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I'm trying to develop a system that will monitor long term CO2 levels indoors and will run on a battery.

A lot of the conventional CO2 sensors are really power hungry and expensive. However, I came across this (http://www.ccmoss.com/gas-sensors) which claims it can measure 'equivalent CO2' in the environment where the main source of VOCs is from humans.

This seems great with a power consumption of <1mW, but it's £5.96 per sensor which is quite pricey!

Has anyone heard of anything similar, or have an idea of how 'equivalent CO2' is calculated so I could maybe use a cheaper VOC sensor and do the calculation myself?

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£5.96 ($7.74 USD) is actually incredibly cheap for a C02 sensor. I used a Futurelec C02 sensor in a project, and it runs $34.90 with no processing capabilities at all - just a simple voltage output.

You'll have to read the datasheet for your specific device to see what "equivalent C02" means.

Here is a PDF from Gas Sensing Solutions, a UK-based sensor manufacturer, and they say:

Some manufacturers of air quality sensors are now providing an output in "CO2 equivalent units." This measure is considered misleading and may confuse many new to the indoor air quality industry.

There is currently no recognized procedure to directly quantify the output of these sensors to carbon dioxide or the differential between inside and outside concentrations that is used to determine ventilation rate.

...so read the manufacturer's documentation carefully to fully understand what you're actually measuring.

I'm going to deviate from objectivity for a moment and give you an opinion: I'm skeptical of the devices you linked to. I have trouble believing they can deliver a high level of accuracy for that price, given that bare sensors alone (as you noted) are significantly more expensive. The fact that they provide a dubious "C02 equivalent" measurement rather than an absolute value makes me question their usefulness. I would want to learn a lot more about how they work and exactly what information they're providing before I purchased one. I'm sure someone with more experience working with air quality measurements could provide good use cases for these, but I can't help but be wary of them.

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Not much as answer but just as experimentation with cheap things,

you could try measuring resistance or conductance of water in presence of air, since CO2 react with water and give H+ and CO3-- , that conduct electricity... as well, this CO2 can come back to air also.


You could also try to increase surface-area of water, such as with glass-wool or capillary tubes etc,

as well you may require amplification of electronic signal. You may apply on that water,

instead DC, an oscillating voltage or a low-voltage AC could be tried ((keeping in mind all- possible hazards of AC-source, for all-sorts of reader)), to get larger and much-linear relationship of voltage and current, and better conduction in certain voltage limit than DC.


Effect of temperature would count in this.

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