NIGHTTIME GREENHOUSE HUMIDITY CONTROL WITH A COOLED WETNESS SENSOR

High night-time humidity in greenhouses may lead to the spread of fung al diseases. Traditionally, dehumidifying the greenhouse air in mild c limates is accomplished by means of periodic ventilation. More efficie nt operation can be achieved by triggering ventilation with a humidity sensor or a wetness sensor. In this study, a thermoelectrically coole d wetness sensor was used to turn on a fan, a heater or a combination of both, to dehumidify a small experimental greenhouse. Increased sens or cooling increased the condensation safety margin, namely the differ ence between canopy temperature and the dew-point temperature of the g reenhouse air. The safety margin was increased by about 0.8 K for ever y 1 K of sensor cooling. The results show that the need for dehumidifi cation increases with atmospheric humidity and decreases with wind spe ed (which promotes natural infiltration). (C) 1997 Elsevier Science B. V.