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Comment by fotta | Hacker News Reader

I have an SCD30 which is a dual-channel NDIR sensor and am pretty happy with it. Once a year I’ll take it outside to recalibrate it and it’s never drifted more than a few ppm. I make sure to turn off the auto-calibration and manually calibrate it because auto-calibration can lead to weird behavior if you don’t know what it’s doing (it re-baselines the lowest observed ppm in a time window as 400ppm).

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FWIW, Sensirion's AEs told me they have empirical data showing that CO2 drops to outdoor levels at least once a week even in closed environments. Based on that they recommend using the auto calibration.

Anecdotally, I see my CO2 levels hover around 600ppm inside even over night.

I think that really depends on the environment it’s deployed in. I used to live in a poorly ventilated former warehouse apartment with a mini split for HVAC and at night my levels were around 700. I live in a much better ventilated apartment now and it’s made a noticeable difference in QoL.

700 is great, I see spikes up to 5000 in one of our rooms.

levels in the range 400-1000 are considered good quality

700 is pretty good, what are you on about?

If your sensor recalibratws to a baseline of 400 each day and you have a baseline of 700 that is a drift of 300 ppm per day or ca 100 000 ppm per year.

You see why this is a problem?

It's not going to drift by 300ppm per day - it'll pick the lowest value it saw and assume it was 400ppm. If that happens to be 700ppm every day, then it'll be wrong by 300ppm every day.

There are many cases where the level does not drop to ambient levels, e.g. offices in basements, low-energy houses etc.

We even saw schools where the classrooms were not able to drop to ambient levels over the whole unoccupied weekend because the ppm levels on Friday afternoon were beyond 3000ppm.

The automatic baseline calibration period of some of the NDIR CO2 sensors can actually be adjusted through the software. I believe the Aranet allows that and also we do have that feature on the AirGradient dashboard. So you can switch it to a much longer period or switch it off completely.

It always comes down to how well you can ventilate. If you can open windows / doors to different sides of a house / building there is usually some natural air flow, and the stronger it is, the faster it will bring CO2 levels down. In the apartment here it can be less than 5 minutes to have CO2 levels to ambient levels, but it also takes longer if the air is "standing" even outside. For that you will need active ventilation then, clearly, same for having windows to the same side of the building only etc.

There is also another thing: In multi-tenant buildings you may have others airing out their apartments which means you will not get ambient levels, but elevated ones due to added CO2 from the other apartment(s). For example, I see >500 ppm CO2 then. Also, even the ambient levels are not always the same; I have seen values between 416 and 432 ppm depending on the day, wind, and other factors (living in a larger city).

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