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Title: UNIVERSAL MICROWAVE MOISTURE SENSOR

Author
item TRABELSI, SAMI - UNIV OF GEORGIA
item Nelson, Stuart

Submitted to: World Congress on Microwave and Radio Frequency Processing
Publication Type: Abstract Only
Publication Acceptance Date: 9/1/2002
Publication Date: 9/22/2002
Citation: TRABELSI, S., NELSON, S.O. UNIVERSAL MICROWAVE MOISTURE SENSOR. FINAL PROGRAM AND ABSTRACT BOOK, THIRD WORLD CONGRESS ON MICROWAVE AND RADIO FREQUENCY PROCESSING, SYDNEY, AUSTRALIA. p. 20 2002.

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, processors, etc. 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. Research has shown that a single calibration based on the dielectric properties of grain measured at microwave frequencies can reliably sense the moisture content of wheat, oats, and soybeans. Because these three commodities have very different kernel shapes, sizes, and composition, this technique offers promise for a universal calibration for grain, soybeans, and similar crops. New research has answered important questions related to the practical use of these kinds of measurements for moisture determination. These findings provide additional incentives for the development of practical microwave moisture sensing instruments that can serve as new tools for the maintenance of high quality in such agricultural products, thus benefiting farmers and consumers as well.

Technical Abstract: Sensing moisture content in granular materials accurately and in real time is of prime importance in many industries, including food and agricultural, pharmaceutical, and mining. Microwave dielectric-based sensors have been used for on-line monitoring and control of moisture content. They have the advantage of being nondestructive and, in some instances, contactless. However, their anticipated widespread use, at least in agriculture, has never materialized. For granular materials at least four limiting factors can be distinguished: 1) effects of ionic conductivity, 2) errors related to bulk density change, 3) need for an individual calibration for each kind of grain, and 4) the instrument-dependent calibration algorithm. This paper presents a microwave dielectric calibration method that addresses all of the four. The method is based on measurement of the dielectric properties at microwave frequencies. At frequencies above 3 GHz, the effect of ionic conductivity is expected to be negligible. A permittivity-based calibration method is proposed that provides moisture content for several kinds of grain from a single moisture calibration equation without need for bulk density compensation. Because this moisture calibration function is expressed in terms of intrinsic properties of the material and is both independent of the material and the bulk density variations, the calibration algorithm for moisture content determination should also remain valid for instruments of different designs. The two components of the complex permittivity can be derived from impedance, transmission-line (reflection and/or transmission), or resonant cavity measurements. Results of measurements on wheat, corn, and soybeans, carried out with two different free-space systems, one operating in the near field and the other in the far field, show potential for transferability for moisture calibration algorithms across instruments. The coefficients in the resulting calibration equations are sufficiently similar to allow their combination for practical use with these granular materials.