Paul Le Duff (Le Duff, Plouescat, France)

We’ve been using ProDrain for over two years now and we wouldn’t know how to work without it. We have a semi-closed greenhouse with artificial lighting. In a normal greenhouse, plant transpiration is mainly the result of solar radiation: the correlation is almost 90%. In our greenhouse, this correlation is perhaps only 50%. The constant air flow from the air tubes and the heat from the lamps have a tremendous effect on plant transpiration. Normal irrigation control is based on the radiation sum during the day and time settings at night. In our case, this type of control would not even begin to meet the irrigation needs of our plants. Of course, we also measure the drain water volume, but this represents control ‘after the fact’ and a reading that is not available at night. The undersaturation level measured by ProDrain tells us exactly how much water the plants are absorbing 24 hours a day. Another thing we learned from the ProDrain system was the effect of releasing air that was still too cold into the greenhouse. When the ambient temperature was too high and cooling was required, we would  sometimes observe that the cooled air released from the tubes brought plant transpiration to a complete halt. This was especially the case on partly cloudy days in March and April. On these days, it is still cold outside, while the radiation level is already quite high. The sun quickly heats up the air in the greenhouse and the climate control system responds to this change. If the ambient temperature is still rising when the fans are at full speed, then the outside air drawn into the greenhouse becomes increasingly colder. The air is too cold in fact, because it causes a breakpoint in plant transpiration. We now control the tube air temperatures based on a maximum difference with the ambient temperature. This ensures that the plants keep transpiring, even if the ambient temperature rises above our cooling or ventilation setpoints.

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