Development of Moisture Sensor for Rockwool Culture

Abstract

This study was carried out in order to develop a moisture sensor for rockwool culture. The moisture sensor consists of a small cylindrical plastic probe (50×4×4 mm), in which a thermocouple and nichrome wire were buried and isolated from each other by a plastic resin. The probe was placed in the rockwool block, heated by the nichrome wire during 3 min, and the temperature was measured just before and after heating by the thermocouple. When the heat was transmitted to the rockwool block, the difference between the two measured values decreased in proportion to the thermal conductivity which depended directly on the degree of water saturation in the block. This difference was indicated as mV by the controller. Consequently, the reciprocal of the mV value showed a close linear correlation with the degree of water saturation. It was essential for standard monitoring in the rockwool culture that the probe remained in close with the lower part of the rockwool mat. The degree of water saturation which was measured automatically gave adequate information for the irrigation control system of rockwool culture.