Calibration of a microwave system for measuring grain moisture content

A method of moisture content determination in grain is discussed based on simultaneous measurements of two parameters of the electromagnetic wave transmitted through the layer of material. These parameters, attenuation, A, (decrease of the wave amplitude), and phase shift, F, (the phase delay in the material), are directly related to the amount of water in the material. Both parameters, however, are also dependent upon the material temperature and its density at the time of measurements. It has been observed empirically for many moist materials and agricultural and food products, including grain, that by measuring attenuation and phase shift, the effect of density variation can be minimized, thus leading to a density independent calibration equation. Temperature of the material is usually easy to measure, and it can be introduced as another variable into the calibration equation, making the measuring system both temperature- and density independent. This technique, however, is not limited to the frequency reported in this paper. It can be applied for much lower frequencies, as long as the ionic conductivity of the material is not a disturbing factor