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
(3) That which diffuses through the outer packaging during storage
These factors are considered in British Standard 1133: Section
(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
(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
(a) Variations between daytime and night-time temperatures reach 20
(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
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
SILICA GEL SACHETS FOR HOME USE
Silica gel can often be useful in domestic applications, but quite
hard to find. To this end, we have three sizes of sachets available
in smaller quantities, thus:
50 x 10 gram self-indicating (blue to pink) silica gel sachets.
25 x 25 gram self-indicating (blue to pink) silica gel sachets.
10 x 100 gram self-indicating (blue to pink) silica gel sachets.
Kindly mention the requirement to enable us to give the quotation
Please remember also that there are applications to which silica gel
isn't really suited. These are mainly damp but large and unsealed
spaces such as: bathrooms, basements, sheds, caravans and cars
(which are in regular use). Even quite large amounts of silica gel
in areas such as these can quickly become exhausted and any
improvement is often very short term only.