Squirrel Cages Although there are some makeshift applications that work fairly well, it is hard to beat a squirrel cage or shaded pole blower. The models in the 200 cfm (cubic feet per minute) range are relatively inexpensive and work in closet sized growing chambers. The 465 cfm blower is probably the most popular one sold. It usually costs under $100 for a very safe and reliable exhaust system. A pump with a capacity of about 980 cubic feet per minute will discharge the air equivalent in a 10 ft. X 10 ft. X 10 ft. room in about one minute. However, the incoming air mixes with the outgoing air, so one minute of operation results in a 50% reduction of stale air. A three minute cycle would change the air almost completely. Maxi-CO2 The 1500 ppm figure is the one given most often in the literature as being ideal. It is by no means absolute, however use white widow. One expert puts the ideal at 2000 ppm, and a pair of researchers who obtained excellent results on a Danish ebb-and-flow table used a mere 1300 ppm. The author of this book feels that 1500 ppm is a good general figure.