Some general questions about Co2

VPD is about the temperature of the leaf not the atmosphere. you can typically expect at least -3F at the leaf due to the evaporation taking place. if you can use aninfrared thermometer to get a good offset to use with your probe data, that would probably be accurate enough? i don't know what the optimal VPD is. I've read varying amounts... I've seen some suggest as high as 1.5 and as low as 1.2 in flower phase. Each had a lower suggestion fore mature vege. It will compliment the overall concentration your fertilize at (more so with soilless/hydro). E,G. A higher vpd would need a lower concentration over time since the plant drinks more in those conditions. In order to maximize your CO2 you need to tightly control temperatures and RH. I believe you want about 30C, but you can look this reference material up, corresponding RH would be based on your target VPD.. find a reputable source like the bruce bugbee suggestion below (or above, however these answers are ordered). Seal the room up .. think of gas escaping like leaving a window open with the AC on. freshen the air occasionally? maybe get a hepa air filter? obvioulsy losing the ability to exhaust comes with some costs too. have some sort of safety feature... something that'll warn you if the co2 rises higher. don't die doing something stupid... and if you do, please do so before you reproduce :P kidding.. no one deserves that, so be smart about it.