[greggsy]

If I understand correctly, they’re calibrated at the factory, which is probably in China, which might have vastly differing CO2 levels depending on whether the winds blowing up or downstream of any heavy industry or metropolitan centre?

[lancewiggs]

You can calibrate the Aranet4 at any time by putting it outside, and it will set that as 420ppm. Obviously the ambient CO2 levels are raising slightly each year, so this is not entirely perfect.

We have tried a number of different CO2 sensors over the years (mainly from Ali Express), and recently purchased 2 Aranet4’s. We were pleasantly surprised that they report identical readings in the same location.

[ericd]

Not an expert, but I looked into this a bit a while back to see if one could reasonably tell emissions via differences in atmospheric CO2 concentration, iirc the conclusion was that CO2 concentrations don’t vary much outside, air mixes pretty well, pretty quickly.

Even seasonal fluctuation is just a few percent.

[gsich]

I don't think this is what is meant by "calibration".