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.
NIn 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.