This Smart Watch Keeps An Eye On Ambient CO2 Levels

Human respiration takes in oxygen and in turn, we exhale carbon dioxide. Thus, an uptick of carbon dioxide levels around us can indicate we’re in the presence of other humans, and also, perhaps, the pathogens they carry. To explore this phenomenon, [C Scott Ananian] developed a mod for the Watchy open-source smartwatch, which lets it detect carbon dioxide.

The idea behind the build is simple. If you’re around increased CO2 levels, it may be because you’re surrounded by people, and thus more likely to be exposed to COVID-19. To detect CO2, the watch relies on a Sensiron SCD40 or SCD41 sensor. This is read by the Watchy’s ESP32 microcontroller, and results are graphed on the watch’s e-Paper display. The Watchy is also given a nice new aluminum case to fit the additional hardware.

It’s cool having a graph on your wrist of the ambient concentration of CO2, and at the very least, it could make a good talking point next time you’re at a particularly boring party. You’ll also be more than ready to advise other partygoers if the carbon dioxide level is reaching dangerous levels.

We’ve seen similar builds before, which are useful not only for pandemic safety but also for monitoring if you have any leaks from CO2 storage in the house. If you’ve been working on your own ways to track dangerous gases, be sure to drop us a line!

20 thoughts on “This Smart Watch Keeps An Eye On Ambient CO2 Levels

    1. The band is a Leatherman Tread, comfort has been described as being similar to regular metal link watch bands but the tools aren’t very useful and it’s surprisingly expensive.

  1. I love it that someone needs to have a watch to detect if other people are around. It’s kind of like a weather station, “Hey let me see if it’s raining” instead of you know, look and see.
    Walks down the street looks at his wrist “I wonder if there are people around?”

    Joking aside, it’s a cool look and awesome design.

    1. Well, it was quite interesting going to a red Sox playoff game and being able to correlate the volume of cheering with the co2 level. (It did rise, but not enough to be concerning.). I’m sure it would be even more interesting to watch in an indoor stadium.

    2. It’s probably more for detecting quality of ventilation than sheer presence/absence of people. Having a CO2 sensor myself, it’s really fascinating data. The fresh air intake for my furnace isn’t enough to keep levels low, but it does help. It also helps to have the furnace fan on “circulate” (run 1/3 the time regardless of whether heat/cool is required), and of course open windows help more than anything but make my pm2.5 sensor and asthma less happy. There’s also some research with some related pathogen that keeping CO2 under 600 is correlated with having enough ventilation to substantially reduce risk.

      This is a nice build, definitely benefits from the more compact scd40/41. I have the older, bulkier scd30.

  2. CO2 you can detect with your body long before it is high enough to matter, what you cannot detect is O2 or CO levels both of which have far great impact on your wellbeing. If the above contradicts your beliefs then go and do some reading about human physiology.

    1. “The idea behind the build is simple. If you’re around increased CO2 levels, it may be because you’re surrounded by people, and thus more likely to be exposed to COVID-19”. I don’t think they are worried about the actual levels of CO2 just using it as a theoretical proxy.

      1. Yes, though it also does have some mental performance impacts at entirely plausible levels like 1500ppm. But it’s definitely correlated with a lot of other measurements, which is why people use other sensors as “effective CO2” sensors: basically all mostly used for humans vs ventilation.

  3. How the CO2 sensor works is pretty amazing (photoacoustic sensing). It pulses IR light at a specific wavelength (one of CO2’s absorption bands) on the molecules inside the metal cavity. This gives CO2 molecules additional energy which increases the pressure and then that change in pressure is detected by a sensor that is used to measure minute changes in pressure (a microphone). And it has a relative humidity sensor and a temperature sensor so that it can compensate for pressure changes that would otherwise skew the ppm measurements.

    1. That type of sensor is called a Golay cell. Simple in principle, actually building a good one is a bit of a pain. Early ones actually used a mechanical rotating shutter to modulate the light source.

      This was the gold standard for bedside and anesthesia gas monitoring in the ’80s, with 5-figure instrument price tags. It’s great to finally see affordable sensors hit the market.

  4. The open-source Watchy kit has been around awhile, it is made by SQFMI which is an abbreviation for Squarofumi [skwair-oh-foo-mee] Limited with offices in Hong Kong and New York City.[1][2] The current Watchy kit does not come with a CO2 sensor, that’s what [C Scott Ananian]’s mod is for. The problem is getting a Watchy in the first place. SQFMI’s Watchy kits used to sell on Tindie for $49.99 USD each [3], but every time I looked it was Out of Stock. However, today I saw Watchy kits up for sale at Mouser Electronics for $59.00 each.[4] It too is Out of Stock, but it does say this: “On Order: 3,196 Expected 12/28/2021. Factory Lead Time: 20 Weeks.” Mouser also sells SQFMI’s optional CNC-cut anodized aluminum Watchy case.[5] The standard Watchy kit comes with the “Yatari” injection-molded plastic case. Mouser’s aluminum Watchy case page says: “Stock: 985 Can Ship Immediately. Factory Lead-Time: 9 Weeks.” Having the aluminum Watchy case in-stock is an encouraging sign for the 3,196 Watchy Kits expected at Mouser in the end of December this year. Mouser is accepting pre-orders for the Watchy kits. Maybe I’ll actually be able to get a Watchy this time around :-) Note: I have no affiliation with any of the entities named herein.

    * References:

    1. Squarofumi (SQFMI)

    2. SQFMI Shop

    3. Watchy Kit on Tindie $49.00 ea.

    4. Watchy Kit on Mouser $59.00 ea.

    5. SQFMI CNC anodized aluminum case for Watchy on Mouser $49.00 ea.

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