Active Air CO2 Replacement Regulator for Hydrofarm CO2 SystemThis product is a replacement part for the old style HF COSYS, and is not a complete unit system. CO2 is an odorless, invisible, and non flammable gas. The average level of CO2 in the atmosphere is about 300 ppmv (parts per million). If the level decreases down below 200 ppmv in an enclosed growing area, plant growth slows to a halt. Through the years of testing and research, the optimum enrichment level of CO2 for plant growth has been established at