6 Determining moisture levels
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6
Determining moisture levels
The moisture analyser is used as a quick and reliable means of
determining the moisture content in powders and liquids by the
thermogravimetric process.
6.1
Fundamental principles
The term moisture does not just relate to water, it also encompasses
all substances which evaporate when they are heated. Alongside
water, they also include
– fats
– oils
– alcohol
– solvents
– etc...
There are different techniques for determining the moisture of a
material.
Thermogravimetry is the technique used in the moisture analyser. In
this technique, the sample is weighed before and after heating so as
to determine the moisture content from the difference.
The conventional drying oven technique works on the same principle
except that, the measurement takes much longer. In the drying oven
technique, the sample is heated from the outside inwards by a stream
of hot air so as to draw out the moisture.
In the case of the halogen radiation used in the moisture analyser, the
radiation mainly penetrates into the sample where it is converted into
heat energy, heating the sample from the inside out. A small portion
of the halogen radiation is reflected by the sample; this reflection is
lower in dark samples than in light samples. The penetration depth of
the halogen radiation depends on the permeability of the sample. In
the case of low-permeability samples, the halogen radiation only
penetrates into the upper layers of the sample, which may lead to
incomplete drying, charring or combustion. Consequently, the sample
preparation is extremely important.