[throwaway9d0291]

For CO2, which I've researched the most out of the various things that interest me, I'd want a Senseair K30 or S8 Commercial, an Amphenol Telaire T6613 or T6713, or a CozIR-A or CozIR-LP sensor.

For low cost, I'd go with the Senseair S8 Residential. It's not fantastically accurate but it's still a "real" sensor and you can get it for $20 from China.

For other aspects of air quality, I haven't finished researching the options yet.

[joshefin]

Thanks for sharing. I wanted to get Winsen MH-Z19B, but Senseair S8 (residential) looks like a better, more reliable option, for a DIY project. One comparison: https://www.letscontrolit.com/forum/viewtopic.php?t=3736&sta...

[throwaway9d0291]

If you'd asked me a few days ago I'd have said that the MH-Z19B is okay on a budget but now that I know you can get an S8-0053 for $20, I don't think I can recommend it.

If it's difficult for you to get the cheap S8s for some reason though, per that thread it looks like an MH-Z19B with calibration off is a decent choice.