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DESICCANTS |
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Application
Characteristics
Desiccant
unit
Desiccant
types
Humidity
Indicators
Military
specifications
Application
of desiccant bags
Practical
calculation method
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Application
Machinery,
optical and electronic apparatus, military goods
and weapons systems, pharmaceutical products and
many others need protection against corrosion and
deterioration caused by high humidity and
condensation. This protection is needed during
both storage and transport, all over the world,
but especially when climate zones with high
temperatures and Relative Humidity levels are
encountered.
The
use of desiccants, preferably in combination with
a barrier material, provides this protection,
simply and cheaply, over long periods of time.
In
Europe, a system of norms for the methods and
articles was introduced a number of years ago,
called the DIN classification system. The use of
desiccants is included in this system, under norms
DIN 55473 and 55474. The desiccants we sell comply
with these 2 DIN norms.
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Characteristics
Desiccants
are granules of between 0.3 and 6.3 mm in which
there are countless numbers of pores. One gram of
desiccant can have a surface area of one m2. The
less water they contain themselves, the more
active they are i.e. the more water vapour they
can adsorb from the surrounding air.
Air
dried by desiccants will not release condensation
on sensitive surfaces when the temperature drops.
Sufficient
protection is achieved when the Relative Humidity
(RH) in a closed package does not exceed 40%
during storage or transport. This is easily
achievable with the correct use of desiccants.
Rust forming commences when the RH exceeds 40%.
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Desiccant
unit
According
to DIN 55473, a desiccant unit is the measure for
the adsorption capacity of the desiccant. At a
temperature of 22C, at least the following
quantities of water vapour must be
absorbed:
3 grams at 20% Relative Humidity
6 grams at 40% Relative Humidity
This
unit weighs approximately 33 grams.
Desiccants
are packed in sachets containing 1/6 of a unit (5
grams), 1/3 (10 grams), (15 grams) and larger
bags containing 1 (30), 2 (60), 4 (125), 8 (250),
16 (500), 32 (1000) and 80 (2500) units (grams).
Please
bear in mind that, depending on the size of the
area that is to be protected, 10 bags of 8 units
are generally more effective than 1 bag of 80
units.
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Desiccant
types
The
desiccants described here are activated,
calcium rich clay and silica gel which are not poisonous,
chemically indifferent and environmentally
friendly.
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Humidity
Indicators
These
are usually small pieces of cardboard with a
number of coloured circles impregnated with cobalt
chloride, each indicating a certain percentage
Relative Humidity. The percentages range from 10%
to 80%. As the humidity increases, the chemically
impregnated circles change from blue (dry) through
lavender to pink (humid). When it decreases, the
process is reversed. They are also supplied in
aluminium or plastic "windows", which
can be screwed into the packaging material and
through which the Relative Humidity inside the
package can be read.
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Military
specifications
Apart
from complying with DIN 55473 and 55474, our
desiccants also comply with:
MIL
- D - 3464 (US Army)
TL
6850 - 008 (German Army)
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Application
of desiccant bags
1.
The area to be protected must be hermetically
sealed or as close thereto as possible. Outside
openings must be closed off with Alu-foil or
rubber strips (e.g. sliding windows).
2.
The desiccant bags must be placed as high as
possible inside the area that is to be protected,
spread evenly from front to back.
3.
Internal compartments or lockers must be opened.
4.
Try to avoid that articles of wood, paper,
cardboard etc. are left inside the area. (These
can contain up to 20% of their weight in water
which will be released into the air when it warms
up and will condensate when it cools off). If
there are wooden struts or supports inside the
object/package, additional bags of desiccants have
to be used.
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Practical
calculation method
Determine
1.
The Moisture Vapour Transmission Rate (MVTR)
of the packaging material
-polyethylene
200 micron
=
0.4 grams per square meter per 24 hours
-
Alufoil A30
=
0.1 grams per square meter per 24 hours
2.
The surface area of the packaging material
in square meters.
3.
The total number of days for which
protection is required.
Formula
MVTR
x surface area x days
________________________
=Number
of desiccant units
6
Example
0.4
x 24sq.m. x 60 days
________________________ =96
Units
6
NB
the figure 6 below the line is the amount of
water in grams, 1 unit has to absorb to keep
the RH at 40%.
In
case there is cardboard/ wooden dunnage
inside the package, add 20 units of
desiccant per Kilogram of dunnage.
In
case it concerns a hermetically sealed
object (e.g. metal cases etc.), 5 unit of 30
grams must be used per m3 of space.
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