A Method for measuring Greenhouse Cover Temperature using a Thermocouple

Temperature of the greenhouse cover Tc is an essential parameter needed for any analysis of energy transfer in the greenhouse. Measuring the correct value of Tc is difficult due to the transparency of the covering materials and the effects of solar and thermal radiation and air movement on the cover surface. Therefore, Tc, in most cases, has been estimated theoretically by applying an energy balance to the greenhouse. In this study, a simple measuring method was proposed to predict Tc using a thermocouple junction (copper constantan, type-T) adhered directly to the cover surface. A correction factor was provided in the form of a regression correlation to exclude the effect of solar and thermal radiation on the thermocouple junction. The effect of air movement on the junction was also examined for air current speeds up to 5 m s−1. The results showed that solar and thermal radiation have a significant effect on the junction temperature and resulted in an overestimation of Tc, while the air movement around the junction had no significant effect. This method, in addition to an infrared (IR) thermometer and a protected thermocouple junction with a passive radiation shield, was applied to measure the cover surface temperature of an experimental glasshouse under hot sunny and cloudy conditions. Protection of the thermocouple junction from solar and thermal radiation coming from the surroundings caused a large overestimation error in the measured value of Tc under sunny conditions. Values of Tc measured by the proposed method were in good agreement with those measured by the IR thermometer under cloudy and sunny conditions. The proposed method can be used to measure the greenhouse cover temperature or any surface temperature with a maximal error equal to the measuring accuracy of the IR thermometer (±1%). The different surfaces of the cover (side walls and roof) have almost the same temperature under any weather conditions.