Cotton at 30 % relative humidity and cotton at 80 % are, mechanically, two different materials. The fibre absorbs water from the air, the water plasticises it, and its strength, elongation, dimensions and even its electrical behaviour change with it.
This is why every textile test method points back to the same short standard: ISO 139, the standard atmosphere for conditioning and testing.
What the standard atmosphere is
The temperate standard atmosphere is:
20 ± 2 °C and 65 ± 4 % relative humidity
A tropical alternative exists at a higher temperature for regions where the temperate condition is impractical to hold. Whichever you use, it goes on the report — results from the two are not comparable.
Note how tight the humidity tolerance is: ± 4 %. A room with a domestic humidifier and no control loop does not hold that. Neither does a laboratory with a door that opens onto a dye house.
Equilibrium is a measurement, not a waiting time
"We left it overnight" is not conditioning. The standard defines equilibrium by mass:
Weigh the specimen at intervals of at least two hours. It has reached equilibrium when the change between successive weighings is no more than 0.25 % of the mass.
Thin, open fabrics reach this quickly. Heavy wool, dense coated fabrics, wound packages of yarn and rolled carpet can take a great deal longer — sometimes days. A cone of yarn conditioned for four hours is conditioned on the outside only.
If you test to a schedule rather than to equilibrium, build the check in: weigh a sacrificial specimen from each material type and record it.
Approach from the dry side
This is the detail that gets missed, and it is not a technicality.
Textile fibres show hysteresis. A specimen that reaches 65 % RH by drying down from a wetter state holds more moisture than an identical specimen that reaches 65 % by taking up moisture from a drier state. Same room, same reading on the hygrometer, different moisture content — and therefore different strength.
ISO 139 resolves this by requiring that specimens reach equilibrium from the dry side. Material that arrives wet, or that has been in a humid store, is pre-dried first, then allowed to pick up moisture in the standard atmosphere.
If your incoming samples arrive in sealed bags from a humid climate, pre-drying is not optional.
The room is part of the instrument
A conditioning room or cabinet needs:
- Active heating and cooling — heating alone cannot hold 20 °C in summer.
- Humidification that does not produce visible mist, and controlled dry air for dehumidification.
- Air movement, so that specimens in the middle of a rack see the same conditions as those at the edge.
- Continuous recording of temperature and humidity, not a spot reading when someone remembers.
That last point matters for audits. A thermohygrograph or electronic data logger gives you a record you can attach to a disputed report. Without it, you cannot prove the conditions were held when the test ran — and "we always keep it at 65" is not evidence.
Keep the logger record for at least as long as you keep the test reports. When a customer challenges a result from four months ago, the trace is the fastest way to close it.
Where it shows up
| Property | Effect of higher moisture |
|---|---|
| Cotton tensile strength | Increases |
| Most synthetics | Little change |
| Dimensional stability | Specimens relax and shift |
| Pilling behaviour | Softer fibre, more pills form |
| Electrostatic behaviour | Charge dissipates faster |
The direction is not the same for every fibre, which is exactly why the standard fixes the atmosphere rather than trying to correct for it.
Five questions for your own lab
- Do you record temperature and humidity continuously, or by spot check?
- Do you determine equilibrium by mass change, or by clock?
- Do heavy and packaged materials get longer conditioning than fabric swatches?
- Do wet or humid incoming samples get pre-dried?
- Does the conditioning atmosphere appear on the test report?
A laboratory that answers yes to all five can defend any result it issues. One that cannot is, in effect, reporting the weather.