THERMAL CONDUCTIVITY SENSING FOR MONITORING AND CONTROL OF MOISTURE CONTENT IN INERT SUBSTRATES: A COMPARISON OF TOMATO CROPPING ON ROCKWOOL AND ON POLYURETHANE

The thermal conductivity of inert substrates like rockwool and polyurethane is a function of the density and the moisture content of the substrates. Using a probe, based on the line source method, the thermal conductivity can be measured in relation to different moisture contents at constant density. The thermal conductivity of these substrates is limited to a range between the conductivity of air for the dry material and that of water for the fully saturated material (respectively from 0.026 W/mK to 0.6 W/mK). For polyurethane (1 m x 0.15 m x 0.06 m) and rockwool slabs (1 m x 0.15 m x 0.075 m), empirical calibration curves were constructed over the full moisture content range.

In the greenhouse setup, the thermal conductivity measurements are fully automated and connected with the irrigation system of the greenhouse. This makes it possible to measure on-line the thermal conductivity of substrates used to cultivate plants, to calculate the moisture content of the substrate, and finally to let the computer decide wether it is necessary to irrigate the plants or not.

In this study the moisture content was controlled at around 70 % on volume basis, for commercial tomato cropping both on polyurethane and rockwool substrate. The reliability of the procedure was investigated. The effect of the substrate on the quality of the tomato fruits was examined, based on non-destructive quality descriptors.

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