How do you measure CO2?

i dont see very accurate measurements without professional equipment generally on anything. be it ppfd, co2, humidity etc. so in a sense you’ll get what you pay for. why do you want to measure co2? if you’re not providing them with co2 and pulling in fresh air from outside its safe to assume you have 400ppm co2, if not pulling in fresh air and the tent is in the house 600ppm is probable, if the tent is in your bed room or living room with the doors closed and you’re in the room for the most part of the day then in that case around 700-800ppm is a good guess. these baseline guesses are almost as effective as a cheap co2 meter :)