chain is on the factory page →
Air permeability, and the question most datasheets dodge
Air permeability is measured by clamping a flat sample over a circular opening, drawing air through it, and holding a fixed pressure differential across the fabric while the flow rate is read. The result is reported as a flow per unit area — that is, how much air the fabric passes for a given push.
It is used as a proxy for the open structure of the cloth. It correlates with how freely liquor will pass, how fast a cake will form, and how much of the fabric is still open after service. It is not a pore size, it is not a retention rating, and it does not predict what a specific slurry will do to the fabric. What it is genuinely good at is comparison: same method, same pressure, same unit, new cloth versus used cloth, or candidate A versus candidate B.
A permeability figure with no test pressure differential and no unit beside it is not a specification — it is a number. Ask it of every supplier, including us. A supplier who cannot answer is quoting a figure they did not measure.
Why the pressure differential is not a detail
The same piece of cloth reports a different figure at a different pressure differential, because more push means more flow. The relationship is not proportional, and it is not the same from one fabric construction to the next — a dense multifilament and an open monofilament do not respond to a higher pressure in the same way. So a permeability measured at one pressure cannot be scaled to another pressure by arithmetic. Two figures measured at two different pressures are simply not comparable, no matter how similar they look.
Pressure differentials you will see quoted differ by convention rather than by physics: roughly 200 Pa is usual where metric fabric datasheets are written, while US customary datasheets are commonly quoted at half an inch of water gauge, about 125 Pa. Those two conventions produce different numbers for the same cloth. This is industry-level convention, not a measurement of ours.
Unit arithmetic — the part that is convertible
Units, unlike pressures, convert exactly, because they are the same physical quantity written differently. At least five are in circulation for this one property. Multiply by the factor below to land on m³/m²/h.
| Unit as written | What it says | To m³/m²/h | Where you meet it |
|---|---|---|---|
| L/dm²/min | Litres of air per square decimetre per minute | × 6 | Common in European fabric datasheets |
| m³/m²/h | Cubic metres of air per square metre per hour | × 1 | Reference column |
| m³/m²/min | Cubic metres per square metre per minute | × 60 | Same family, minute basis |
| CFM (ft³/min per ft²) | Cubic feet per minute per square foot | × 18.29 | US customary datasheets |
| mm/s | Face velocity through the fabric | × 3.6 | Stated as a velocity, not a volume |
These factors are unit arithmetic — dimensional conversion, nothing measured. They are valid only between figures taken at the same pressure differential. Converting the unit does not convert the test.
If your plant specification, your QA department or your machine's documentation names a particular test method, state the name in your enquiry rather than the number you expect. Whether that specific method can be run is something to establish before an order, not after a delivery. We do not name standard designations on this page that we have not verified in the standard itself.
Breaking strength, warp and weft — and which one you should be reading
A strip of fabric is cut to a defined width, gripped in two jaws, and pulled at a controlled speed until it breaks. Two things come out of it: the force at break, and how far the strip stretched on the way there. Both are measured separately in the two yarn directions, because a woven fabric is two different materials depending on which way you pull it.
Warp
The yarns that run along the loom. They are held under tension throughout weaving, which tends to make the warp direction the stronger and the less extensible of the two.
Weft
The yarns laid across the loom. The weaker direction in a typical construction — and, on a wide belt, the direction that carries the running load.
Why the weft figure usually governs on a belt
A vacuum belt is wide. The fabric has to be produced with its full belt width across the loom, which means the warp runs across the machine and the weft runs in the direction of travel. The direction of travel is the direction under tension in service: drive pull, take-up load, and every acceleration the belt sees. So the load on the machine is carried by the weaker of the two directions, and a datasheet that leads with the impressive warp figure is describing the direction nothing is pulling on.
Read the weft figure first. Then read elongation. A cloth that passes on breaking strength but stretches further under working load will walk out of the machine's take-up travel long before it ever approaches breaking — which reaches the maintenance superintendent as a tracking problem, not as a fabric problem.
Which direction actually runs in the direction of travel depends on how a given belt is fabricated. On a narrow belt or on a cloth cut from a wider roll the convention can invert, so it is worth confirming rather than assuming — for your belt it is settled on the order drawing, not on a web page.
Width, developed length, squareness
Dimensional inspection is the least glamorous check, and it decides whether a belt runs at all. Three measurements matter, and they are taken on the finished belt, not on the roll it came from.
Finished width
Measured across the web at several positions along the length, not once at one end. A belt that is wider at one point than another will find the edge guides at that point first.
Developed length
The length of the endless loop as fabricated, including whatever the seam consumes. This is the number that has to agree with the machine's centre distances and its take-up travel.
Squareness
Whether the seam, and the cut edges, sit at a true right angle to the direction of travel. Checked by comparing the two diagonals of the laid-out belt.
If the seam is not square to the belt's direction of travel, one edge reaches the roller fractionally before the other on every single revolution. The belt is steered by that difference, continuously and in the same direction, so it walks — and it keeps walking however carefully the rollers are aligned. This is why a belt that tracks off on a machine with verified roller alignment should send you to the belt's diagonals rather than back to the machine. Diagnosing belt tracking →
The same three measurements are what you take off the old belt before you order a new one, and a measurement taken wrongly is why a replacement does not fit. How to measure your old belt →
Surface, seam and edge — what each one is looking for
Visual inspection sounds subjective, and it is the easiest part of a supplier audit to leave unspecified. It is worth insisting on a written statement of what was looked at, because each of the three inspections is hunting a different failure.
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Surface inspection
Looking for: Broken or missing yarns, slubs and knots, contamination pressed into the face, streaks or bands where tension or calendering varied across the width, and any area where the finish looks different from the rest of the roll.
Why it matters: A local defect in the face becomes a local difference in permeability, and that becomes a wet stripe in the cake.
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Seam inspection
Looking for: Squareness of the joint to the belt edges, alignment of hooks or loops, uniform pin engagement, stitch or weld continuity across the full width, and the absence of a step in thickness at the joint.
Why it matters: The seam carries both the running load and the vacuum seal, and it is the part an operator can inspect without stopping the line.
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Edge inspection
Looking for: Continuity of the edge seal or reinforcement along the whole length, treated width consistent end to end, no fraying at the cut, and no hard ridge that would ride on the vacuum-box seal.
Why it matters: Frayed edges leak vacuum, and that loss is easy to misread as a machine seal problem rather than a cloth edge one.
Seam construction is a specification decided before fabrication, not a detail settled in the shop. How seams fail, and what it tells you →
What to ask any supplier for
Seven fields. A test report that carries all seven can be checked, repeated and compared. A report missing any of them is a claim, not a measurement. Apply this to every quotation you receive — this one included.
| Report field | Why you need it |
|---|---|
| Test method name | Without it the number cannot be compared with anyone else’s number, or repeated by your own lab. |
| Instrument / rig | Different rigs clamp different sample areas and reach the set pressure differently. |
| Pressure differential | Easy to leave off a report, and the field that changes the result most. |
| Unit | Five units are in circulation for the same property. A bare number is not a specification. |
| Sample count and positions | One reading in the middle of a belt several metres wide says nothing about its edges. |
| Test date | Ties the result to the material that was actually made, not to a historic datasheet. |
| Lot / roll reference | Lets you trace a belt that misbehaves back to the roll it was cut from. |
Two further habits worth keeping. First, a certificate is a document with a number and a validity date — ask for the file, not for a logo on a website. Second, when a figure looks impressively better than a competing quotation, check the pressure differential before you check the price.
What we have not confirmed yet
Everything above is public test methodology, which is why it can be published today. Our own test methods, instruments, reported units and per-lot report format are being confirmed in writing and the result is not published yet. Until it is, the table below reads UNKNOWN. We would rather ship it empty than fill it with figures nobody can trace back to a method.
| Item | Value | Unit | How it gets confirmed |
|---|---|---|---|
| Air permeability test method | UNKNOWN | — | Named in writing by the factory, then published here with its pressure differential |
| Air permeability instrument | UNKNOWN | — | Confirmed with the method |
| Reported permeability unit | UNKNOWN | — | Fixed once the method is confirmed; conversion arithmetic is above |
| Tensile test method and sample geometry | UNKNOWN | — | Named in writing by the factory |
| Warp / weft reporting convention | UNKNOWN | — | Confirmed with the tensile method |
| Dimensional tolerance on width and length | UNKNOWN | mm | Confirmed per belt on the order drawing |
| Squareness tolerance | UNKNOWN | — | Confirmed per belt on the order drawing |
| Per-lot report format | UNKNOWN | — | Confirmed with the factory; format published when it exists |
| Sampling positions across the width | UNKNOWN | — | Confirmed with the method |
| Certifications held and their scope | UNKNOWN | — | Published as files with numbers and validity dates, or not at all |
No certification is claimed anywhere on this site. Where a certificate applies to your order it is sent as the document itself, with its number and validity dates, so you can verify it at source. The production chain behind these checks — weaving through packing — is set out on the factory page, with the same discipline: process in writing, values left empty until they are confirmed.
Ask us the questions on this page
If your specification names a test method, an acceptance limit or a reporting format, put it in the enquiry. We will tell you plainly whether it can be met and what still has to be confirmed before you order — which is a more useful answer than a number with no method behind it.
Every belt is fabricated to your machine dimensions — we quote per RFQ. No online checkout.
Samples available — freight terms confirmed per request.
Where to go next
Factory & QC
The nine production steps these checks sit inside, and what is controlled at each one.
Measure Your Old Belt
Width, developed length and squareness taken off the belt you are replacing.
HVBF Filter Cloth
The cloth family these tests describe: weave, polymer, seam, edge and width options.
HVBF Spec Sheet
The written specification sheet, and which of its fields are still open.