# Why are CO2 sensors so expensive when CO sensors aren't?

Please excuse me if Earth Science is not the correct place to ask this question.

I read a New York Times article about increasing $$\small\mathsf{CO_2}$$ levels in the atmosphere: "Carbon in Atmosphere Is Rising, Even as Emissions Stabilize" (26 June 2017)

According to the article, there are not enough $$\small\mathsf{CO_2}$$ monitoring stations for scientists to have a detailed understanding of where $$\small\mathsf{CO_2}$$ is being emitted/absorbed (e.g. by landmasses and oceans)

This got me thinking, would it be possible to design a dual-use product that integrates a $$\small\mathsf{CO_2}$$ sensor (e.g. a solar powered garden lamp) from which we could crowd source $$\small\mathsf{CO_2}$$ data for scientists? Perhaps a bit naive, since the sensors may not be accurate enough for scientific studies, and if it's a consumer product they would likely have a dense install base in metropolitan areas and less in the countryside/oceans.

But that isn't the biggest flaw in this idea. No, that came when I looked up $$\small\mathsf{CO_2}$$ sensor prices:

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I was able to find some Arduino compatible sensors from a Google search for around 50USD at the low end, but looking at electronics distributors, a more reasonable price for a raw $$\small\mathsf{CO_2}$$ sensor seems to be at least 70USD.

Carbon Monoxide detectors which you are encouraged to install in your home are much less expensive!

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1. How can it be that $$\small\mathsf{CO_2}$$ sensors are so expensive compared to CO detectors?
2. Is this due to the fact that carbon monoxide sensors use a detection method which cannot accurately determine the PPM of CO?
• Detectors are not well-structured measurement devices. Measuring real CO requires sophisticated equipment than just home based detectors. Your question is not about Earth science, it is an enquiry into the price of sensors/detectors. – Gemechu Fanta Garuma Jun 26 '17 at 23:29
• I think this is a good question that emerges from interest towards earth science, but is actually as much about the sensor/detector engineering. Some earth scientist with an measurement device background could have a great answer to this. – Communisty Jun 27 '17 at 6:27
• Technically, this question is not a question on Earth Science. However, it is related to dealing with some current Earth Science-related questions. A similar question is dealt with in some project on urban air quality (a discussed at scientific conferences): Can we equip public transport buses, citizens or bikes with small, robust and maintenance-free (and cheap) devices to measure air pollutants such as NOx or PM with adequate accuracy? Currently, the answer is no. Either the instrument do not provide adequate accuracy or they are too expensive or ... . – daniel.neumann Jun 27 '17 at 12:59
• As @GFG pointed out (but to emphasize it): You are right with your second question. There is a difference between a detector, which should just give a qualitative response (CO above a rough threshold: YES/NO), and measurement device, which should give a quantitative output on the CO concentration. – daniel.neumann Jun 27 '17 at 13:03
• I would suspect the major reason CO detectors are less expensive than CO2 detectors is simply that millions of people DO install them, enabling mass production and economies of scale. Also, they don't need to be all that accurate, just sound an alarm if the concentration goes over some minimum threshold. – jamesqf Jun 28 '17 at 4:53

## Carbon monoxide (CO), natural gas & hydrocarbons, VOCs, ammonia, etc.

In this Chemistry SE answer and this Electronics SE question I discuss how the family of MOS gas sensors work. They can measure a large variety of different gasses that can react with atomic oxygen (O rather than $$\small\mathsf{O_2}$$).

I'll quote from this question:

...sintered composite based on the semiconductor material $$\small\mathsf{SnO_2}$$, which is the metal oxide. The different sensors have different admixtures and operating conditions to achieve different sensitivities to different gasses.

The principle of operation of MOS-type gas sensors is explained in Figaro sensors' Operating principle; MOS type.

The Figaro link shows the operating principle in the GIF below:

GIF (Source: Figaro)

When the $$\small\mathsf{SnO_2}$$ is heated by the (usually 5V but not always) heater supply, oxygen in the air will dissociate at the surface and oxygen ions will stick to the surface, depleting the semiconductor and cutting off the flow of current. When a carbon-containing gas (e.g. $$\small\mathsf{CO}$$ or $$\small\mathsf{C_xH_y}$$ (but not $$\small\mathsf{CO_2}$$)) comes along, it will combine with the adsorbed oxygen and remove it from the surface. The more gas, the lower the equilibrium concentration of adsorbed oxygen and the higher resulting current flow through the semiconductor.

## Carbon dioxide ($$\small\mathsf{CO_2}$$)

The $$\small\mathsf{CO_2}$$ sensor works on a totally different principle because $$\small\mathsf{CO_2}$$ is fully oxidized carbon, it will not react further with atomic oxygen.

Instead the $$\small\mathsf{CO_2}$$ sensors are either solid electrolyte chemical cells like the MG-811 or newer versions (see this question) or they are Non-dispersive Infrared Light sensors (NDIRs) based on optical absorption of a certain wavelength of infrared light, using a hot filament or other light source to produce IR and a narrow multilayer optical filter to select one of the strong absorption lines of $$\small\mathsf{CO_2}$$. The more $$\small\mathsf{CO_2}$$, the more the reduction in the transmission of light in the optical path of the sensor.

I think the core of the answer should lie in what was already stated in the comments. These detectors are not scientific instruments. For air quality monitoring much more sophisticated analyzers are needed to measure the concentration of the gases. The example in the link works on a basis of gas filter (filled with CO) and it can just as well be used for $$\small\mathsf{CO_2}$$ if you fill the filter with this gas instead. Analyzers like this can cost around 20 000 USD (very rough estimate based on prices in my national currency).

This is what is relevant to Earth Science SE, the questions about the price should really be asked on a different site.

• I would also like to know what makes the scientific and more accurate instruments so expensive compared to the simple commercial ones (parts, working principle rights/complexity, large scale production.. ?). Given the special use of CO2 analyzers in Earth Science this is probably as relevant here as it would be in Engineering SE. – Communisty Jun 29 '17 at 11:56
• @Communisty It's worth comparing your average off-the-shelf CO detector and how it's used to the equipment and practices used at places like the Keeling Curve operation on Mauna Loa: scrippsco2.ucsd.edu/assets/publications/… Mauna Loa is used because it's isolated from nearby transient sources of CO2 such as rush-hour traffic or growing/decaying agriculture and someone is on site to monitor conditions. – jeffronicus Jun 29 '17 at 20:57

First you need to make the difference between a detector and a sensor, while the first one only shows that some gas is present, the second has to show the amount of gas.

Usually a sensor needs calibration, where you must give two points with different concentrations, this is not easy and you need a known source of concentration. Also this means that a sensor requires more electronics for its human-machine interaction.
A detector only needs to trigger an alarm, nothing else.

In the Arduino compatible sensors I've found, CO, methane, and other common raw sensors are as little as US\$1 from China. The cheapest $$\small\mathsf{CO_2}$$ sensor (also from China) I found was around US\$35. Otherwise, raw sensors components are around US\$100. The only real difference between the raw sensors and scientific instruments are that the instruments are obviously finished units that are certified and calibrated, like (for example) are raw Geiger tubes (US\$5) vs calibrated and certified Geiger counters (US$300). I have several DIY Geiger counters (US\$35) and calibrated Geiger counter instruments.