> CO2 was measured with infrared but water also absorbed it, so you need to heat things up enough to not have water. It can be small, but not watch small.

Can't you just measure CO2 "naively"; but then also, separately, measure rH; and then use the rH value to grab a research-calibrated LUT to pass the raw CO2 value through?

(I presume this is why all the little standalone CO2-sensor boxes you can buy also have rH displays. They're measuring it anyway to normalize the CO2 value, so they may as well make it a feature and display it.)