3 ms·

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,

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.

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.