Seems similar to the SCD40 photoacoustic approach.
I used that for an open-source CO2 monitor I designed:
Great project, lovely design and super cool that it's also open source hardware.
We at AirGradient open sourced our monitors around 2 years ago and this has been the best decision for our company.
Nice to see this miniaturization of photoacoustic spectroscopy - something I've done a bit of in the past. It is an underappreciated technique. Ordinarily one measures the difference in optical throughput with and without a sample. If it is a weak absorber, it is a difference between two large numbers. PAS is zero background. No absorption, no pressure wave, no signal. Any absorption stands out clearly against that zero background.
We did some extensive testing of photo-acoustic and optical NDIR CO2 sensors indoors as well as outdoors and also in comparison to scientific reference instruments.
I just updated the blog post that I wrote last year [1] with the specs of this new Infineon sensor and also our experiences with doing CO2 measurement outdoors.
In summary we can say that under normal indoor conditions (e.g. small range of temperature, relative high range of CO2 etc.), the photo-acoustic NDIR sensors perform well.
However, outdoors they significantly underperform optical NDIR sensors to the extent that they are barely usable. See for example this chart [2].
It's hard to say why exactly they perform so poorly outdoors but I believe that they rely on quite complex internal algorithms that probably have been only developed with typical indoor conditions in mind and as soon as some parameters like temperature are out of the typical range, they significantly drop in their performance.
Furthermore, we also detected that they are sensitive to interference from low frequency noise which could for example come from ventilation systems, refrigerators etc. This is not surprising as they rely on their internal microphone to do the photo-acoustic measurements.
All of this and the rock-solid performance we see with optical NDIR sensors made us quite wary about the photo-acoustic sensors and not use them in our open-source hardware monitors. As I said, I believe they work quite well in general but because we saw also some really strange and unexpected behavior of these photo-acoustic sensors, I'd now always prefer optical NDIR sensors if possible.
[1] https://www.airgradient.com/blog/co2-sensors-photo-acoustic-...
[2] https://www.airgradient.com/blog/co2-sensors-photo-acoustic-...
Interesting. But why would you monitor CO2 outdoors? Even the NDIR ones just return "400" when they detect outdoor air and use it as calibration.
Though i believe you can turn off the auto calibration which is probably a good idea in busy city areas where I imagine outdoor CO2 is slightly higher than 400. And manually calibrate them with an actual known source.
Outdoor CO2 levels can go up to 700-800ppm in rural areas due to plants creating CO2 [1]. I was initially surprised it's that high but it was verified with reference instruments.
Why measure CO2 outdoors?
We are able to detect emission sources pretty well now with our outdoor monitors [2] that have the SenseAir S8 NDIR sensor built in. You can see some real data in the blog post I wrote about launching our global CO2 map [3].
So setting up a dense network of these sensors in a city would allow to measure if for example the introduction of low-emission zones or switching to electric buses etc. would work.
Another use case is to check for leakage in underground CO2 storage facilities.
All-in-all it's still experimental (to some extent) but we know the accuracy is there and we can see more and more use cases as outlined in my blog post. So we now work hard to get more and more of these sensors out there to get more data to identify additional use cases.
[1] https://www.airgradient.com/blog/performance-of-low-cost-co2...
[2] https://www.airgradient.com/outdoor/
[3] https://www.airgradient.com/blog/airgradient-global-co2-map/
Finally an actually working (cheap?) CO2 sensor?
So many of those actually measure humidity, temp and VOCs and try to derive some sort of CO2 reading out of those.
True. The cheap ones are trying to guess CO2. Those are called "indoor air quality sensors".
Small CO2 sensors have been available for years, for about $50. Compare [1].
Life of this new device is only 10 years, which is short for HVAC systems. A hotel might have a thousand of these. Older devices say "15+" years.
All these devices have a calibration problem. They drift. They try to correct by treating the lowest value they ever see as "normal" (that's about 400 ppm CO2 today, vs 300 PPM in 1950) and recalibrating. So they're not useful for observing a general increase in CO2. They're also not useful for greenhouses, where CO2 levels may drop below ambient CO2 due to photosynthesis. Manual recalibration is possible but requires feeding in pure nitrogen and a known nitrogen/CO2 mixture.[2]
Devices which don't need that re-calibration exist.[3] They're more complicated. Also don't seem to be stocked by the usual distributors.
[1] https://rmtplusstoragesenseair.blob.core.windows.net/docs/pu...
[2] https://www.co2meter.com/blogs/news/7512282-co2-sensor-calib...
[3] https://www.murata.com/en-us/products/sensor/co2/overview/te...
Do you know of anywhere at all to get one of the devices that don’t require re-calibration?
It's not really "finally". They introduced a similar sensor, the PASCO2V01, a couple years ago. That one has been available on a breakout board with the necessary support hardware from SparkFun for over a year [1].
Comparing the datasheets for the PASCO2V01 and the new PASCO2V15 the old one actually seems a little better as far as CO2 measuring performance goes. They are the same on most things, but the old one has slightly better accuracy.
The new one is ±(50 ppm + 5%) between 400 ppm and 3000 ppm.
The old one is ±(30 ppm + 3%) between 400 ppm and 5000 ppm.
The big difference is this:
> Infineon has recently introduced the PASCO2V15, a new 5 V sensor to improve air quality monitoring in building environments.
Both of them require a dual voltage power supply. They both want 3.3 V for their digital components and a higher voltage for their IR emitter.
For the older one that higher voltage is 12 V. For the newer it is 5 V.
Iirc they were merged into esphome last year too.
Sensirion has the SCD40, which appears to be based on the same principle. It's much cheaper than the SCD30.
Yes, SCD30 series are optical while SCD40 series are photoacoustic. STC series are thermal conductivity based. STC and SCD40 are smaller than SCD30 but less accurate if memory serves (check datasheet).
This was my thought exactly. I used sensors that were about $25/each in the past and those worked well but this would be seemingly way easier to integrate and get ahold of.
$25 for a CO2 sensor component that actually works is not bad at all.
Sensirion SCD41 should be pretty good, right? You can get them for ~25 USD on aliexpress ...
I'm not sure I would source non-Chinese electronic components from Aliexpress... (unless it's just to play with them, definitely not for a product): you might need to check their reliability and quality. Pretty sure that Sensirion themselves are not selling there, so they are probably either clones, fakes, recycled ones, or if you are lucky authentic ones that for some reason ended up there (but I can't imagine a way).
> or if you are lucky authentic ones that for some reason ended up there (but I can't imagine a way).
Chinese fabs are notorious for running secret shifts that make their customers’ chips to sell themselves. They’re the exact same chip but made without authorization, using the customer’s exact design.
That said, Sensirion has their own CMOS fabs in Switzerland so that’s very unlikely to be the case here.
I got mine from M5Stack and works quite well, I’m submitting data every 5 seconds using EspHome/HomeAssistant
The Senseair S88, which was released earlier this year, costs ~$22/piece, or ~$13/piece if you order more than 100.
They've had a 12v version for a while, and it's quite nice despite the high voltage requirement. I made a little breakout with a boost converter. Sensirion has a slightly smaller sensor as well, SCD41 that I think works on similar principles.
Neither are cheap, around $25-40 each in small quantities. The infineon one has a full blown microcontroller handling the operation of the sensors.
To keep accuracy you would need to have a CO2 gas setup which isn't cheap either, but for indoor use I don't think it matters.
I’ve been considering designing a wearable that monitors CO2 and PM2.5 continuously, but I’m unsure if people would wear it in conjunction with an Apple Watch or similar.
This is super interesting for me - but I'd love to put it on a bicycle (they are often locked up and stationary - for example) but move at faster speeds. Maybe this means they are only useful at tracking information when locked/stationary?
I really like the idea of using cheap (?) devices in a sort of mesh to feed back telemetry data on pollution. Pollution is everyone's concern, so visualising that would be cool.
Interested to hear if you had any more thoughts on this!
Related: cheap device to track pollution + mesh + visualization: https://luftdaten.info/
Once had this thing on a balcony of a shared flat in Heilbronn, Germany. Wondered what that was, previous tenant told me about it and it was never removed from there.
I'm not sure how accurate that would be on your wrist, because the proportion of recently exhaled air would be so much higher, since it's only a foot or two from your mouth.
CO2 monitors often have little silent fans to draw in fresh air as well, for accuracy.
I don’t think it’d make a difference. I’ve got one on my desk in front of me and can’t detect a difference with me at the desk vs not.
I believe Aranet4 contains this NDIR optical sensor https://www.co2meter.com/collections/sensors/products/006-0-...
Like that, but cheaper and more accurate.
Datasheet can be found here: https://www.infineon.com/dgdl/Infineon-PASCO2V15-DataSheet-v...
Have been using scd30/31/40. Great sensors. This one requires a bit more power but would be interesting to see price as it seems it actually measured CO2. (A lot of other sensors simulate it with measuring alcohols and assume people breathing which gives poor results)
The problem with the common CO2 sensor modules is they don't have DC accuracy. Meaning they rely on the device being present in place where it regularly (e.g at least once a week) gets exposed to fresh air, which the module sets as its baseline. This works because fresh air has roughly the same CO2 concentration everywhere.
Hopefully this method doesn't have the same restriction.
I worked in a building 500 ft from a busy highway and when I cleaned my desk it always had black dust on it.
Along these lines of air quality, can anyone recommend a similarly advanced PM2.5 / PM10 sensor under $100 / ea?
I haven't gotten their PM sensor unit but have a CO2 sensor from CO2.click. About to pull the trigger on a PM sensor but just deciding which one. The founder there is active on a few places including Mastodon and I really like my CO2 sensor from them.
Edit: sorry missed your price guidance. They are quite a bit more so probably not what you're after!
Have been using a Plantower PMS5003 for a while with ESPhome and it’s pretty good.
Would be nice if they packaged these up in laptops - fan is already there and always running.