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 .
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
(3) That which diffuses through the outer packaging
during storage or transit.
These factors are considered in British Standard 1133: Section 19: 1968 as
The area of the moisture barrier : A (in square
(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
(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).
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).
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
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
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
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 send a cheque (made payable to Ravi Industries.)
to the above address if you would like to place
an order (please state the sachet size that
you need and include a return address). Or telephone
with debit/ credit card details.
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.