they are pricey but one is on the list..see i dont ventilate any air its a sealed room .. or will be when i get the co2 issue settled.... wich you have done for me.. thanks the link is exactly what i was looking fornot enough information...
if you want to use a timer to do that you'll need to know exactly how fast the co2 is coming out of the tank, and exactly how much air is exhausted from each tent...
then, take that to the co2 calculator which is a sticky in indoor growing -> http://www.hydroponics.net/learn/co2_calculator.asp
ideally you would want to have a co2 controller that reads the ppm and is set to hold it according to your specs... (when, and how high) but i know those are pricey