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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.

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  • $\begingroup$ You really kindof need a reference sensor. The full scale range of these things is not that high, so constructing a reference signal (ie mixing CO2 and N2 from a tank at controlled temperature and pressure) would probably cost more than buying a better digital sensor for calibration. $\endgroup$
    – Pete W
    Jan 5, 2022 at 15:20
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    $\begingroup$ ps - if you want cheap CO2 for playing around, beg/scrounge for waste dry ice from a business that gets stuff shipped in it $\endgroup$
    – Pete W
    Jan 5, 2022 at 15:22
  • $\begingroup$ Thanks @PeteW for the handy suggestion about dry ice. $\endgroup$
    – rockfort
    Jan 7, 2022 at 11:14

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If you're really only looking for a rough calibration, you may be able to set up a dilution system. 10,000 ppm is 1%, so you'd be aiming for a gas mixture of 1% CO2 and 99% air. Use an air compressor and small flowmeter to get the air at some known flowrate, say 100 ml/minute. Then put some dry ice in a mostly-sealed box. As it sublimates it will displace the air and eventually be filled with pure CO2. Now you need 1 ml/min drawn out of the box and combined with the air flow. You'll need a little pressure to drive it, so you'd need a small pump to draw the CO2 out and push it through the flowmeter into the combined stream.

Acquiring all those bits and pieces may well cost more than buying or renting a calibrated reference sensor.

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  • $\begingroup$ Neat idea, creating the intended mixture like that. Indeed there are flow meter + regulator combo devices available for small diameter hose, these are affordable and could come handy for other projects too. $\endgroup$
    – rockfort
    Jan 7, 2022 at 11:42

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