MEASURING PRINCIPLE

One of the properties of microwaves is the ability to travel through non conductive materials.In materials with bipolar molecular structure, (i.e. water), the electric field of microwaves can induce oscillations whilst travelling through the medium. During this process, the micro waves loose some of their energy. This loss in energy increase with the amount of water that medium contains, with the result that as the water concentration increase, less energy will reach the other side of the medium. Water not only absorbs but also reflects some of the micro wave energy. By utilising these two effects (reflection and absorption) one is able to perform moisture measurements with micro wave techniques.

During transmission, (where the substance to be analysed is placed between the micro wave emitter and detector) the micro wave intensity arriving at the detector decreases with increasing of moisture. The moisture content can then be calculated by taking into account the absorption of the dry substance and some geometrical factors.

In reflection the procedure is equivalent, exept that both micro wave emitter and detector are mounted on the same side of the substance.

Advantages of micro wave moisture determination

• MWs are not sensitive to pH and conductivity of the material to be measured.
• Total moisture ( surface as well as inherent) is measured.
• Contactless measurement: No abrasion of the sensors, no sticking of material, no interference with the process.
• The measurement gives instantaneous results ("real time measurement") and can be utilised for process control.

The Material to be measured

Nearly all bulk goods can be measured by means of microwaves. There are very few materials with restrictions: Problems can however arise in the rare case where the water molecules are not able to oscillate, for example if they are captured in capillaries. For the same reason crystal water and ice do not respond to the measurement.

Some examples of material which can be measured are coal, sand, gravel, wood chips, potatoes, pommes chips, tobacco, seedcorn etc.

The microwaves do not heat or change in any way  the material, due to the extreme low energy emitted ( it would need approximatelly one thousand emitters to cause any measurable effect).

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