Comparison of density-independent expressions for moisture content determination in wheat at microwave frequencies

The accuracy of moisture content determination in grain from measurement of electromagnetic parameters is known to be dependent on the bulk density of the sample used for the measurement. Various means of limiting this effect have been considered in the past. This paper identifies various density-independent expressions developed for this purpose and compares their effectiveness in minimizing the density dependence of the predicted moisture content. The same procedure and data set from more than 180 measurements on hard red winter wheat, Triticum aestivum L., at frequencies of 11.3 and 16.8 GHz on samples in free space were used for the comparison. Several density-independent expressions were identified that predict wheat moisture content with a standard error of calibration of about 0.2% moisture for moisture contents ranging from 10.6 to 18.2% w.b. Although these results were obtained for static samples of grain, the principles can be extended to grain flowing through a tubular conduit, or moving on a conveyor, for continuous measurements. (C) 1998 Silsoe Research Institute.