alt Hacker NewsFor the purposes of indoor ventilation monitoring you can calibrate by occasionally exposing the sensor to fresh air. Either taking it outside or just the room not having people in it. The sensor will treat the lowest reading it gets as 400ppm since this is what outdoor air is.
A sensor mounted in the office will get calibrated every night when the office is empty.
The outdoor CO2 is rising every year. It is not fixed at 400 ppm. The calibrations you speak of are fake. A good sensor can be expected to remain within 10% of reference for ten years.