Free-space reflection method for measuring moisture content and bulk density of particulate materials at microwave frequency.

A measurement system based on free-space reflection method is designed for simultaneous and independent determination of moisture content and bulk density of particulate materials. The proposed system consists of microwave cavity oscillator, horn antenna, slide rail, sample holder, mixer, and digital meter. Sand and rice with different moisture contents and bulk densities are chosen as samples. Calibration models for moisture content and bulk density are proposed according to the measurement of the position of the minimum of the traveling-standing wave and the ratio of the maximum-to-minimum field strength of the traveling-standing wave at different temperatures. The moisture constant, ranging from 0% to 24.6%, is obtained with a coefficient of determination (R(2)) greater than 0.982 and a standard error of prediction (SEP) value of less than 0.695%. The bulk density, ranging from 0.501 g/cm(3) to 1.822 g/cm(3), is determined with a R(2) ≥ 0.961 and a SEP value ranging from 0.0144 g/cm(3) to 0.0382 g/cm(3) for different samples.