Density- and structure-independent calibration method for microwave moisture determination in granular materials

The moisture content of granular materials can be determined indirectly and nondestructively from measurements of their dielectric properties at a given frequency and a given temperature. Several sensors have been developed based on this principle and successfully used in different industries including pharmaceutical, construction, and food and agricultural products processing. However, the effects of both the bulk density and structure are troublesome and cause significant errors if they are not taken into account or eliminated by defining appropriate calibration functions. It is possible to determine bulk density by an additional device, such as a /spl gamma/-detector, and compensate for its effect in the reading output of the meter. The structure, i.e., size and geometry in this instance, has a more subtle effect and although it is known to affect the scattering and absorption properties of the material, it is not possible to quantify this effect by measurement or theoretical means, and hence it cannot be compensated for in the final meter output. The only method applied to this time requires individual calibration for each particular material and instrument. In this paper, a new method is presented for moisture determination in granular material independent of bulk density and structure. This unified calibration method is based on the combination of two concepts, i.e., density independence and structure independence, embodied in a function fully defined in terms of the dielectric properties.