The Desiccant Method
The desiccant method is used to protect merchandise against damage from moisture. To protect the merchandise, a desiccant is added in a closed envelope (or a closed space). The desiccant takes moisture out of the air, generally reducing the relative humidity to a safe value of approx 40% at 20° C when correctly dimensioned.
Migration of Moisture
Air stores much more water at high temperatures than at low temperatures. At maximum relative humidity of 100% and 30° C, this will amount to 30.4 g/m3, but at 5° C it will only be 6.8 g/m3. Since this moisture cannot escape through the outer envelope when temperatures drop, it is deposited in the merchandise or on the packaging materials. If the merchandise does not warm up as fast as the air, condensation will in fact form on the surface of the merchandise.
Conversely, when temperatures rise moisture migrates out of the merchandise and/or the packaging materials into the air. Thus, for example, during packaging at the warehouse a moist wooden pallet can increase the originally low relative humidity in the envelope from, for example, 30% to 80%. Acute temperature fluctuations are not only caused during the transport of merchandise through different climate zones. The difference between a day with strong radiation by the sun and the following night can make for a difference of over 40° in
The four moisture types
For correct dimensioning, all "four mois-ture types" have to be taken into account. In this closed system, the moisture values can either be precisely calculated according to formulas or estimated according to experience. This procedure is described in detail in the dimensioning section.