Fibre and yarn are sold by mass, and part of that mass is water. That makes moisture a commercial number as well as a technical one — and it is surprisingly easy to report the wrong one.
Two definitions, two denominators
ASTM D2495 (moisture in cotton by oven-drying) sets out the two terms clearly:
- Moisture content — the water in the material as a percentage of the mass of the moist material ("as is" or "wet basis").
- Moisture regain — the water as a percentage of the mass of the moisture-free material ("dry basis").
With W the original mass and D the dry mass:
Moisture content % = (W − D) / W × 100
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Moisture regain % = (W − D) / D × 100
The numbers are close for dry fibres and drift apart as moisture rises. For example, a moisture content of 12.0 % corresponds to a regain of 13.6 % — the pair used for wool yarns in ASTM's commercial regain table. Converting is simple:
Regain = Content / (100 − Content) × 100
The mistake is not in the arithmetic. It is in a report that says "moisture 8.5 %" without saying which one. Write "moisture regain" or "moisture content" in full, every time.
"Percentage points", not "per cent"
D2495 also flags a second ambiguity. If regain rises from 5 % to 7 %, that is an increase of two percentage points, or an increase of 40 % of the original value. Both statements are arithmetically true and they sound very different in a claim letter. Use percentage points.
Commercial regain is a contract value, not a measurement
Trading uses an agreed commercial moisture regain to calculate invoiced mass. ASTM D1909 tabulates such values — for example, 0.4 % for polyester, 4.5 % for nylon and 13.6 % for wool in its 2004 table, with different values for cotton yarn depending on whether it is natural, dyed or mercerised. The ISO 6741 series covers the determination of commercial mass for consignments of fibres and yarns.
D1909 makes an important point: the actual regain of a material in the standard atmosphere bears no consistent relation to the commercial regain. Equilibrium regain depends on the material's history. So a laboratory should never "check" a commercial regain by measuring a conditioned specimen and expecting the same figure. The commercial value is used in the calculation; the oven-dry mass is what you measure.
Note also the scope of ISO 6741-1: homogeneous consignments of fibres and yarns of a single generic type. Blends, beamed yarns, coated yarns and retail put-ups are outside it.
The oven method and its two weighing routes
The reference way of finding the dry mass is to dry the specimen in an oven and weigh it. ASTM D2495 uses a ventilated oven at 105 °C and gives two routes:
- Weighing inside the oven, using an oven balance, so the specimen is never exposed to room air.
- Weighing in closed containers that are capped in the oven and cooled in a desiccator.
What it does not allow is the common shortcut: taking a hot, dry specimen out on an open tray and weighing it on the bench. A dry fibre starts to take up moisture from the room immediately, and the "dry" mass you record is too high.
A further subtlety: if the air supplied to the oven contains moisture, an oven-dried specimen still holds a little water when its mass stops changing. D2495 notes this and points out that a truly moisture-free state requires desiccated air. For commercial purposes the oven-dry mass is accepted; for research, know which you have.
Halogen moisture analysers: fast, but calibrate them
A halogen moisture analyser combines a balance and a radiant heater. It dries a small sample in minutes and reports mass loss. That speed makes it ideal for process control — but three cautions apply:
- Mass loss is not only water. Spin finishes, oils and other volatiles also leave at high temperature. Too high a setting reports them as moisture.
- The end point is set by you — a temperature, and a switch-off criterion. Two analysers with different settings give different results on the same sample.
- The sample is small. A few grams of a heterogeneous package or bale may not be representative.
The practical approach is to set the analyser up against the oven method for each material type: choose the temperature and end criterion that reproduce the oven result, record them as a method, and recheck periodically.
Handheld meters
Portable meters with material scales give an instant reading on bales, packages and fabric rolls without taking a sample. They are indicators, best used to spot wet lots and trends. When a figure goes on an invoice or a dispute report, confirm it gravimetrically.
Checklist
- Content or regain — written out in words?
- Dry mass found by oven, with weighing in the oven or in closed containers?
- Halogen analyser settings validated against the oven for this material?
- Commercial regain treated as a contract value, not a measured one?
- Differences reported in percentage points?
Our moisture range includes halogen moisture analysers — among them a model with 0.1 mg readability and a 50–230 °C range — a portable textile moisture meter and laboratory drying ovens. They are supplied from our workshop in Istanbul with a one-year warranty.