5 ms·

Do you see consistency between your CO2 detectors? Early on in the pandemic when I started working at home, I became very cognizant about the impact of CO2 and

by breput 5y ago

Do you see consistency between your CO2 detectors? Early on in the pandemic when I started working at home, I became very cognizant about the impact of CO2 and built a little project with a microcontroller and the MHZ-19B CO2 sensor. Calibrating the sensor is an ongoing challenge and while I have 18 months of data, I'm still not 100% confident about the accuracy of the data at any given time. Still...I literally can feel when the CO2 level is above 800 - 1000ppm and it isn't always as easy as opening a window to counteract. There are certain conditions - maybe the barometric pressure or temperature gradient, but sometimes forced ventilation is necessary to get into the "comfortable" 400-600ppm range.

dbish 5y ago
They seem accurate within 100ppm or so which is enough to give me the information I need. Looking at their specs their claim is 75ppm https://support.getawair.com/hc/en-us/articles/360060843933-What-is-the-Technical-Accuracy-of-Awair-Element-Sensors- https://support.getawair.com/hc/en-us/articles/360060843933-...
- breput 5y ago
  Yeah, that is more than adequate. When I posted my project online I got the not-unexpected feedback that the sensor was hard to calibrate. I don't care about 400 vs. 600 ppm. I do care about 600 ppm vs. 1600 ppm.
jfim 5y ago
I'm not sure about the MHZ-19B, but I've been using the SenseAir S8 and the data that comes out of it has been rather noisy when powered by a switching power supply. If you have an oscilloscope, you might want to check whether your 5V rail changes in voltage when taking measurements. For example, mine looked like the picture in the middle of this blog post: https://blog.jean-francois.im/2021/05/08/building-a-simple-air-quality-monitor/ https://blog.jean-francois.im/2021/05/08/building-a-simple-a... If that's the case, I've found that the quality of the measurements fare better when there's a linear regulator supplying the power. I bought some LM7805s off DigiKey and they should arrive sometime soon, ping me if you want to know how that experiment went.
- breput 5y ago
  That's great advice and I will definitely look into it. I have a servo and a LCD panel on the same 5V supply as the ESP32, and while I think the USB power supply I have is sufficient, it is definitely worth investigating.
- breput 5y ago
  FWIW, this is my current temperature/CO2 plot: https://imgur.com/a/LhUoPcS https://imgur.com/a/LhUoPcS The temperature graph is pretty easy to understand as you see the A/C kick on twice during the day. The CO2 level is maybe not as obvious but I can see when I was in this room and when I closed the window when I was leaving.
  - bruce343434 5y ago
    I imagine co2 not being perfectly mixed and maybe wafts of it hitting the sensor? The overal trend is clear though. Try transforming the data with a moving hour average or something.
  - jfim 5y ago
    Oh that's not too bad, mine looks like this: https://imgur.com/a/7weijF7 https://imgur.com/a/7weijF7 with a sampling rate of 1Hz, without smoothing. As another poster pointed out, with automatic background calibration and opening the windows daily it doesn't get too out of whack. Out of curiosity, do you have any more details on how you built yours, I'm curious to see how you have done things differently from mine. :-)
Youden 5y ago
For nearly all sensors, you need to do one of two things: - Make sure auto-calibration is disabled and manually calibrate the sensor once a year or so by taking it out into fresh air and running its fresh-air calibration routine. - Leave auto-calibration enabled and make sure that the sensor is exposed directly to fresh air for at least an hour every single day. These sensors are first and foremost designed for use in HVAC systems in commercial buildings, which usually (always?) have an overnight fresh-air purge, so to keep them calibrated, the manufacturers just assume the sensor will always see fresh air at least once a day calibrate them based on that assumption. In a residential environment, that usually doesn't work. And FWIW, I've run a bunch of Sensirion SCD30 sensors for a few months and they worked quite well, with auto-calibration disabled. They're supported by ESPHome as well. Sensirion also recently released their SCD4x sensors [0] which measure CO2 with some kind of black magic [1] instead of the usual NDIR. The coolest thing is that the sensor is relatively tiny and built for SMD assembly. [0]: https://www.sensirion.com/en/environmental-sensors/carbon-dioxide-sensors/carbon-dioxide-sensor-scd4x/ https://www.sensirion.com/en/environmental-sensors/carbon-di... [1]: https://www.sensirion.com/en/about-us/company/pasensr-technology/ https://www.sensirion.com/en/about-us/company/pasensr-techno...

by breput 5y ago

dbish 5y ago

They seem accurate within 100ppm or so which is enough to give me the information I need. Looking at their specs their claim is 75ppm https://support.getawair.com/hc/en-us/articles/360060843933-What-is-the-Technical-Accuracy-of-Awair-Element-Sensors- https://support.getawair.com/hc/en-us/articles/360060843933-...

breput 5y ago
Yeah, that is more than adequate. When I posted my project online I got the not-unexpected feedback that the sensor was hard to calibrate. I don't care about 400 vs. 600 ppm. I do care about 600 ppm vs. 1600 ppm.

jfim 5y ago

I'm not sure about the MHZ-19B, but I've been using the SenseAir S8 and the data that comes out of it has been rather noisy when powered by a switching power supply. If you have an oscilloscope, you might want to check whether your 5V rail changes in voltage when taking measurements. For example, mine looked like the picture in the middle of this blog post: https://blog.jean-francois.im/2021/05/08/building-a-simple-air-quality-monitor/ https://blog.jean-francois.im/2021/05/08/building-a-simple-a... If that's the case, I've found that the quality of the measurements fare better when there's a linear regulator supplying the power. I bought some LM7805s off DigiKey and they should arrive sometime soon, ping me if you want to know how that experiment went.

breput 5y ago
That's great advice and I will definitely look into it. I have a servo and a LCD panel on the same 5V supply as the ESP32, and while I think the USB power supply I have is sufficient, it is definitely worth investigating.
breput 5y ago
FWIW, this is my current temperature/CO2 plot: https://imgur.com/a/LhUoPcS https://imgur.com/a/LhUoPcS The temperature graph is pretty easy to understand as you see the A/C kick on twice during the day. The CO2 level is maybe not as obvious but I can see when I was in this room and when I closed the window when I was leaving.
- bruce343434 5y ago
  I imagine co2 not being perfectly mixed and maybe wafts of it hitting the sensor? The overal trend is clear though. Try transforming the data with a moving hour average or something.
- jfim 5y ago
  Oh that's not too bad, mine looks like this: https://imgur.com/a/7weijF7 https://imgur.com/a/7weijF7 with a sampling rate of 1Hz, without smoothing. As another poster pointed out, with automatic background calibration and opening the windows daily it doesn't get too out of whack. Out of curiosity, do you have any more details on how you built yours, I'm curious to see how you have done things differently from mine. :-)

Youden 5y ago

For nearly all sensors, you need to do one of two things: - Make sure auto-calibration is disabled and manually calibrate the sensor once a year or so by taking it out into fresh air and running its fresh-air calibration routine. - Leave auto-calibration enabled and make sure that the sensor is exposed directly to fresh air for at least an hour every single day. These sensors are first and foremost designed for use in HVAC systems in commercial buildings, which usually (always?) have an overnight fresh-air purge, so to keep them calibrated, the manufacturers just assume the sensor will always see fresh air at least once a day calibrate them based on that assumption. In a residential environment, that usually doesn't work. And FWIW, I've run a bunch of Sensirion SCD30 sensors for a few months and they worked quite well, with auto-calibration disabled. They're supported by ESPHome as well. Sensirion also recently released their SCD4x sensors [0] which measure CO2 with some kind of black magic [1] instead of the usual NDIR. The coolest thing is that the sensor is relatively tiny and built for SMD assembly. [0]: https://www.sensirion.com/en/environmental-sensors/carbon-dioxide-sensors/carbon-dioxide-sensor-scd4x/ https://www.sensirion.com/en/environmental-sensors/carbon-di... [1]: https://www.sensirion.com/en/about-us/company/pasensr-technology/ https://www.sensirion.com/en/about-us/company/pasensr-techno...