You definately need a meter when using Co2. You cannot know what the ppm is without it. You're right the Co2 will fall to plants but most of us have too much air movement in the room to depend on this method. How large is your area? If it's closet sized the Co2 will find it's way to plants, but if it's a average bedroom size you need the entire room to have at least 1000ppm. Hopethis helps.