Carbon dioxide is absorbed by the plant through its pores. In small spaces, the present carbon dioxide will soon be used up. Therefore the air in the growroom has to be replenished regularly. For this you need to buy an exhaust fan. You have to make sure however that it is powerful enough to replenish all the air at least 20 times per hour. The fan can be connected to a time clock or thermostat and/or hygrometer. To provide for an optimal gas change for the plant we also recommend to place an oscillating fan in the growroom, in order to have a constant air flow along the plants.
In urban areas the carbon dioxide concentration might increase to a higher value than the normal 0.03% which is 300 ppm (parts per million). From regular horticulture we know that adding extra carbon dioxide to a concentration of 0.15% highly stimulates the growth and the speed of photo-synthesis. This results in faster and higher yields. This yield increasing effect is most powerful with intensive lighting and inert substrate cultivation, such as rockwool. Another effect that has been reported by growers is the fact that a higher carbon dioxide concentration makes the plants less sensitive to higher temperatures.
A third effect is that there is less need to ventilate (unless the humidity is too high) because you don t depend on carbon dioxide from the outside air. In greenhouses the exhaust gas of oil-fired central heating is conducted back into the greenhouse. To raise the carbon dioxide concentration in grow rooms, it is usually supplied from bottles. There are two ways to provide for more carbon dioxide in the growroom.
(I) The cheapest way is to buy a pressure regulator that can be adjusted so that after ventilation (when carbon dioxide is dispelled from the room) the right amount of carbon dioxide will be released inside again. The exact quantity you need is something you have to work out yourself. You calculate this as follows: Length x Width x Height of the growroom in metres gives the volume of the room in cubic metres. One cubic metre is 1000 litres. If for instance you want to increase the concentration from 0.03% to the required level of 0.15%, you need to add 0.12% carbon dioxide. Suppose your grow room measures 2 x 2 x 3m , which is 12000 litres. 0.12% of 12000 litres is 14.4 litres. So to this room, 14.4 litres of carbon dioxide should be added to obtain an optimal gas concentration. This needs to be done after every exhaust period. This only needs to be done during the “day period”, because the plants only use carbon dioxide when the light is on. One kilo of carbon dioxide is approximately 500 litres. So a 10 kilogram bottle contains approximately 5000 litres. This means that a grow room of 2 x 2 x 3m needs two bottles per grow period.
(II) The second system to keep the concentration of carbon dioxide at the right percentage is by the use of a carbon dioxide meter and a computer controlled pressure regulator. The concentration of gas is constantly measured and the computer makes sure that with a too low concentration, the right quantity of gas is added. The ventilator could also be connected to this computer. This system is not cheap but once it has been installed you don’t need to worry about it anymore.