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Direct digital control of air humidity for plant research



Direct digital control of air humidity for plant research



Biotronics 13: 29-38



Direct digital control (DDC) system was employed in a growth chamber for accurate control of air humidity in wider regions. Control elements were composed of cooling brine coil, dehumidifying brine coil, reheating coil (electric heater), heating coil (electric heater) and steam boiler. Volumes of circulated brines, electric currents in heaters and steam volume were manipulated by PID control action (velocity algorithm). Furthermore, capacities of cooling and dehumidifying coils were optimalized for high accuracy by the feed-forward control of brine temperatures, where the optimum temperatures of respective brines were set on the basis of the desired values of air temperature and humidity. This system made it possible to control relative humidity from 10 or 20 to 85% within .+-. 3% RH in an air temperature control region of 0 to 45 .+-. 0.3.degree. C (the lowest humidity was 10% RH at air temperatures higher than 10.degree. C, but was 20% RH at 0.degree. C): This control region covered saturation deficits of 0.73-8.77 g m-3.

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