Summary: Are there any worthwhile methods for rough calibration of CO2 sensors without proper chemistry lab equipment, calibration gas, or other known-good sensor available? It would only be used as an air quality indicator.
Full story: I have a couple of SEN0159 analog CO2 modules based on the MG-811 sensor. According to the manufacturer, calibration has to be done manually for each individual sensor, i.e. to determine the parameters for the conversion function from voltage to concentration percentage. There are two sets of parameters mentioned as examples in the documentation, but these differ wildly from one another, so indeed these particular values cannot be trusted.
For the calibration, one would need to expose the sensor to at least two samples of known CO2 concentration. One of these would obviously be the baseline value, in fresh outdoor air, which can be treated as 400 ppm (or nearby meteorological station data could be consulted). For other type of sensors, a zero ppm calibration point would be available to use with e.g. pure nitrogen gas. This particular sensor has a measurement range of 400 to 10000 ppm though, meaning that some other point has to be used within those bounds.
So the challenge is to obtain an approximately known CO2 concentration in some container (with the sensor in it). I am asking in Engineering with the hope to get some creative answers using common workshop/household tools, cheap material, and widely available CO2 sources like soda siphon, carbonated water, burning something etc, maybe even simple exhaled air.