Thanks, I read about the need of recalibrating sensors, although didn't imagine that would be needed so often. Thankfully I recently moved to a very clean air area with trees and such, so exposing the sensor to clean air would just involve moving it a few meters outside.

Mine is indeed not perfectly calibrated since it sometimes gives value that are lower than the outside air, but the scheme of things, being off by 20 or 50ppm isn't going to change anything

Mine is indeed not perfectly calibrated since it sometimes gives value that are lower than the outside air, but the scheme of things, being off by 20 or 50ppm isn't going to change anything

If you're getting values lower then atmospheric then the readings are if by much more than 20 to 50 ppm.

why ?

This is pretty much irrelevant though. I have a CO2 monitor for a closed space. Heavy exercise with the door closed will set off the 1500 PPM alarm every time. Do I care that it is off by maybe +/- 200 PPM? Not at all. It still absolutely serves its purpose of sounding an alarm when the air gets stale.