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This question emerges from Earth Science SE since recently there was asked this. There so far no one had the engineering/ measurement device expertise to answer this.

I am aware that these are fundamentally different in the case that the purposes of CO2 analyzer is to accurately monitor the levels of CO2 in the atmosphere probably in making a difference between a few parts per million (ppm) in air (as there is currently around 400ppm in the atmosphere and it is a well mixed gas in the atmosphere). On the other hand CO sensors probably need to be able to tell when the concentration of CO exceeds some predetermined threshold where it is safe for humans to breathe.

I would like to hear the answer on why and what causes the large price difference. My guesses are: complexity of manufacturing of CO2 analyzer, parts or the large scale production of CO sensors. It is probably the sum of many but from an expert on this or an enlightened opinion I would like to hear from this more accurately.

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Part of the answer is that CO is chemically reactive, while CO2 is not.

"Cheap and simple" CO sensors either mimic the reaction of CO with the oxygen-carrying chemicals in the bloodstream and detect the color change during the reaction, or they contain a simple fuel cell which combusts the CO to CO2 and measures the electrical current produced. The fuel cell method is now the most commonly used.

CO2 sensors measure the absorption of infra-red light by the CO2 (i.e. they use the same physics which causes the greenhouse effect). This is an accurate method, but not cheap technology. There are cheaper sensors available which use chemical reactions, but the calibration and accuracy is not very stable and they have a relatively short life (e.g. 2 years).

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