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This is not unique to the SCD40 series. This applies also to NDIR based sensors. They also need to 'see' outside air on a weekly basis.

It's just the way CO2 sensing tech works.

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Not necessarily. The SCD series can have auto calibration turned off. You take it outside once and baseline that to 400ppm. The drift on the sensor is rated very low (+-50 ppm over the sensor’s lifetime). This is what I do with my SCD30.

Interesting, I didn't know that, I've only used them in auto mode. I also have SCD30s, the 40s are a pain in the ** to solder and they were hard to get because they are new.

I wonder why they don't just calibrate them off the factory line then? I mean, 50ppm is not much. In auto mode you won't get much better accuracy than that because you don't always air the house well enough to get down to 400 (let alone in a city's CO2 bubble).

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