Resonant microwave sensors for instantaneous determination of moisture in foodstuffs

Open microwave resonators are very well suited for online moisture monitoring during the production of foodstuffs. In order to allow for rapid measurements, the required information is derived from the dielectric behaviour of the material. Unlike for example the method of Karl Fischer titration, the dielectric microwave method determines the moisture content indirectly and is thus dependent on density changes. The present paper outlines special features of open resonators for density independent moisture measurement. It is demonstrated, how unfavourable design of the window for coupling out the fringing field of the open resonator leads to non-linear behaviour of the characteristics of resonant frequency and reciprocal quality factor vs density and moisture changes. Several engineered open microwave moisture sensors are presented and discussed. More in detail, a surface sensor, a surface sensor equipped with a small container on top, a self contained hand-held surface moisture sensor and a grooving-probe as well as a multi-hole sensor are shown. The electronics and the signal processing of the sensors are briefly described. Finally experimental moisture measurements and a density independent calibration are given for rolled oats. Moisture measurements on whole kernels of oats, barley and wheat demonstrate the only weak dependence of the microwave moisture measurement approach on the special type of material.