imo, (and a lot of others too) unless you're using tanked co2 (or burner) you're wasting your time/money. lets say you wanted around 1000 ppm. producing numbers like that with dry ice, or some yeast mix or whatever, is near impossible. you don't have to spend a grand on a co2 set-up, to make it worth using. all you really need is a/some tanks, a regulator, and a repeat cycle timer. a regulator is around a hundred. tank(s) are another hundred. a good repeat timer maybe sixty, and assorted other fairly cheap supplies.....