Moisture

The loss of energy is therefore a measure of the water content of the permeated material.

If the loss is not too high, some of the microwaves will arrive on the opposite side and can be measured there.

The microwaves not only lose their energy, they are also reflected in the medium they penetrate, what can be used also as a measure of the water content of the material.

By using the transmission measurement, when the material to be measured is placed between the transmitter and the receiver, the entire material is recorded.

When it comes to reflection, the microwaves have to travel twice, so the detected layer thickness is smaller.

It is also difficult to keep the distance to the measured material constant - an important condition for a reflection measurement, therefore the transmission method is more meaningful on a conveyor belt, the most frequently used measuring location.

Advantages of microwave moisture measurement:
● The measurement is not influenced by changing pH or conductivity of the measuring medium
● The total water content, not just that on the surface, is recorded
● the method is absolutely contactless. There is no wear on electrodes or caking.
● The measurement results are immediately available and can therefore be used for direct process control.

Almost all bulk goods can be measured with this method even there are few exceptions: problems can arise with materials in which the adhering water is not able to oscillate, e.g. if it is "trapped" in capillaries.

For the same reason, crystal water cannot be recorded, as well even ice does not react like water, but like another solid.

Some examples of materials that led to successful measurements: brown and hard coal, sand, wood chips, potato chips, tobacco, seeds, etc.

Because of the extremely low energy emitted by the transmitter, the microwaves are not able to heat the measured material: approximately one thousand transmitters would be required to bring about a measurable increase in temperature.

Most of the experience with coal with a grain size of approx. 0-20 mm is available regarding the measuring accuracy.

The accuracy that can be achieved depends on the exact measurement conditions, e.g. whether it is filter cakes or homogeneous moisture distributions.

An accuracy of ± 0.5% was achieved for coals and the best results to date were achieved with ± 0.1% in sand, however, with other measured materials one can assume a value of ± 0.3% or better, e.g. this value was reached in copper ore.

Because the transmission method is preferable in any case, the measuring section usually consists of a microwave transmitter and a receiver, where the transmitter is placed under the conveyor belt and the receiver above and a bracket for installation, which has the necessary stability and prevents geometric changes.

Alternative measurement methods:
● neutron radiation
● IR radiation (active, in contrast to temperature measurement)