On studies of plant growth using growth cabinet or greenhouse, there have been paid little attention to carbon dioxide concentration of air. However it is supposed that owing to circulated re-using of air for air-conditioning in growth cabinet of phytotoron, the carbon dioxide concentration is quite decreased and the plants do not grow properly. Therefore, it is necessary to cultivate plants under the normal concentration of carbon dioxide. The same problem should be considered in greenhouse.
Even in the case of carbon dioxide nutrient system which practically used recently, it is necessary to keep the effective and economic level of the carbon dioxide concentration.
The authors tried to make the automatic controlling equipment of carbon dioxide concentration in closed system culture.
The heat conductivity principle was used for determining CO2 concentration. The circuit and piping of it are shown in Figs. 1 and 2. After the standard air (outside air or carbon dioxide free air) is sucked into the reference and detector cells of the bridge, the bridge is balanced by the balancing resistance r2. And then, the bridge is unbalanced a fixed value which correspond to a given concentration of carbon dioxide of the cabinet air with the regulating resistance r1 and the cabinet air is sent to the detector cell. Untill the carbon dioxide concentration in the growth cabinet reachs a given value, the bridge is retained in the unbalanced condition and the magnetic valve of carbon dioxide bomb opens and the carbon dioxide is sent into the cabinet. The syncronous rectifying circuit is equiped so as not to open the magnetic valve by the opposite unbalance of the bridge when the concentration becomes higher than the fixed value.