Have you ever experienced spikes in the S8's readings? Mine seems to underflow and wraparound every once in a while for no apparent reason or when exposed to fresh air for prolonged periods of time. I tried recalibrating it on the 400ppm baseline a few times but the issue hasn't gone away.

How does the calibration work? I saw comments saying you have to get a canister with a known co2 content and run it through to calibrate. Do these home sensors just assume the outside level is a fixed value and calibrate to that?

It assumes that the sensor is exposed to fresh air every once in a while, and that fresh air is at 400 ppm CO2. It'll basically slowly lower its baseline calibration over several days, and if the value ever drops below 400 ppm it'll increase the baseline calibration relatively rapidly.

It can be disabled via a command IIRC.

Isn't this approach flawed as it look like seasonal lows of CO2 [0] are above 400ppm are are going to continue rising making the alignment drift even further out. Is there any way to adjust the calibration with the expected low CO2 readings?

[0] https://www.co2.earth/seasonal-co2-cycle

In theory yes, but in practice any sensor that's not regularly calibrated against a well known reference (eg. a chamber with 0 ppm CO2) will always be off somewhat.

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.

>Yes the senseair S8 is very reliable. Our professional air quality sensor uses the same CO2 sensor

Thanks!