The general rule is that a minimum of 1.2 units of desiccant per 1 cubic foot of volume is needed to adequately protect a given space. For more information about desiccants, AGM’s "Selecting the Right Desiccant" has you covered.
As an example, a container measuring 15” x 15” x 12” has an internal volume of 2,700 cubic inches, or about 1.6 cubic feet. Such a container will need approximately 1.9-units of desiccant (1.6 multiplied by 1.2) to keep dry. However, since 1.9-unit bags are not common, it’s recommended to round up – in this case to a 2-unit bag, as it is better to use more desiccant than less.
For quick math, try:
A unit of desiccant is defined in the US military specification MIL-D-3464, which codifies standards for dehydrating agents, like desiccants. In the specification, it is stated that 1 unit of desiccant is equivalent to the amount of desiccant required to adsorb 3 grams of water vapor in an environment of 20% relative humidity and temperature of 25C (77F) AND 6 grams of water vapor in an environment at 40% and temperature of 25C (77F).
The mil-spec definition means that a unit of desiccant is a different quantity for different desiccants because different desiccants feature different adsorption characteristics.
For most applications, a simple estimation using either the Desiccant Unit Calculator or the reference chart below is adequate to determine how much desiccant you need. For example, storing firearms or other similarly sized metal objects, cameras, or perishables around the house can safely be stored using an estimated desiccant quantity.
However, many government and professional projects involve components with significant moisture sensitivities. Examples include UV telescope optics used on some satellites, storing and shipping hypersonics or rocket engines, etc. For projects like these, AGM Engineering works closely with project engineers to determine necessary desiccant requirements as well as pressure relief equipment. AGM’s Hydra20 is a service offered for reviewing a project based on specific environmental and usage variables that accurately prescribes desiccant amounts and changeout intervals to protect equipment for any length of time.
Accurately determining the amount of desiccant needed to properly dehumidify a space can be tricky due to the numerous variables to consider, such as:
While it is possible to accurately calculate desiccant requirements and amounts (see AGM's Moisture Analysis Service), the process is far more involved and requires precise measurements in all of the variables listed above, as well as many others. As a result, these calculations are most often done for equipment highly sensitive to moisture or frost. For many projects, however, estimating desiccant amounts is enough. Or, at least, enough to get started - even for advanced tech projects!
For precise desiccant calculation, AGM's engineering team offers free moisture analysis. Using our extensive materials-data library in coordination with proprietary software, AGM engineers can quickly review seal, desiccant, and structural material options to help you make smarter decisions. AGM moisture analysis determines optimal moisture protection for hygroscopic and moisture sensitive materials, as well as helps you hit your desiccation targets across any length of time.
Below is the quick reference guide to estimating desiccant units. The units specified apply to bulk, packaged, and custom desiccant quantities and, thus, the guide is a good multi-purpose estimator. However, seeking additional advice in determining appropriate quantities is strongly recommended in the two following scenarios:
Finally, it should be noted that there are many different types of desiccants, not all of which are equally applicable to every scenario. Thus, it is important for projects involving precise dryness targets that AGM Engineering review the project before desiccant types and quantities are fully committed and specified.
|Rigid Barrier Dimensions||Units Req.|
|Cubic Inches||Cubic Feet||Gallons|
The above example and reference guide pertain to rigid containers only, which are defined as moisture-impervious containers made from metal, plastic, glass or combinations of different materials. Furthermore, the above guide does not take into consideration relative humidity (RH) or other on-site atmospheric conditions. As a result, further information is often required to accurately prescribe appropriate desiccant amounts for many shipping and storage containers.
For help protecting your application, or for information about determining the amount of desiccant needed for non-rigid containers and containers that may be at risk for high moisture vapor transmission, call AGM at (520) 881-2130, or email us with the link below!
AGM Container Controls, Inc.
3526 E. Fort Lowell Rd.
Tucson, AZ 85716