Desiccant is a drying agent that is commonly to use for absorbing moisture in the air from the moisture barrier bag. After the barrier bag bag is well sealed, the desiccant will absorb the moisture from the air remaining in the bag. The desiccant remains dry even when it is completely saturated with moisture vapor.
In general, desiccant is sold by the “Unit” or fractional Unit. One unit of desiccant will absorb a specific amount moisture. A unit weighs about 28 grams or 1 ounce. The pouches of desiccant are placed into metal pails to keep the desiccant dry during shipping and storage. For example, 4PLDES500 from SCS is a 4 units of desiccant.
There are different units of desiccant available to select. How much desiccant do you need? Here are two methods that you may consider.
Method 1: per MIL-P-116
Formula: Number of desiccant required (units) = 0.011 x bag area in square inches
For Example, 10” x 20” barrier bag
Finding Bag Area 10” x 20” x 2” sides = 400sqin
By applying formula,
Number of desiccant required = 0.011 x 400sqin = 4.4 units
Therefore it suggests to use 4.5 units of desiccant
Here is a table for recommended desiccant usage:
. Information taken out of EIA-583, Table 1, Page 8.
*To measure interior bag surface area, multiply length x width x 2
**MIH = Maximum Interior Humidity (%)
Method 2: per EIA 583.
This method allows you to tailor the desiccant to your specific needs by using below formula.
Before making the calculation, you should have the following information:
Bag area, Bag MVTR (moisture vapor transmission rate, or how much moisture the bag lets in over time), Months of Storage, Maximum Interior Humidity (MIH)
Formula: Number of desiccant required (units) = 0.231 x Bag Area x Bag MVTR x Months divided by Moisture Capacity
For example, 10” x 20” barrier bag with a 0.02 MVTR, a 12 month storage time and a MIH of 20%
By applying formula,
Number of desiccant required (units) = 0.231 x 400sqin x 0.02 MVTR x 12 divided by 4.8g/unit = 4.6units
Therefore, it would be suggested to use 4.5 units of desiccant