Commercial Moisture Standard
Trade contracts for papermaking raw materials require a uniform baseline for calculating transactional weight. The standard known as air dry pulp represents sheeted or baled fibre adjusted to a theoretical equilibrium of ninety percent bone dry material and ten percent moisture. This convention ensures that buyers pay for fibre rather than water.
Baled Weight Calculation
Determinations of invoice weight rely on testing procedures defined by international standards such as ISO 801. Samples are extracted from selected bales across a shipment to establish the average moisture level of the lot. The commercial weight of air dry pulp is then computed by multiplying the gross wrapped weight by the ratio of the tested dry matter percentage to ninety percent.
In practice, a batch with eighty-five percent dry matter contains less air dry weight than its actual weight. If a shipment of twenty tonnes has a tested dryness of eighty-eight percent, the invoiced weight decreases accordingly.
Process Energy Requirement
Thermal drying at the pulp mill consumes steam to bring the web to its target moisture equilibrium before baling. If the mill over-dries the sheet below the standard level, it wastes thermal energy and risks damaging the strength of the cellulose fibres. The processing limit of air dry pulp is dictated by the point where natural atmospheric re-absorption begins, meaning that over-dried sheets quickly reclaim moisture from ambient air during storage or transit.
This equilibrium prevents additional mass changes.
