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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 instrume

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-vs-ndir-updated/ https://www.airgradient.com/blog/co2-sensors-photo-acoustic-... [2] https://www.airgradient.com/blog/co2-sensors-photo-acoustic-vs-ndir-updated/#outdoors https://www.airgradient.com/blog/co2-sensors-photo-acoustic-...

wkat4242 2y ago
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.
- ahaucnx 2y ago
  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-sensors-outdoors/ https://www.airgradient.com/blog/performance-of-low-cost-co2... [2] https://www.airgradient.com/outdoor/ https://www.airgradient.com/outdoor/ [3] https://www.airgradient.com/blog/airgradient-global-co2-map/ https://www.airgradient.com/blog/airgradient-global-co2-map/
  - Rebelgecko 2y ago
    Who do you calibrate these CO2 sensors in areas where the background level is so high?
    - ahaucnx 2y ago
      Not sure what you mean with calibration here. Above is not so much about calibration but correlation. Basically comparing the monitors to the reference data.
      - hyperknot 2y ago
        I think he meant How?, meaning normally a calibration is placing indoor monitors outdoors for 20 mins.
        2y ago
moogly 2y ago
The Infineon press release talks about indoor monitoring only, and it is even present in the title, but I have noticed that you guys consider HN to be a very important marketing channel, so I get it.
- ahaucnx 2y ago
  If you take the time and actually read the blog post I linked to, you will see that mostly it is about indoor CO2. However, to understand the performance of these different sensor technologies it is important to point out that they fail under certain environmental conditions, e.g. outdoors.
  - deleted 2y ago
    [deleted]
dr_kiszonka 2y ago
If you have time, could you write a bit more about the humming phenomenon?
- OldGuyInTheClub 2y ago
  Photoacoustic is usually done with a chopped or pulsed light source at a few hundred Hz (ie. audio frequencies). If the molecules don't absorb at that wavelength or if the molecules are not present, the light just goes through. If light is absorbed, there will be slight heating when the beam is on resulting in a sound wave at the chopping/pulsing frequency. This is detected by the microphone. For weak absorptions, the intensity of the sound wave will be linear in the concentration. I think this is what is meant by "humming": There is a sound that is generated when the target molecule is present.
OldGuyInTheClub 2y ago
Optical linewidths are influenced by temperature and pressure. Outdoors, humidity can be a factor as well. When I did photoacoustic spectroscopy, we used two or more wavelengths in the measurement; one on resonance, one off resonance to help with background discrimination. I am not sure whether these ultracompact instruments have that capability. Additionally, contaminants on any surface the light strikes will generate spurious signals. Lock-in (synchronous) detection at the optical source modulation frequency can help get rid of a lot of noise but if the noise appears at that frequency, it will corrupt the measurement. This classic paper from 1986 covers a lot of what can be done with the technique and how to do it correctly. Reviews of Modern Physics is behind a paywall but there's a copy on the web at the URL below. Applications of photoacoustic sensing techniques Andrew C. Tam Rev. Mod. Phys. 58, 381 – Published 1 April 1986 http://users.df.uba.ar/dkunik/photothermal/review/tam86.pdf http://users.df.uba.ar/dkunik/photothermal/review/tam86.pdf
simonebrunozzi 2y ago
If money is not a problem (let's say, up to $1,000), what's the best CO2 indoor sensors you would suggest today?

wkat4242 2y ago

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.

ahaucnx 2y ago
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-sensors-outdoors/ https://www.airgradient.com/blog/performance-of-low-cost-co2... [2] https://www.airgradient.com/outdoor/ https://www.airgradient.com/outdoor/ [3] https://www.airgradient.com/blog/airgradient-global-co2-map/ https://www.airgradient.com/blog/airgradient-global-co2-map/
- Rebelgecko 2y ago
  Who do you calibrate these CO2 sensors in areas where the background level is so high?
  - ahaucnx 2y ago
    Not sure what you mean with calibration here. Above is not so much about calibration but correlation. Basically comparing the monitors to the reference data.
    - hyperknot 2y ago
      I think he meant How?, meaning normally a calibration is placing indoor monitors outdoors for 20 mins.
      - 2y ago

moogly 2y ago

The Infineon press release talks about indoor monitoring only, and it is even present in the title, but I have noticed that you guys consider HN to be a very important marketing channel, so I get it.

ahaucnx 2y ago
If you take the time and actually read the blog post I linked to, you will see that mostly it is about indoor CO2. However, to understand the performance of these different sensor technologies it is important to point out that they fail under certain environmental conditions, e.g. outdoors.
- deleted 2y ago
  [deleted]

dr_kiszonka 2y ago

If you have time, could you write a bit more about the humming phenomenon?

OldGuyInTheClub 2y ago
Photoacoustic is usually done with a chopped or pulsed light source at a few hundred Hz (ie. audio frequencies). If the molecules don't absorb at that wavelength or if the molecules are not present, the light just goes through. If light is absorbed, there will be slight heating when the beam is on resulting in a sound wave at the chopping/pulsing frequency. This is detected by the microphone. For weak absorptions, the intensity of the sound wave will be linear in the concentration. I think this is what is meant by "humming": There is a sound that is generated when the target molecule is present.

OldGuyInTheClub 2y ago

Optical linewidths are influenced by temperature and pressure. Outdoors, humidity can be a factor as well. When I did photoacoustic spectroscopy, we used two or more wavelengths in the measurement; one on resonance, one off resonance to help with background discrimination. I am not sure whether these ultracompact instruments have that capability. Additionally, contaminants on any surface the light strikes will generate spurious signals. Lock-in (synchronous) detection at the optical source modulation frequency can help get rid of a lot of noise but if the noise appears at that frequency, it will corrupt the measurement. This classic paper from 1986 covers a lot of what can be done with the technique and how to do it correctly. Reviews of Modern Physics is behind a paywall but there's a copy on the web at the URL below. Applications of photoacoustic sensing techniques Andrew C. Tam Rev. Mod. Phys. 58, 381 – Published 1 April 1986 http://users.df.uba.ar/dkunik/photothermal/review/tam86.pdf http://users.df.uba.ar/dkunik/photothermal/review/tam86.pdf

simonebrunozzi 2y ago

If money is not a problem (let's say, up to $1,000), what's the best CO2 indoor sensors you would suggest today?