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Air Quality Shorts: Part 3 - CO2 Sensors

CO2 Sensors Are Having a Moment.

CO2 sensors have increased in accuracy in recent years, and have also gotten smaller, as well as costing less. By the time the Pandemic ramped up in 2021, I had been working on the subject of Air Quality and started reading that these humble sensors were being used as a 'proxy' for risk of Coronavirus in public buildings, schools, offices, restaurants, etc. (Link).

The Guardian - Covid in classrooms get CO2 monitors

To be clear,CO2 sensors cannot 'detect' viral particles, in fact, neither can the Particulate Sensor - and even if it could, it wouldn't be able to distinguish between a Flu Virus or a COVID-19 Strain. Yet the realisation that if we have a room filled with a lot of people exhaling CO2, and no windows open, this is a very good 'proxy' for creating an alert to encourage people to open windows, switch on (non-recirculating) AC, or simply have less people in the room in future.

For the list price of around £35-£45 you can get a Sensirion SCD40 CO2 sensor, or you can find it in a breakout board, such as Adafruit’s (ID:5187), or for around £100-£150 you can buy an off-the-shelf product, to just plug-in and start in your home or office. (Cheaper ones are available, but are less likely to be ‘True CO2’ sensors). In case you were curious, I took one apart, so you don’t have to...

Sensor with can lid removed

Removing the top of the Sensor, you can see the tiny hole where the gas-exchange takes place, through the white stick-on membrane. This really brings home how small the particles are - 3 atoms are about as small as it gets in air quality monitoring, so this is an amazing achievement for a discrete (ie not broadband MOX sensor), to give pretty accurate (±1.8%) over a range of 400-5000ppm.

As mentioned in the video, the sensor is the shiny square in the top right of the sensor array in the central green PCB, shown here on a Development PCB.

Jude holding ESDK C02 Sensor board and enclosure

The SCD40 was used in the ESDK as shown, and could not only be used as a proxy for CO2, but is also a good ‘air quality for living’ sensor. For example, I was working in my shed and realised I can go from 400ppm (backgroundCO2levels outside), to over 1000ppm in about 20mins, which is deemed ‘mildly unhealthy’. In 1 hour I can be at 4000ppm which is ‘seriously unhealthy’, and from experience, I had a pounding headache and felt dizzy and was sweating. HR Managers should take note - productivity is bad if the air quality is bad.

Jude sketch book checking out the maths

Being a Nerd About Town, I was curious to see if this checked out on the maths of the situation, and remarkably it did: By working out the likely gas exchange in the volume of my shed, it was indeed 'talking myself stupider'... a realisation which has all the jokes you'd imagine with my Wife! Anyway, this ended up having a big impact on my working habits, and sleeping habits (you’ll sleep better with the window open, even just a crack), but of course went on to inspire the Good Air Canary...

Jude with the Canary

The Canary, which you may well have seen if a regular at DesignSpark, but here it is in action if not. (Link).

Jude with specification of sensor

Key design in recommendations guide

Key design in recommendations guide with more illustrations

In the video, I go through some of Sensirion’s datasheet - but also the design guides that influenced the design of the ESDK’s CO2 sensor also. In short, isolating from heat, sunlight and too much draught are all things to consider carefully.

It should be said that I have seen very accurate projects, and also examples from where someone made a CO2 Ring, with the sensor mounted without any enclosure or real adherence to the design specification. It may well be that its error is ±200ppm (as opposed to ±50ppm if installed ‘correctly’), but for projects like this, accuracy is not truly the goal - it is a statement of what our lives might be like as sensors shrink and become integrated into our apparel, and possibly even bodies. So the integration of this (or any other AQ sensor) needs to be aligned to what you are trying to achieve. As they say, better to know the rules and break them, than be ignorant and not know what you’re invalidating. I for one look forward to seeing what other projects come as a result of these small yet powerful sensors.

The Air Quality Shorts Series.

The following series will give you an overview of sensors of interest. Right now we have:

But stay tuned, as we also have more sensors in development! So follow this Article for updates...

Winner of the 2020 Alastair Graham-Bryce "Imagineering" Award (IMechE), Jude thrives in high risk collaborations, uncertainty and pressure - drawing from global networks and experiences to deliver high profile campaigns and digital/physical products. A leading Creative Technologist & Physical Prototyping Expert, Jude has worked for NHS, Dyson, LEGO, and a number of start-ups. He is one of the eight featured inventors in BBC Two's Big Life Fix.
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