User manual
USER MANUAL
13
Moisture in substances is an essential quality factor of technical and economical
importance.
Methods of determining moisture may be grouped in two main categories: absolute and
deductive.
Absolute methods are based on simple relations, e.g. weight decline during drying.
Thermogravimetric analysis used in moisture analyser LEXUS AXIS is an example of
this method.
Deductive (indirect) methods measure physical quantity related with moisture, e.g.
electromagnetic waves absorption, electrical conductance, acoustic wave speed. Some
of these methods, unlike thermogravimetric analysis, enable to determine water content.
Thermogravimetry
- lat. thermo – heat, gravi – weight, metry – method
Thermogravimetric
analysis – a process of determination of a substance mass
decline as a result of heat-up. The sample is weighed before and after heating-up, the
difference is calculated in relation to initial weight or final weight (dry mass).
Moisture in
s
ub
s
t
a
n
c
e
s
Thermogravimetric analysis includes all ingredients evaporating from substances during
heating-up, which results in weight decrease.
In result of the above, determining of moisture content in substances is not equal water
content. Beside water, moisture consists of all other volatile matter: fats, alcohol,
aromas, organic dissolvent and other substances resultant as en effect of thermal
decomposition.
Thermogravimetric analysis does not distinguish water from other volatile matters.
Infrared radiation drying is more effective than traditional methods (e.g. in an oven) as
the radiation deeply penetrates the substance, which shortens drying time.
11.1 Infrared
radiation sou
r
c
e
ATS/BTS series moisture analyser uses 2 halogen heaters (rated power 100W,
l=78mm) in serial connection as a radiation source. The heaters emit also visible
radiation, which does not affect drying process.
11.2 Infrared
radiation
drying
d
e
sc
r
i
p
t
i
on
Sample drying is a result of absorption of infrared radiation, which results in sample
temperature increase and evaporation of volatile matters.
Infrared radiation penetrates surface layers, the depth depends on penetrability of a
sample (different in various substances). Part of radiation is reflected by the sample
surface. Penetrated layers absorb the radiation and convert its energy into heat. Emitted
heat propagates inside the sample. Effectiveness of the propagation depends on
thermal conductivity of the sample. The better the conductivity, the faster drying process
and volatile matter evaporation. During drying process sample parameters change, its
thermal conductivity decreases so there is a risk of burning the sample. Some
parameters may be estimated “by sight”, e.g. smooth and light surfaces reflect radiation
better. This must be taken into account when setting drying parameters.
11.3 Drawing and
preparation
of a
s
a
m
p
l
e
As sample of given substance must be representative, drawing and preparing a sample
is very important process as it affects repeatability of measurements. The most common