What Is the Desiccant Unit System?

While all desiccants pull moisture from the air, the amount of product needed to reach the same relative humidity (RH) level will vary. For example, with high-performance materials like molecular sieve, you can get away with a much smaller packet to achieve the same results that you'd see with a much larger packet of a traditional clay desiccant. Even the geographic source of the material can have an effect on the strength; montmorillonite clay from one part of the world might have different adsorbtion rates and strength compared to another depending on the specific conditions in which it was formed.

In response to the need for a system to measure sorbents by their capabilities rather than quantities, the U.S. Department of Defense created a Military Specification, or Mil-Spec, to enable collaboration and accuracy when using these materials. The resulting Unit System has become standard in the desiccant industry. According to MIL-D-3464 Rev. E, a Unit is the amount of a desiccant required to adsorb 3.0g of moisture from a container with 20% relative humidity (RH), and 6.0g from a container with 40% RH at 25° C / 77° F.

Unit values under MIL-D-3464E range from 1/6th unit to 16 units, so if you see an 80-unit pouch claiming to fit military specifications, that seller does NOT know their business or does NOT respect the Mil-Spec system.

Manufacturers have embraced the unit system in part because for purchasers in the United States Military, it is difficult to buy outside the approved military spec for desiccants. The standardization of the unit system helped ensure that they can rely upon the same results, every time. This reliability is worth its weight in gold to some customers, and a responsibility we at IMPAK take seriously.

A hypothetical example that one of our desiccant experts likes to use is this: If you owned a small ranch in Wyoming and discovered that 15 grams of the clay on your property, when baked out, could adsorb 6 grams of moisture at 77° F and 40% relative humidity, you might want to start a new business. 15 grams is much less than the 33 grams of clay in a standard 1 unit packet, and with that low of a volume of desiccant material comes a similar reduction in coverstock (the material used for bags or packets) and external packaging for those bags or packets. And that's on top of the the usefulness of offering a much smaller amount of clay that a potential customer needs to account for in their packaging.