Inexpensive Microwave Moisture Sensor for Granular Materials

Authors: Trabelsi, Samir; Nelson, Stuart

Submitted to: IEEE IGARSS Annual Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 5/20/2007
Publication Date: 7/9/2007
Citation: Trabelsi, S., Nelson, S.O. 2007. Inexpensive Microwave Moisture Sensor for Granular Materials. IEEE IGARSS Annual Proceedings.

Interpretive Summary: Moisture content of cereal grains and many other agricultural products is the most important characteristic that determines their suitability for storage, and it is also important in determining the selling price. Electronic moisture meters are used almost universally in the grain trade for determining the moisture content of samples taken from lots being delivered to grain elevators, transportation facilities, and processors. However, reliable moisture sensing equipment for continuously monitoring grain and seed moisture content on harvesting equipment, when loading and unloading grain, and in processing is needed to provide better moisture content information for managing these operations. It has been shown that microwave measurements on grain and seed can provide reliable moisture information independent of packing variations that take place while grain is moving in a chute or conveyor. New research has revealed that a single calibration based on the dielectric properties of grain measured at microwave frequencies can reliably sense the moisture content of wheat, corn, grain sorghum, soybeans, and barley. Because these five commodities have very different kernel shapes, sizes, and composition, this new technique offers promise for a universal calibration for grain, soybeans, and similar crops. A prototype microwave moisture sensor has been assembled with off-the-shelf microwave components and tested for measurements on corn and wheat over wide ranges in moisture content. The relatively inexpensive instrument provided moisture content independent of bulk density and kind of grain with accuracies better than 1 percent moisture content. The results provide an incentive for the development of practical microwave moisture sensing instruments that can provide new tools for the maintenance of high quality in such agricultural products, thus benefiting farmers and consumers as well.

Technical Abstract: A prototype microwave moisture sensor is described that was assembled from relatively inexpensive microwave components and tested for sensing moisture content in corn and wheat. Components include off-the-shelf voltage-controlled oscillator, isolator, power splitter, two 19-dBi microstrip patch antennas and I/Q demodulator connected through a multichannel connector box to a personal computer. It uses a free-space measurement technique for determination of dielectric properties of grain samples and a unified calibration algorithm for determining moisture content independent of grain bulk density and kind of grain. Results with corn and wheat of moisture ranges between 11 and 25% and 10 and 19%, respectively, provided moisture content with a standard error of calibration lf 0.8% moisture content for either grain type with a single calibration.