Laboratory

ISO 139 Conditioning Time to Moisture Equilibrium

Equilibrium is approached exponentially and never actually reached. Conditioning time is really a question about how close is close enough.

Sample State Regain
%
%
g/m²
Approach Kinetics
h

Depends on thickness, density and air movement; measure it on your own goods.

pp

Conditioning Time

— h

To come within the stated tolerance of equilibrium

Approach to Equilibrium

Regain Gap to Close
— pp
Half-Time of Approach
— h
Regain After 24 Hours
— %
Moisture Taken Up
— g/m²
Conditioning Time
— days

A sample arriving wetter than equilibrium desorbs rather than absorbs, and textiles are hysteretic — the equilibrium reached from above is not the same as from below, which is why standards specify pre-drying. Thick, dense or tightly rolled goods equilibrate far more slowly than the time constant of a single open layer.

ISO 139 Conditioning Time to Moisture Equilibrium — free, with the formula and a worked example, at Textile School.