In a sealed moisture-control packaging, the amount of desiccant is related to the packages' moisture permeability, the storage period, and the volume of the container. Taking silica gel as an example, the following empirical formula can be used to calculate the amount of silica gel needed:
m1 -- Amount of silica gel needed, kg;
K -- Coefficient of environment;
A -- Full packaging area，m2;
t -- Shelf life, month;
R -- Moisture permeability of packaging materials, g/(m2*24h);
m2 -- Cushioning material weight, kg;
If the moisture permeability is zero for packaging material, and no cushioning inside the container, then R = 0、m2 = 0 , the following empirical formula can be used:
m = 0.5 V
Coefficient of environment table
| Environment Condition | K | ||
|---|---|---|---|
| Condition | Avg.Temp ℃ | Avg. RH % | |
| Extreme high temperature and humidity | 35 ~ 40 | > 90 | 12 |
| High temperature and humidity | 30 | 90 | 20 |
| High humidity | 25 | 80 | 30 |
| general condition | 20 | < 70 | 60 |
The moisture permeability can be found by:
1.Metal and glass container: R = 0;
2.PE/Aluminium foil/Fiber cloth composite material: R ≈ 0;
3.The moisture permeability of a plastic film is related to its thickness, environment condition, and its R value can be test by GB1037 - 1988 "The sheet-cup method for testing water vapor permeability of a plastic film"
Example #1 A precision optical instrument, 0.2 kg cushioning, put inside a metal container, how many silica gel needed?
Answer: It's a metal container, so R = 0.
m1 = m2 / 2 = 0.2 / 2 = 0.1 (kg)
Example #2 Inside a package, the package's moisture permeability is 4g/(m2*24h), the packaging area is 1m2, shelf life is 2 month, the package contains 1 kg cushioning for protection, the environment condition is 35 ~ 40℃, and RH >90%, how many silica gel needed?
Answer: Through coefficient of environment table, K=12
m1 = (1 * 4 * 2)/12 + 1/2 = 7/6 (kg)
Example #3 Moisture permeability is 20g/(m2*24h), full packaging area is 1m2, shelf life is 2 month, no cushioning, the environment condition is 35 ~ 40℃, and RH >90%, how many silica gel needed?
Answer: Through coefficient of environment table, K=12, no cushioning, so m2 = 0
m1 = (1 * 20 * 2)/12 + 0/2 = 10/3 (kg)
Example #4 A 0.1m³ metal container contains a product, no cushioning, how many silica gel needed?
Answer: It's a metal container, so R = 0. There's no cushioning inside the container, therefore, m2 = 0
m1 = 0.5V = 0.5 * 0.1 = 0.05 (kg)