WATER CONTENT MEASUREMENTS WITH NON-DESTRUCTIVE DI-ELECTRICAL METHODS IN ROCKWOOL SUBSTRATES

The method is based on capacitance measurements from which the di-electrical properties can be derived. With these data and a temperature measurement, the watercontent and the electrical conductivity can be calculated. With aid of an electronic circuit the capacitance measurement is compensated for EC's up to 8 ms/cm.

Specially for glasshouse crops, sensors are developed which can penetrate very easily into the rockwool substrate and which measure the average quantities of water content, EC and T in a volume of about 7.5*7.5*7.5 cm³.

Two types of equipment were developed to measure in glasshouse practice.

The first one is a hand-held meter for incidental determination of water content, EC and T in the rockwool slabs, for example to determine the variation within a slab, within a glasshouse, or between various crops.

The second is a computer controlled measuring system which can cope with up to 100 sensors. This system is also appropriate to couple the measuring signal to the irrigation system or irrigation computer.

With the computer controlled apparatus the water content as a function of time can be displayed very easily. So the water consumption of the crop can be studied. Also EC variations within the substrate can be investigated.

Thus a new method for automatic measurement and irrigation control in rockwool substrates is now available.

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