The cheapest one that is not eCO2 (estimated) will do the job of telling you when you’ve not ventilated enough just fine. No need to spend hundreds to thousands on lab-grade tech.

+- 50ppm, or even 100 or 200 is nothing. You will mainly care if the ppm is around 500 or around 1000-2000 (depending on preference).

The devices that have alarms are quite nice. Leave them on a desk and they’ll tell you when to open the window. The ergonomics of how you’ll use it are maybe more important than accuracy when it’s <200ppm off.

Aiegradient is great and they make indoor and outdoor monitors. Fully open source. They are the authors of this article and to a lot of research and advocacy which is great.

TFA Dostmann make an “Airco2ntrol mini” which is USB powered (and you can pull the readings over USB too) and has a nice little red-amber-green light system on the front (as well as the LCD) for easy reference. It’s about £85. It has a dual NDIR sensor (not sure which one exactly) which the article says are usually the most accurate type of consumer-grade sensors.

I’ve had mine for years, very pleased with it… just sits on my desk and lets me know when it’s time to open the windows.

For NDIR sensors, the Aranet4 Home is an often recommended one. I have a couple, and they are great little units. A number of scientists recommended them during the heart of covid as a proxy for covid risk in indoor spaces (obviously, an imperfect proxy, but since there are no cheap, portable devices to measure viral concentrations in the air, you work with what you've got).

I use the Airthings (the $300 one). It detects more than just CO2, has an e-ink display, and has a decent app.

Yes. Look up quingping co2 monitor on Aliexpress, the white one with BW screen. It’s NDIR based, and can last a month on battery and then sync the measurements to the cloud, if you want to leave it somewhere. Around $70.

When I looked a few years ago the Await element was best consumer grade one on the market at a good price. I’ve been happy with it so far.

I like my qingping - works with both - homekit and home assistant.