[scaphandre]

The 'Netatmo' home weather stations record CO2 level. Mine shares over wifi. I get around 400ppm with windows open for a while, or 1200ppm with windows closed.

I don't know what the calibration mechanism is, nor the specificity.

It's around $100 on Amazon - https://www.amazon.com/Netatmo-Weather-Station-for-Smartphon...