The Auto Calibration feature for Monnit CO2 sensors
Monnit CO2 sensors offer an Auto Calibration feature which allows the sensor to periodically calibrate itself based on readings during the Auto Calibration time period.
How it works
In this example, we will consider a CO2 sensor with the Auto Calibration period set to 3 days. The sensor saves the lowest consistent 1 hour average of CO2 concentration and the sensor will calculate, save, and apply an offset that would make this lowest average value equal 400 ppm. In this example, if this lowest average is equal to 375 ppm, after auto calibration that same 375 reading will now read as 400 ppm. If the low average value is 575 ppm and auto calibrate happens again, 575 readings will now become 400 ppm. This happens on the sensor so it does not change any historical data.
A key detail to this operating correctly is that the sensor must be exposed to a 400 ppm (fresh air) environment for a continuous full hour during the Auto Calibration period (3 days in this example) in order for this to work properly. If the lowest actual concentration to which the sensor is exposed to is higher than 400 ppm (say 675 ppm) the sensor will now be inaccurate after the Auto Calibration is applied.