The vast majority of air is inert, that is it does not readily react with anything. Only the 20% Oxygen (O2) is used for respiration (using energy) and the 0.03% Carbon Dioxide (CO2) for photosynthesis (storing energy as sugar). In fact, the plants mass is mainly obtained through CO2 as opposed to matter drawn up from the soil. As the level of CO2 is relatively low, although increasing daily, it is the next limiting factor after light. Outside, wind brings fresh air, mixing and moving the used air, but under cover this must be taken on by the cultivator. Peak growth has been reported at CO2 levels of 1500 to 2000 ppm (1.5-2.0 Volume percentage). Any higher than this will effect photosynthesis negatively.
Firstly air needs to be moved over the plants gently to remove the boundary layer effects of still air and to mix with the volume of gas in the growing area. This boundary layer of air immediately around the leaf does not diffuse into the surrounding air, this layer is of a very small volume so CO2 is quickly exhausted. Simple air movers and air circulatory fans are employed for this.
Next, the larger volume air in the grow room must be replenished periodically to maintain gas levels. This is achieved with extractor fans pulling air out and drawing fresh air into the area. The fan is placed in a high location to pull out hot, moist, used air whilst fresh, cool air is drawn in from below. These fans can then be co-ordinated to maintain temperature and humidity levels.
The constituents of dry air can be expressed as volume percentages, which will translate to the partial pressures out of the total atmospheric pressure
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