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Microwave moisture sensors represent a sound solution for real-time, nondestructive determination of moisture content in grains, seeds, nuts, and other agricultural products. One of the most critical phases in developing these sensors is their calibration against standard methods, in this instance, the oven-drying technique. In this study, dielectric properties of different varieties of wheat, and corn were measured at 10 GHz and an average room temperature of 25.2°C to investigate the influence of variety in calibrating microwave moisture sensors. Dielectric properties measured for all varieties tested for a given material were nearly superimposed, and the same was observed when plotting density-independent calibration function, ψ, as a function of moisture content. Performance of individual moisture calibration equations in predicting moisture content was compared to that from a single moisture calibration equation established for all varieties combined through computation of the standard error of calibration (SEC). In most instances, for both wheat and corn the SEC corresponding to individual varieties were nearly the same as that corresponding to all varieties combined. In addition, it was observed that a unified calibration can be established for both wheat and corn whereby moisture can be predicted from a single moisture calibration equation based on measurement of the dielectric properties at a single microwave frequency. Here, such an equation provided moisture content in wheat and corn with a SEC of 0.87%. [ABSTRACT FROM AUTHOR]

A brief historical perspective on dielectric properties of agricultural products is presented, including their use for rapid measurement of moisture content in grain and in considering potential dielectric heating applications. General principles are discussed as they relate to dielectric properties of materials and energy absorption from radio-frequency and microwave electric fields. Measurement principles and techniques for determining the dielectric properties of agricultural materials are described, and appropriate references are cited for detailed information. Dielectric properties data for some grain, insects, fruits, and vegetables are presented that relate to applications for selective heating of insects in grain and potential maturity and quality sensing in fresh fruits and vegetables. Principles are discussed for the use of microwave dielectric properties for sensing moisture content in grain, and relationships between dielectric properties of granular and powdered materials and their bulk densities are described. Index Terms--Dielectric properties, agricultural products, grain, moisture measurement, insects, fruits, vegetables, permittivity, granular materials, powders, permittivity-density relationships.