Microwave measuring methods can be applied to determine the
properties of materials and hence it is possible to develop microwave sensorsfor processing industry. The first applications were in moisture measurement
but lately many new sensor applications have appeared.
Microwave sensors especially suitable in forest industry
(wood, paper), in chemical industry (plastics, chemicals), in food industry
(tobacco, butter) etc. In developing sensors several problems must be solved.
The dielectric properties of the material in question must be known. They can
be measured or in some cases determined theoretically by applying mixing
theories.
The sensor is often some kind of a resonator. Its structure
must be such that the electric field penetrates the material as required, and
allows free flow of material. Displacements occurring normally in the material
flow or dirt in the sensor must not cause measuring errors. Several examples of
lately developed sensors are given.
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