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 sens ing in many food processing and agriculture-related industries. They are su itable for on-line realtime monitoring and control. However, with particula te materials, bulk density fluctuations cause significant errors in moistur e content determination. To overcome this shortcoming, density-independent calibration functions are needed. In this paper, a new approach is presente d in which both bulk density and moisture content are determined directly f rom measured microwave dielectric properties. A simple relationship between bulk density and the dielectric properties is identified, and a new densit y-independent function for moisture content prediction, exclusively depende nt on the dielectric properties of the material under test (epsilon', epsil on "), is proposed. The validity and applicability of this function are dem onstrated with an extensive data set obtained from measurements on a granul ar material (wheat), over wide ranges of frequency (11-18 GHz), temperature (-1 degrees C-42 degrees C), moisture content (10.6%-19.2%, wet basis), an d bulk density (0.72-0.88 g/cm(3)). Explicit calibration equations for mois ture prediction at different frequencies and temperatures are provided. Alt hough data obtained by a transmission microwave measurement technique were used, this new approach remains valid in general for other techniques, prov ided that epsilon' and epsilon " are determined accurately.