Is that Co2 sensor reliable? I've been googling sensors lately and CO2 seems to be a mine field. Hard to tell what is a CO2 sensor and what is a VOC/CO2 equivalent sensor...
Only need to keep in mind that the baseline calibration sometimes needs 1-2 weeks to kick in but you can also force a manual calibration.
In general NDIR CO2 sensors are quite reliable but you should stay away from TVOC sensors that give an eCO2 (estimated CO2) value. These are often very off and I would not recommend to use TVOC sensors for CO2 measurement.
It can be disabled via a command IIRC.
This automatic calibration just ensures that the sensor is in the right ballpark, but if a 10-20 ppm absolute difference in CO2 concentration is important for your application, you'll need a better sensor and/or calibration procedure than the automatic background calibration.
Realistically, for home applications, it'll be more like CO2 < 500 ppm, things are good, above 700, might be time to consider opening the windows, above 1000, definitely time to open the windows. For that kind of application, being off by 10-20 ppm doesn't really matter too much.
On a side note, it is pretty disheartening to think that the 400 ppm CO2 background level was accurate a few years ago when the sensors were designed, and that in the span of a few years that background reference is now off.
Thanks!
Electrochemical sensors respond to CO2 and other VOC gases and give you an "eCO2" reading, and you can never be really sure if it measured CO2 or something else, and require frequent calibration. They suck.
NDIR CO2 sensors work by emitting IR light and measuring the CO2 by its IR absorption band, and are much more accurate at singling out only CO2 among other gases.
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
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/se... [2] https://rmtplusstoragesenseair.blob.core.windows.net/docs/pu...
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-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/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.