Open microwave resonators are very well suited for online moisture monitori
ng during the production of foodstuffs. In order to allow for rapid measure
ments. 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 f
eatures of open resonators for density independent moisture measurement. It
is demonstrated, how unfavourable design of the window for coupling out th
e fringing field of the open resonator leads to non-linear behaviour of the
characteristics of resonant frequency and reciprocal quality factor vs den
sity and moisture changes. Several engineered open microwave moisture senso
rs 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 br
iefly described. Finally experimental moisture measurements and a density i
ndependent calibration are given for rolled oats. Moisture measurements on
whole kernels of oats, barley and wheat demonstrate the only weak dependenc
e of the microwave moisture measurement approach on the special type of mat
erial. (C) 2001 Elsevier Science Ltd. All rights reserved.