Laboratory Sampling
A fibre bed conditions slowly because the vapour is adsorbed before it can travel, not because air is slow.
— h
From the stated initial regain to within tolerance of equilibrium
This is a diffusion estimate for a slab exchanging through its faces, so it applies to stacks, flat specimens and open beds. A wound package is a cylinder with a much longer path than its wall thickness suggests, and this will read optimistic for one. The isotherm is treated as linear over the interval travelled, which is reasonable near the standard atmosphere and poor at the extremes where the curve turns sharply. Sorption hysteresis is not modelled: the standards require approach from the dry side precisely because the equilibrium value itself depends on direction. Heat of sorption is ignored - real conditioning warms the specimen slightly and self-retards a little at the start. Where the answer matters, the standards' own criterion of successive weighings remains the acceptance test; this predicts how long to expect before that criterion is met.
Fibre & Fabric Conditioning Time to Moisture Equilibrium — free, with the formula and a worked example, at Textile School.