you can get a C02 / CM02 PPm meter and mount it in the tent.
i would suggest putting it right into the tent (below the plants if possible). how long does it last? years. i leave mine in the room untill grungy stuff starts to form (in the bottle), clean it out and do it again
the only thing thats going on inside the bottle is yeast feeding. most people dont know it, but yeast is a bacterial culture. the bacteria in the water feeds on suger, and farts out C02 like no-ones buisness (thats why yeast+ suger makes bread rise, C02).
so, in one sense it is like using a little animal, only this is billions of microscopic life forms.