You can’t prevent moisture from entering an enclosure. It’s in the sealed-in air. It’s in the enclosed materials. It even permeates through the walls and seals of the enclosure. And, when a transit case is equipped with a breather for pressure equalization, air will be drawn in from outside, too.
Moisture causes corrosion and provides a fertile environment for mold. But, it becomes an even bigger problem when an enclosure endures dramatic temperature changes, especially cooling. Warm air holds water vapor. But, when the case cools, that moisture condenses. The result can be damaged materials and electrical short circuits.
Given that moisture will always find a way in, the best approach is to mop it up. A number of drying agents – or, more technically, desiccants – will do this. A desiccant placed in the enclosure will draw moisture out of the air and lock it up, preventing condensation damage.
A desiccant attracts moisture to its surface and holds it. There are many different types. Each has different characteristics and prices. Desiccants are specified in terms of the proportion of their weight they can absorb. A good desiccant will take up to 35% its own weight in water. One less effective, but possibly cheaper, may only take up 10%.
Note, desiccants have a finite capacity to adsorb moisture. When they’re “full”, their effectiveness is at an end. Most though, can be regenerated by careful drying.
How much desiccant to use depends primarily on the volume of the enclosure and how much moisture is expected. However, there are no hard-and-fast rules. Many factors determine the most appropriate type and quantity of desiccant for an application. A trial and error approach may be satisfactory in some applications. However, with high value packages, or when there’s no margin for error, the best approach is to consult with an expert.
Damaging moisture is always present in a transit case. So, it needs to be controlled with a desiccant. By anticipating the likely conditions, it’s possible to determine the appropriate type and quantity to use.