Moisture Measurement
Barbara Kanesberg and Mantosh Chawla, July/August 2002
Moisture occurs in nearly all substances. In our last column, we discussed
ways in which inappropriate moisture levels can negatively affect the properties
of the host material (e.g. the substrate) as well as impact cost and product
quality. There are many techniques for the measurement of moisture. Because
these have largely grown up in discrete industries, there is a good deal of
confusion and ambiguity concerning techniques and measurement units, calibration
problems, and moisture interaction with host materials.
Techniques and Measurement Units
Depending upon the technique, different units are used to measure and
define the moisture level. These techniques can be broken down into
three major areas:
Standard gravimetric methods employ a known weight sample which is
heated to drive off the free water by evaporation after which the
remaining dry
weight
is measured. The difference in the sample weight before and after heating
is a measure of the moisture content. Weight is the unit of measure and
moisture content is specified as percentage of the bulk weight.
Electrical impedance techniques make use of the huge difference in
the dielectric constant of water compared to most common host materials.
By
applying an
electrical potential to the sample, measurements of the current flow,
resistance to flow,
or the charge capable of being put in to the field of view, a reading
of the relative conductance can be made and the moisture level
gauged. This
technique
uses electrical measure as units to define the level of moisture.
Spectroscopic methods generally use the near infrared part of the spectrum,
more exactly one of the three vibration energy levels of the hydrogen/oxygen
bands. For measurement of moisture in solids a surface reflectance
technique is used. The substrate, in the absence of water, has a
characteristic spectroscopic fingerprint. With careful calibration,
changes in the
spectroscopic
fingerprint
when water is present can be used to indicate moisture level. This
technique can be used either in gases and transparent solids, or,
by surface reflection,
for non-transparent solids. As with gravimetric methods, this technique
expresses the moisture content as a ratio.
Calibration
Calibration is a recurring problem in moisture measurement. Until
it is resolved, fragmentation and misunderstandings will persist,
especially
since it is
very difficult to develop and maintain a primary reference standard
for bulk moisture
content, nor are there any guidelines for units of measurement.
The instrument user for each and every product for which measurement is required
necessarily carries out calibrations
and
comparisons. This normally involves extracting samples from the
process, recording the
reading
on the instrument, then oven drying the sample to constant mass.
The error involved in this approach can be large, though the accuracy
and precision of the measurement techniques are so variable that
this “noise” is
often masked. However this remains the principal practical method of calibration
due to lack of any primary, secondary or tertiary reference standard. There
is also the problem of checking the instrument for drift or variation and,
if either of these occurs, knowing what exactly needs to be done to remedy
the situation. This becomes of paramount importance when looking at on-line
moisture determination. For on-line techniques to have practical value in a
production situation, re-calibration must be easily performed by the user without
complicated reference to third parties for traceable reference standards.
Moisture Interaction with Host Materials
Water may be physically or chemically bound to the host material.
Physically bound. This is the state most associated with water
in which it sits with the host material throughout its volume,
whether
absorbed
or adsorbed.
Apart from solids, it also covers the liquids that form creams,
oils, emulsions and colloids, or in some liquids as solutions
such as methanol
and ethanol.
Chemically bound. This is where the water forms part of the
crystal lattice or it hydrogen bonds to another charged species.
Also
included in this
state would be ice, where water hydrogen bonds to itself. Techniques,
calibration and moisture interaction with the host material
can contribute to ambiguity
in moisture measurement.
Next month: A continued discussion of the most common techniques
for moisture measurement.
Barbara Kanegsberg at BFK Solutions, 16924 Livorno Dr., Pacific Palisades, CA
90272, 310-459-3614; barbara@bfksolutions.com;
contact Mantosh Chawla atPhotoemission Tech., (PET), 3255 Grande Vista Dr.,
Newbury Park, CA, 91320; 805-499-7667
