Moisture Measurement, Part 3
Barbara Kanegsberg, Mantosh Chawla with Ed Kanegsberg, September 2002
Achieving appropriate moisture levels in the substrate impacts quality and
costs. Last month, we discussed
techniques and measurement units, calibration issues, and interaction of moisture
with the substrate. This month, we
will contrast two techniques used for moisture measurement: gravimetric
and electrical impedance. The first is a primary method for moisture
determination; the latter, a secondary method. Primary techniques are quantitative,
extractive,
and typically destructive. Secondary techniques may be adapted for on-line
measurements. Both have their
place in establishing and maintaining product quality and in contamination
control. Gravimetric
measurement is a common
primary method for moisture determination. This technique is performed
off-line, often at an analytical laboratory. Moisture removal is
accomplished using a dry chamber, microwave drying, or infrared drying.
Sample preparation
and determination are time consuming. As for all primary methods,
accuracy of the gravimetric measurement is dependent upon sensitivity,
accuracy,
precision of instrumentation, skill of laboratory personnel, and
an accurate, consistent sampling protocol. Out of control processing, if
continued
during lab testing by the primary method, may potentially result in
product loss.
Further, by the time test results are obtained, the process may have
changed substantially. Gravimetric moisture determination can, however,
provide
good process control and this well-established and universal method
is
often the only way to provide the basic calibration required for
on-line process measurement methods.
Electrical impedance measurement is a secondary technique for determining
moisture levels in that a property of the analyte or substrate
is measured without extraction
of water from the sample. As with some other secondary techniques,
the electrical impedance technique provides the advantages of continuous
or rapid sampling
measurement and real-time process monitoring and control. While secondary
techniques must be calibrated against a primary reference method,
even a relatively unsophisticated
continuous moisture analyzer provides process trend information.
It is often productive to complement a secondary technique, such as electrical
impedance,
with periodic sampling for a primary determination by the gravimetric
method
or by Karl
Fischer titration.
Electrical impedance measurement relies on the high relative permeation
(dielectric constant) of water compared to any other host substrate.
Techniques involving
capacitance, resistance, or conductivity are somewhat similar. Electrical
impedance measurement can be used to detect near-surface moisture
(moisture in the top
10 to 12 mm). Because it is a penetrating measurement, it can be
used to measure non-homogeneous products. The large measurement area
can
indicate a representative
bulk average moisture. Compared with other on-line techniques, electrical
impedance is relatively inexpensive, reliable, and robust. The mechanical
sensor designs
suit a wide range of process conditions and can be used in high temperature
environments.
Numerous techniques have been developed to determine impedance, including
radio frequency (RF), microwave, and time domain reflectometry.
Variations in instrument
design and sophistication influence accuracy and reproducibility.
To measure the relative dielectric impedance it is necessary to
electrically couple
the material to the sensing circuit. Placing the material between
two
parallel electrodes is an option, but not readily adapted to on-line
application.
In
contrast, an RF operated sensing circuit can propagate RF energy
through the material and thus couple to the product without physical
contact.
Alternatively, planar fringe field electrodes provide a single-sided
measurement structure
less obstructive to the process.
Electrical impedance has limitations. Impedance measurements cannot
be made with moving substrates. Further, the technique is not
suitable for
moisture
levels above ~14 to 15%. Impedance is influenced by such variables
as temperature, density, and product shape. Measurement of these
variables and combined
dielectric/loss impedance minimize or eliminate these effects.
Further, the influence of
sample geometry and presentation (variations which can occur
in the bulk
packing density
or in the volume of fibrous materials), may be minimized with
application-specific sensor design. True dielectric moisture instruments
are rare. Most
low-cost instruments do not separate the dielectric (capacitance)
and loss (resistance)
components, with the potential for compromising repeatability
and long-term stability.
In the next column, we will discuss details of the primary Karl-Fisher
technique and two additional secondary on-line techniques,
near infrared reflectance
and millimeter wave.
In the next column, we will discuss more of the common secondary
methods for moisture measurement.
Next month: A discussion of the most common techniques for moisture measurement.
Contact Barbara Kanegsberg and Ed Kanegsberg at BFK Solutions;16924 Livorno
Dr., Pacific Palisades, CA 90272,310-459-3614;barbara@bfksolutions.com;contact
Mantosh Chawla at Photoemission Tech., (PET), 3255 Grade
Vista Dr., Newbury Park, CA, 91320;805-499-7667
