Principles of grain and seed moisture sensing through radio-frequency and microwave dielectric properties

Abstract. The basic principles are presented for sensing moisture content in grain and seed through measurement of their permittivities, or dielectric properties, and the correlation of those properties with moisture content. Correlations of dielectric properties with moisture content will be presented for cereal grains and oilseeds over wide ranges of frequency from 1 MHz through 15 GHz in the microwave region. The influence of grain and seed bulk density and temperature on the dielectric properties will also be discussed. The historical development of grain moisture meters using these principles will be covered briefly. At microwave frequencies, both the real and imaginary parts of the relative permittivity, i.e., the dielectric constant and the loss factor, are conveniently utilized with a density-independent moisture calibration function. With microwave measurements, it appears that adjustments and corrections for different grain types are unnecessary in the calibration process, and temperature can be included in that process. In addition, at microwave frequencies, errors due to uneven moisture distribution in the kernels will be less than those encountered at frequencies below the microwave range. The density-independent nature of moisture sensing in grain and seed provided by microwave frequencies also makes the monitoring of moisture in flowing grain much more successful than it has been with capacitive sensing at lower frequencies. The use of grain and seed permittivities measured at microwave frequencies shows promise for simultaneous sensing of moisture content and bulk density in both static and flowing materials, providing moisture content independent of bulk density. Because of advantages offered by measurement at the higher frequencies, commercial development of new moisture meters for grain and seed can be expected to improve the reliability and utility of such instruments in the grain and seed industries.