3 ms·

Pardon my lack of knowledge, but what's wrong about an equivalent type sensor?

the_pwner224 5y ago
Those basically measure VOCs and use that to guess what the CO2 concentration is (the two are often correlated, or something like that). So the 'CO2' reading they give is often inaccurate, and can be heavily manipulated by the presence of certain gases/VOCs. The one that AirGradient has chosen is a true CO2 sensor, AFAIK.
NortySpock 5y ago
VOC: Volatile organic compounds -- aerosolized oils, greases, perfumes, "bad smells", solvents, exhaled breath which would contain normal biological material, or anything else having to do with carbon compounds A VOC/CO2 equivalent sensor is measuring VOCs and using them as a proxy to estimate CO2 concentration levels. CO2 is of course fairly non-reactive so it'd difficult (read: expensive) to make a chemical sensor for this. VOCs are more reactive and easier to make a sensor for. Basically, you're getting a sensor for VOCs that, based on some assumptions about outdoor or indoor air quality, is then being used to estimate CO2 levels. This is fine if all you want is a loose estimate, but if you have reason to think your VOC/CO2 ratios are going to be different than "usual" (e.g. a CO2 storage facility, next to a poorly maintained combustion system) or where CO2 detection is of paramount importance (e.g. life-or-death situations), then a VOC detector is not acceptable. EDIT: tried to improve the VOC examples
- parsecs 5y ago
  Some of the CO2 sensors that I worked with use separate signals for deriving estimated VOC and CO2. For example the Senserion SGP30 [1] measures ethanol and H2, and uses H2 to determine eCO2 value. Indeed the Senseair S8 [2] used in AirGradient seem to feature a "more direct" NDIR CO2 sensor, so there's not really any issues there. Is using H2 to detect CO2 "good enough" for all non life-or-death situations? Edit: And to clarify, I was mainly asking the parent comment as to what is their application that demand NDIR or "more direct" CO2 sensors. [1] https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/9_Gas_Sensors/Datasheets/Sensirion_Gas_Sensors_Datasheet_SGP30.pdf https://www.sensirion.com/fileadmin/user_upload/customers/se... [2] https://rmtplusstoragesenseair.blob.core.windows.net/docs/publicerat/PSP107.pdf https://rmtplusstoragesenseair.blob.core.windows.net/docs/pu...
  - jfim 5y ago
    It seems alright if you do use the humidity compensation. It might be off by 100 ppm or so, and wildly off if there are a lot of VOCs in the air, but if you're just using it as a "should I open the windows" kind of thing it'll do fine IMHO. For example, the two sensors I have right now, with the windows open, the S8s report 478 and 455 ppm of CO2, while the SGP30s report 400 and 405 respectively. Without having calibrated instruments it's hard to know if any of these values are good, but they seem at least not too wildly off. The SGP30 is a lot more compact and much less of an energy pig than the S8; the S8 will actually take so much current on the 5V rail that it'll cause the voltage to sag if you're powering it through USB with other devices connected (https://blog.jean-francois.im/2021/05/08/building-a-simple-air-quality-monitor/images/5v-rail-scope.jpg https://blog.jean-francois.im/2021/05/08/building-a-simple-a...). It's also a lot cheaper. However, the SGP30 approximation can be off by a bit, and the calculation of the humidity value to give the sensor is a bit gnarly (https://github.com/jfim/air-quality-monitor-firmware/blob/master/src/main.cpp#L478 https://github.com/jfim/air-quality-monitor-firmware/blob/ma...). It's also pretty obvious that the SGP30 has a hardcoded floor of 400 ppm CO2eq and that it won't ever return a value lower than that.
- ahaucnx 5y ago
  Sometimes even the loose estimates are totally off because most of the time the eCO2 correlates with the TVOC measurement. For example I triggered a TVOC sensor with suncream smell in a very low CO2 room and the CO2 wenth through the roof.

Those basically measure VOCs and use that to guess what the CO2 concentration is (the two are often correlated, or something like that). So the 'CO2' reading they give is often inaccurate, and can be heavily manipulated by the presence of certain gases/VOCs. The one that AirGradient has chosen is a true CO2 sensor, AFAIK.

NortySpock 5y ago

VOC: Volatile organic compounds -- aerosolized oils, greases, perfumes, "bad smells", solvents, exhaled breath which would contain normal biological material, or anything else having to do with carbon compounds A VOC/CO2 equivalent sensor is measuring VOCs and using them as a proxy to estimate CO2 concentration levels. CO2 is of course fairly non-reactive so it'd difficult (read: expensive) to make a chemical sensor for this. VOCs are more reactive and easier to make a sensor for. Basically, you're getting a sensor for VOCs that, based on some assumptions about outdoor or indoor air quality, is then being used to estimate CO2 levels. This is fine if all you want is a loose estimate, but if you have reason to think your VOC/CO2 ratios are going to be different than "usual" (e.g. a CO2 storage facility, next to a poorly maintained combustion system) or where CO2 detection is of paramount importance (e.g. life-or-death situations), then a VOC detector is not acceptable. EDIT: tried to improve the VOC examples

parsecs 5y ago
Some of the CO2 sensors that I worked with use separate signals for deriving estimated VOC and CO2. For example the Senserion SGP30 [1] measures ethanol and H2, and uses H2 to determine eCO2 value. Indeed the Senseair S8 [2] used in AirGradient seem to feature a "more direct" NDIR CO2 sensor, so there's not really any issues there. Is using H2 to detect CO2 "good enough" for all non life-or-death situations? Edit: And to clarify, I was mainly asking the parent comment as to what is their application that demand NDIR or "more direct" CO2 sensors. [1] https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/9_Gas_Sensors/Datasheets/Sensirion_Gas_Sensors_Datasheet_SGP30.pdf https://www.sensirion.com/fileadmin/user_upload/customers/se... [2] https://rmtplusstoragesenseair.blob.core.windows.net/docs/publicerat/PSP107.pdf https://rmtplusstoragesenseair.blob.core.windows.net/docs/pu...
- jfim 5y ago
  It seems alright if you do use the humidity compensation. It might be off by 100 ppm or so, and wildly off if there are a lot of VOCs in the air, but if you're just using it as a "should I open the windows" kind of thing it'll do fine IMHO. For example, the two sensors I have right now, with the windows open, the S8s report 478 and 455 ppm of CO2, while the SGP30s report 400 and 405 respectively. Without having calibrated instruments it's hard to know if any of these values are good, but they seem at least not too wildly off. The SGP30 is a lot more compact and much less of an energy pig than the S8; the S8 will actually take so much current on the 5V rail that it'll cause the voltage to sag if you're powering it through USB with other devices connected (https://blog.jean-francois.im/2021/05/08/building-a-simple-air-quality-monitor/images/5v-rail-scope.jpg https://blog.jean-francois.im/2021/05/08/building-a-simple-a...). It's also a lot cheaper. However, the SGP30 approximation can be off by a bit, and the calculation of the humidity value to give the sensor is a bit gnarly (https://github.com/jfim/air-quality-monitor-firmware/blob/master/src/main.cpp#L478 https://github.com/jfim/air-quality-monitor-firmware/blob/ma...). It's also pretty obvious that the SGP30 has a hardcoded floor of 400 ppm CO2eq and that it won't ever return a value lower than that.
ahaucnx 5y ago
Sometimes even the loose estimates are totally off because most of the time the eCO2 correlates with the TVOC measurement. For example I triggered a TVOC sensor with suncream smell in a very low CO2 room and the CO2 wenth through the roof.