:yeahthat: there's roughly 300-400ppm of C02 depending on location around the world. thats why we use such big fans. not only do they keep our grow rooms cool, they exchange the air in the grow room every minute or 2, so that the ppm's remain in the 300-400 range. when people use tanks or generators, the ppms are jacked way up to 1500ppm. the controllers are there so that the ppm stays that high at all times when lights are on.