well i have the sentinel cppm1 and it says you should calibrate it before use, and that ambient air is 350-450 ppms.
i have had it outside on my balcony at night, about 50 degrees, as well as in my grow room at about 85 degrees, and both times the reader said around 450 ppms, but when i calibrated it outside, then brought it inside, it read about 383 ppms...
then when i unplugged it and took it into my grow room, it shot back up to 470 ppms. and i know that cant be true cuz my plants are suckin up the gas in that mofo LOL