The precise measurement of the permittivity and moisture content of granular products

The authors present results on the effect of moisture obtained by computer-aided measurements of the quality factor and the resonant frequency of a microwave cavity containing three to sixteen grains of the test product. Simultaneous measurements of moisture, temperature, dielectric constant, dielectric losses, and absorbed power were performed on agricultural products (e.g., rice, sarrasin, wheat, barley) and some porous granular ceramic products (molecular sieves or zeolites). The test grains were suspended from a gravimetric balance and were dried over a short period of time inside the test cavity by microwave power. The bone dry weight of the samples used to calculate the moisture level is accurately determined in these measurements: a sudden change occurs in the dielectric loss of the test sample when the grains begin to char. In the case of granular ceramic products (zeolites), more stable results are obtained