Dielectric Properties and Density Relationships for Granular Materials

Abstract

When knowledge of the dielectric properties, or permittivity, of granular or powderedmaterials is important, as in dielectric heating applications or in the sensing of moisture content byradio-frequency or microwave instruments, some reliable relationship between the dielectricproperties and the bulk density of the material is required. Linear relationships between functions ofthe permittivity and bulk density have been observed, and dielectric mixture equations have beenexplored to obtain useful relationships for determining the dielectric properties of granular andpowdered materials. These findings are reviewed, useful permittivity-density relationships areidentified, and an equation is provided that permits reliable calculation of the permittivity of aparticulate material at any given bulk density, if the permittivity is known or measured at anotherknown bulk density. The relationships are illustrated for pulverized coal, wheat, and ground wheat.The useful mixture equation is valid for granular and powdered materials with permittivities in thesame range as those for grain and coal.