A Low-Cost Microwave Sensor for Simultaneous and Independent Determination of Bulk Density and Moisture Content in Grain and Seed

Authors: Trabelsi, Samir; Nelson, Stuart

Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: 4/26/2007
Publication Date: 6/18/2007
Citation: Trabelsi, S., Nelson, S.O. 2007. A low-cost microwave sensor for simultaneous and independent determination of bulk density and moisture content in grain and seed. Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE).

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 wheat and soybeans 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. It also can provide bulk density from sensed microwave dielectric properties with relative errors of about 2 percent. 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 low-cost microwave sensor for nondestructive, rapid sensing of bulk density and moisture content in grain and seed has been built and tested. The sensor operates at a single frequency of 5.8 GHz and uses the principle of free-space-transmission measurement of the relative complex permittivity. Results of complex permittivity measurements with this sensor compared very well to those taken with a vector network analyzer and a pair of horn-lens antennas. Three different calibration algorithms were used to determine bulk density and moisture content from the two components of the relative complex permittivity. With the first calibration algorithm, bulk density and moisture content were determined simultaneously. The second algorithm allowed bulk density determination without knowledge of moisture content and temperature of the sample from a complex-plane representation. Finally, a unified calibration algorithm was used for moisture determination independent of bulk density and independent of the kind of material. Results of bulk density and moisture content prediction from complex permittivity measurement with a low-cost microwave sensor are reported here for wheat and soybeans.