A Wetness Sensor for Detecting Condensation on Tomato Plants in Greenhouses

A wetness sensor, based on the electrical conductivity of a polymer, has been designed and constructed from 0·08 mm-thick flexible copper-coated polyamide film using photographic and etching procedures. The flexible sensors made good thermal contact with the surface of different shapes and sizes of tomato fruits. The space between grid elements was 0·25 mm. The performance of sensors was determined with bare grid elements and elements coated with acrylic latex and vinyl acetate based latex, in different concentrations. Sensors without a coating gave a good and repeatable response. Sensors coated with latex paints had non-reproducible responses, and overestimated the wetness period due to the slow release of moisture absorbed in the paint. The uncoated sensors were used to monitor condensation on tomato fruits in a series of experiments. The experimental results showed that the total wetness period, indicated by the impedance changes of the sensors, agreed well with that deduced from measurements of fruit surface and dew point temperatures. The present design has a suitably quick response to condensation on plant surfaces and this is attributed to the smaller intergrid gap than that used by previous workers and to the absence of a coating material. The greater accuracy in determination of wetness duration will help in the forecasting of microclimate conditions and prediction of disease epidemiology for greenhouse crops.