New density-independent calibration function for microwave sensing of moisture content in particulate materials

Microwave techniques have been considered for a long time for moisture sensing in many food processing and agriculture-related industries. They are suitable for on-line real-time monitoring and control. However, with particulate materials, bulk density fluctuations cause significant errors in moisture content determination. To overcome this shortcoming, density-independent calibration functions are needed. In this paper, a new approach is presented in which both bulk density and moisture content are determined directly from measured microwave dielectric properties. A simple relationship between bulk density and the dielectric properties is identified, and a new density-independent function for moisture content prediction, exclusively dependent on the dielectric properties of the material under test (ε', ε"), is proposed. The validity and applicability of this function are demonstrated with an extensive data set obtained from measurements on a granular material (wheat), over wide ranges of frequency (11-18 GHz), temperature (-1°C-42°C), moisture content (10.6%-19.2%, wet basis), and bulk density (0.72-0.88 g/cm³). Explicit calibration equations for moisture prediction at different frequencies and temperatures are provided. Although data obtained by a transmission microwave measurement technique were used, this new approach remains valid in general for other techniques, provided that ε' and ε" are determined accurately