At AGM, a common problem our customers encounter is the buildup of moisture and condensation inside of their sealed projects. Such occurrences can aggravate engineers who have spent a lot of development time only to have projects experience moisture intrusion problems.
So, just how does moisture enter a sealed enclosure, in some cases, even after the volume has been purged?
There are four ways for water vapor or liquid water to enter a sealed enclosure:
Each ingress method is best mitigated with a different type or combination of desiccant. An effective desiccant will adsorb water vapor in the air, lowering the relative humidity to the point where water cannot condense (the dew point).
We'll take a closer look at each method of ingress, below:
Water vapor in the air can be trapped inside of an enclosure during sealing. Depending on the project and the components involved, it isn't always possible or cost effective to dry-gas purge, and construction inside a dry room is not always available or a guarantee. for this reason, desiccants are commonly used to adsorb moisture from the enclosed space. An easy example is found in pharmaceuticals, where desiccants and oxygen absorbers are included inside of pill bottles to adsorb moisture trapped inside when the bottle is sealed, as well as moisture that enters during repeated openings.
The materials used in many complex constructions, especially within the aerospace, military and space industries, are themselves constructed under specific conditions. For example, circuit boards are commonly constructed in approximately 70% relative humidity conditions. As a result, circuit boards absorb moisture. Once a circuit board is used and enclosed in an assembly, the boards will off-gas into the enclosure. If left unchecked, the off-gassing moisture will build up and can condense, corroding and shorting other electronics, encouraging fungus growth on optical lenses, rusting metal braces and even the housing.
To prevent off-gassing, many enclosures are purged with a dry gas, such as nitrogen, when sealed, in an effort to push out all water vapor. However, dry-gas purging requires flowing gas through an enclosure and can take quite a while to fully accomplish the desired result. Some of our customers have related their own experiences with dry-gas purging their projects only to find humidity or condensation and water inside the project enclosure later on.
A desiccant can be used in order to adsorb the off-gassed moisture. In these situations, however, it is important to consider desiccant adsorption rates, desiccant equilibrium points, and desiccant discharge rates.
All materials have a permeability rate, although different materials have different permeation rates. Moisture Vapor Transmission Rate (MVTR), is a measurement of how quickly moisture vapor will diffuse through a material. In our case, the packing and walls of sealed enclosures. These rates can be quite slow, while others can be fast, such as in tinfoil and fabric, respectively. Factors affecting MVTR include temperature and humidity throughout the course of the enclosure's life. In a sealed enclosure, water vapor will push through packaging in order to equalize pressure. Furthermore, moisture ingress will occur should an enclosure wall develop even a small hole.
The go-to solution for mitigating this issue is desiccant, either bulk or custom. Additionally, humidity indicator cards are often installed in the enclosure to help track internal humidity levels and manage accurate desiccant change-outs.
Similar to component off-gassing, the walls of an enclosure or packing material may contain moisture that can off-gas or is present on the inside surface. For example, a container constructed in the open environment most likely is not thoroughly dry when finished.
Again, dry-gas purging is not necessarily always feasible due to the enclosure size, construction, or cost. In these scenarios, it is generally easier and more cost effective to employ either bulk or custom desiccant, depending on the enclosure size and available internal volume.
AGM Container Controls, Inc.
3526 E. Fort Lowell Rd.
Tucson, AZ 85716