Moisture content, from microwave measurements

An extremely useful technique for measuring the amount of size applied is non-contact on-line determination of water absorption [ 194]. The moisture content of sized warps can be derived from microwave absorption by the water present. 

One of the most successful early measurements was the use of microwave interrogation for the determination of moisture content of foods and materials. An issue of the Journal of Microwave Power was dedicated to the subject of microwave aquametry and contains an extensive bibliography on the subject from 1955 through 1979 (JMP 1980). A more recent compilation has been edited by Kraszewski (1994). 

In our discussion of electromagnetic techniques, we omitted a few available technologies that provide some unique capabilities and, with further development, can attain practical application. One such technique involves the use of a microwave resonance sensor (Kobyashi and Miyahara, 1984) that uses a microwave cavity to measure solids concentration and velocity by monitoring the resonance frequency shift. However, this technique suffers from some shortcomings the frequency shift may be positive or negative, depending on the dielectric properties of the solids, and the cavity is extremely sensitive to changes in moisture content and temperature. 

Distributed circuit methods use coaxial lines, waveguides and resonant cavities at microwave frequencies. The circuits are designed for measuring an attenuation factor and a phase factor, from which sample dielectric properties can be calculated. The sample may form the dielectric medium between the two conductors of a coaxial line (Scaife et al, 1971), or an open coaxial line is brought into contact with the sample surface (Roussy and Pearce, 1995). Fagan et al, (2004) used an open coaxial line method to demonstrate that the moisture and salt contents of processed cheese could be predicted by measuring dielectric properties over a range of frequencies. 

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