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Comment by rogers18445 | Hacker News Reader

Sensirion CO2 Gadget is good with one of the best [small, reasonably priced] sensors (SCD41). Has BLE interface (with android/ios app). Only shows co2. https://sensirion.com/products/catalog/SCD4x-CO2-Gadget/

It does use auto calibration, which means you need to bring it outside for a few minutes every once in a while (7 days IIRC). Not doing it will not break it, but the readings will drift downward (sensor assumes that the lowest reading it gets in a period of time must be 400 ppm) - and will be fixed in a few minutes outside (few minutes of <400 ppm sets lowest reading to 400).

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I’m fond of the CO2 Gadget too, but the autocalibration function takes 7 days to kick in, during which it must be plugged in continuously. This makes it impractical for use as a travel CO2 sensor when moving between two locations at different elevations. E.G., I had my Gadget calibrated for CDMX (2050m), then travelled to SF (50m) where the Gadget read 800ppm outdoors. I left the Gadget plugged in outside for four days, but it never calibrated before I left. I corresponded with an engineer at Sensirion who said it may be possible to add manual calibration via software update, and created a ticket for their product team. I hope they implement that.

That's interesting. For regular use however this may not be a big deal, auto calibration has a secondary mechanism by which it will recalibrate in just a few minutes if ppm < 400.

So after 8 days plugged in, the sensor is sure to be calibrated incorrectly to inside air, then you take it outside and it recalibrates again to correct values within a few minutes?

I highly recommend Dual-NDIR sensors that bring their own volume of calibration gas. Auto calibration is not practical in any serious indoor monitoring use case.

How would that work? Having one sensor sealed with reference gas does nothing to help the second sensor. Is there actually a dual sensor which shuttles the gas between them? Seems complicated.

it's a 2nd sealed chamber and shares some sensor parts. Some parts are different, so it's not perfect of course, but much better as the requirement of regular manual calibration. Sensirion SCD30 for example, where you can even see the Y shape of the gas chambers.

SCD30 also has autocalibration with outside air though?

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