[toasted_flakes]

> it looks at the previous 7 days of data and only if there were at least 5 days where the baselines are within 50ppm of each other, treats the average of those baselines as the fresh air concentration.

Is that documented anywhere? I couldn't figure out why mine is reading garbage, it could be that it didn't see fresh air every day for a full week

[throwaway9d0291]

The main public information is page 13 of the interface description [0]. I got some more specifics from Sensirion directly when I contacted them (the 50ppm minima thing in particular).

Not seeing fresh air for a week isn't enough to throw it off, it needs to not have seen fresh air for a week _and_ the baselines that it has seen have to be within 50ppm of each other.

Have you allowed it to calibrate even once? It needs 7d of uninterrupted power and daily fresh air for the initial calibration.

You can also do forced calibration against a known value using the process on page 14 of [0].

It can also give iffy results if the temperature/humidity values are totally out of whack, which could happen if it's put in an enclosure or something like that. You can set an offset using the instructions on page 15 of [0].

There's also altitude compensation but unless you're doing something really weird that shouldn't affect you so much.

Finally, there's sensor placement [1]. It shouldn't be placed in such a way that there's a lot of air flowing over it directly (e.g. exposed to the air). If it's in front of a fan or something it's going to do weird things.

I'd suggest placing it outside for a few minutes and see what the readings are. If they're far from 400ppm it definitely needs calibrating.

[0]: https://www.sensirion.com/fileadmin/user_upload/customers/se...

[1]: https://www.sensirion.com/fileadmin/user_upload/customers/se...

[petsfed]

"There's also altitude compensation but unless you're doing something really weird that shouldn't affect you so much."

Disagree. I've moved co2 sensors between sea level and 2000m, and measured reference 400ppm gas and seen quite large differences (~150ppm). So it can matter. That's the reason why the SCD30 allows you to set an altitude or pressure.

But its not important enough to constantly feed it pressure data, a once-a-day pressure reading from the NWS would be sufficient.

[toasted_flakes]

Brilliant, thanks. I didn't realise the threshold was 5 days out of a full week, and didn't remember reading about the 50ppm minima.

[lorenzhs]

It's mentioned in the data sheet: https://www.sensirion.com/fileadmin/user_upload/customers/se... and you can disable it: https://www.sensirion.com/fileadmin/user_upload/customers/se...