Well, the problem with most of these off the shelf , indoor sensors, is that they are calibrated to current reading of about 400ppm, that's their base. To corre

Well, the problem with most of these off the shelf , indoor sensors, is that they are calibrated to current reading of about 400ppm, that's their base. To correctly measure CO2 you will need something that does not depend on that value to self-calibrate. What's more, some of them can have +/- 20ppm or worse accuracy. If the average annual ppm increase is 2ppm you can't reliably see the increase on your home device. To do that you need to have something that's NIST calibrated, with correct accuracy, set in a right place, and gather the data for longer period of time. You will need to smooth the data, and know how to interpret it. Source: I develop software for data acquisition systems