Great. Thank you!!

Good question. My casual research into CO2 sensors is that they are generally made for indoor air quality rather than outside atmospheric concentration. Indoor CO2 concentration can soar as high as 1000 ppm where outdoors hovers around the 400 ppm level, so it is important to get a sensor made for the lowest range. The T6613 is specified for operation at 400 ppm so that is a big driver for my decision.

The MH-Z19 has a model specified for 0 to 2000 ppm so it might work fine. I don't know about the CCS811, the data sheet says it measures "equivalent" CO2, which is not a direct measurement, but a quantity inferred after measuring other compounds.

In the long run, there are calibration problems, as the photodetector ages. The best sensors have two light sources and automatically compensate for this effect. Indeed the other candidate I am considering is the Telaire 6615 which has dual sensors. My plan is to use the one of the two Telaires, preferably the dual sensor version, and correct measurements for temperature, pressure, and humidity. Hopefully, it will be accurate for a long time without the need for calibration with a known gas.

FWIW, I am just an amateur and welcome informed opinions. Scientific instruments that measure accurately in the atmospheric range are too pricey for me. The scientific community is just now starting to evaluate low cost sensors like the ones mentioned here, see [1] for a summary of some recent evaluations.

[1] https://www.wmo.int/pages/prog/arep/gaw/documents/Draft_low_...