CannaCentral

Carbon Dioxide (CO2) levels should be high, particularly during the Flowering PHASE . Growth rates can be increased dramatically if the CO2 level is near 700 ppm. 300 ppm (Outside Air) 700 ppm (Ideal) 2000 ppm (Plants Burn) 5000 ppm (Death) CO2 is cheaply produced by burning Natural Gas. However, heat and Carbon Monoxide must be vented to the outside air. CO2 can be obtained by buying or leasing cylinders from local welding supply houses. Beware! The DEA has asked the supply houses to provide lists of large CO2 users. If you are questioned, simply say "I have an old mig welder at the house and I want to try and patch up my (Lawnmower, car, trailer, etc.). If you find that you are making lots of trips to the supply house, complain that your "buddy" keeps leaving the cylinder valve open all night. This should provide a plausible excuse.

I have been doing some research and i find that about 1350 to 1400 ppm works very well for my small area.