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6.2 Microwave Moisture Measurements
When the frequency is rising (in the microwave region) the polarization of the water
molecule becomes more and more difficult because of internal binding forces (the water
molecule „swims“ in the water and is bound to the other molecules).
The microwave field causes an internal friction in the material, which can also be called
dielectric loss. If the power of the microwave radiation is large enough then the material
will be heated. This is the effect which is used in the kitchen microwave oven for heating
food.
Dielectric losses can be measured with special microwave arrangements. The
microwave power necessary for this is much lower than for heating purpose, it can
be < 1 mW. That’s why every kind of danger to human health (electrosmog) can be
excluded.
The maximum of dielectric losses of water corresponds to a frequency of about 20 GHz.
So one would prefer this frequency for getting the maximum measuring effect.
Unfortunately the wavelength of the electromagnetic wave is so small at this frequency,
that a measurement won’t be useful. Moreover dielectric losses are high enough at
lower frequencies too for getting a good detection of water.
So in the microwave region two effects can be used for moisture measurement: the high
permittivity of water (exactly: the real part of dielectric constant) and the dielectric losses
(exactly: the imaginary part of dielectric constant). In other words: The coupling of the
electric function of a microwave moisture meter to the physical properties of water is
very close.
Besides that the microwave frequency range has some more advantages: As can be
shown easily from the basic equations of electro-technics (Maxwell equations), the
influence of ohmic losses (ionic conductivities, e.g. salinity of a building) decreases
when the frequency is raising. Above 1 GHz ohmic losses are negligible compared with
dielectric losses. That means: Microwave moisture measurement is independent from
salinity.
In the preferred frequency range between 2 and 10 GHz the corresponding wavelengths
are within a range that makes it possible to design relatively good focusing applicators
as antennas or stray-field-sensors in a useful size.
Using these applicators penetration depths in the decimetre range can be achieved
because of the directivity of the antennas. By use of stray-field arrangements some
centimetres can be reached.
Conclusion: Microwave moisture measurement arrangements allow a non-destructive
comparison between the water content at the surface of a material and within its
volume.
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