I finally own a CO2 monitor! After seeing Naomi Wu's recommendation, I chose the Sensirion SCD4X CO2 Gadget. I ordered it from Mouser Electronics 8 days ago and after UPS pretended to card me yesterday, they finally delivered it today (next time I'll try FedEx first and hope for better).
The LED has a few different colour states (green/yellow/red, as well as white for initialising and I think blue for updating settings). The brightness level can be changed in 10% increments from 0 all the way up to 100. Data is stored until logging interval is changed or power is removed.
There seems to be good responsiveness and range. Low value appears close enough to background CO2 levels. But it's the relative changes that matters more in day-to-day use anyway. Either way, I'll see if it "auto calibrates" lower in the coming days.
For giggles, I dipped the CO2 gadget into a bowl that had just fizzed up some bicarb soda + vinegar. But I'm not doing this again because I had to delete the day's data in order for the graph to show a more useful y-axis range. The app is quite good actually - you can tell it to delete data older than x days, etc; but since today is day 0, I deleted everything to fix the graph's scale.
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