It sounds like you are looking for something like the air monitor I have been developing over the last two years. It is about the size of a deck of cards and fits in a shirt pocket. The Lithium Polymer battery powers it for 12-16 hours and is recharged with USB-C.
It uses a high-accuracy CO2 sensor (±(40 ppm + 5 %) and you can manually calibrate it to 420 PPM outdoor air. A key innovation is reporting via a dynanmic QR code on its 128x64 OLED display. Just point your phone camera at the QR code and it takes you to a site showing a report with graphs and data tables of recent measurements. There is no app to install or web site to log into. The web page is context-free and all of the measurement data is in the query string. It is nearly ready for production and likely to sell in the $150 price range.
That sounds neat, and I love the idea of displaying a QR code, even though I'm not sure how practical it would be. Is it there to allow viewing more data than would otherwise fit on the display in human-readable form?
For my use case, I could hack a system with a spare phone reading the code on a timer and uploading the results, but this feels like an overkill. How much more complex would it have to be to use that same USB-C charging port for data and power delivery? I'm imagining attaching an ESP32 board to it, which would be powered by your device, and would occasionally wake up, read a CSV file or a data stream over that USB connection, upload it to my home server, and go back into deep sleep. With an option to turn the OLED display off, I imagine it could even extend the battery life.
(Please don't take this as criticism - I fully understand my use case is most likely out of scope of your project, but I'm wondering how close it is to be supported.)
OLED is the cheapest display with enough resolution.
No Bluetooth. A previous version had it, but the development effort was much higher, given it then needed an App also. I wanted something simple for say a school teacher to report values in their classroom.
Data is not stored long term currently. Just since last power on.
The current readings are on the display. The QR code encodes 40 previous values of CO2 plus 40 of the VOC index. I have a debug mode that can display the whole URL, but it is so long that it would be impractical to type it in instead of the simple QR scan.
It uses a high-accuracy CO2 sensor (±(40 ppm + 5 %) and you can manually calibrate it to 420 PPM outdoor air. A key innovation is reporting via a dynanmic QR code on its 128x64 OLED display. Just point your phone camera at the QR code and it takes you to a site showing a report with graphs and data tables of recent measurements. There is no app to install or web site to log into. The web page is context-free and all of the measurement data is in the query string. It is nearly ready for production and likely to sell in the $150 price range.