The aim of using silica gel sachets is to minimize the risk of irreversible damage to items in transit or store caused by condensing water vapour. By adsorbing water molecules, a silica gel sachet will reduce the Relative Humidity inside a package to levels at which condensation will not occur .

Three main moisture sources need to be considered

(1) That from the air contained in the package.

(2) That from the packing added inside of the package (e.g. for      cushioning purposes).

(3) That which diffuses through the outer packaging during storage or      transit.

These factors are considered in British Standard 1133: Section 19: 1968 as

(a) The area of the moisture barrier : A (in square meters).

(b) The type of moisture barrier : R is the moisture vapour transmission       rate of a barrier (in grams per square meter per 24 hours,measured       at 90% relative humidity and 38°C).

(c) The volume of the air inside the barrier : V (in cubic meters of the      container).

(d) The weight and type of any packaging material inside the barrier.       This is known as the dumage factor :  DF (D is the weight of             packing material in grams and F is a factor depending on the type        of damage and its probable moisture content).

(e)  The length of time protection is required : M (in months).

(f)  The type of climate - temperate (average 25°C and 75% relative       humidity) or tropical (average 38°C and 90% relative humidity).

Typical R values are :

10 for a thin poly bag.
3.4 for a heavy duty poly bag (500 gauge).
2 for a heavy duty poly bag (1000 gauge).
0.05 for an aluminum barrier foil.

F values are : 1/5 for timber of moisture content higher than 14%.
1/8 for felt, carton board and similar general packaging work.
1/10 for plywood and timber with a moisture content less than 14%.

They are used in the following formulae :

For temperate climates W = 11 x A x R x M + DF For tropical climates W = 40 x A x R x M + DF For hermetically sealed containers W = 170 x V + DF

Where W is the weight of silica gel required (in grams).

It is difficult to get exact weights of silica gel required from the above formulae owing to the many variables that have to be considered, some of which are difficult to measure. The formulae can, however, be a useful guide to indicate the amount of silica gel required to give adequate protection, it is often prudent to choose the nearest standard size silica gel sachet above the value that W comes out at. For large values of W, a more effective solution than using a single large sachet, is using a number of smaller sachets spread around the package. This will reduce the Relative Humidity at a faster rate.

Items especially at risk.

Condensation of water vapour from air will not occur, even when the relative humidity approaches 100%, providing the air temperature remains constant. So the risk of condensation (and therefore corrosion) arises mainly because of large drops in air temperature, where at the new lower temperature, the volume of water vapour is too much for the air to hold. Special attention needs to be paid in situations where

(a) Variations between daytime and night-time temperatures reach 20 -      30°C.
(b) Seasonal temperature variations are large.
(c) Temperature variations occur due to the intermittent functioning of      encased apparatus such as electrical conductors and resistors, fuses,      circuit breakers or relay motors, which release calories when in use.

The greatest risk of condensation is in high humidity environments, where a drop of just 5°C will on average, cause water vapour to condense. As a general rule it is enough to maintain the relative humidity of the enclosed air below 30% to avoid all risk of condensation.