How often do I need to calibrate the Aranet4 HOME and PRO and how do I do it?

If the device goes outside then it will calibrate itself automatically once a week by presuming that its lowest measurement corresponds to the CO₂ level in outside air.

 

But if it is fixed to a wall and doesn’t go outside, then how often should a HOME or PRO unit be calibrated? This is slightly a “how long is a piece of string” question. Most devices remain consistent for long periods and you don’t want to be calibrating them unnecessarily.

 

One suggestion would be to log a note of the reading for each unit before installation by placing them all outside: if all are showing similar readings +/-50 ppm in the mid 400s ppm then they are all working correctly. Then log the readings of each of them once they have been placed in the desired location/room, and once that room has been ventilated and the air as fresh as it can be expected. This then gives you a reference level for that space when it is well ventilated. If in the future the readings deviate significantly (>100-150ppm) from the reference level at a time when the space should be well ventilated then there are two options: either the device needs recalibrating or the ventilation has worsened for some reason, such as a failed ventilation unit. 

 

This two minute video shows you more about recalibration: