because my google-fu was not good enough to find those.... Regardless, I would be curious on the same questions about real world accuracy on those as well.

I have two of them, one inside and one outside. They both go down to roughly 400 ppm and the indoor one goes very high when it's all cooped up inside. The outdoor one fluctuates since I'm in an urban area. I believe they are accurate to within about 10%.

During the smoke event, they combined with my indoor/outdoor pm2.5 sensors have been extremely useful.

They should go down to 400ppm because the periodic autocalibration which is on by default assumes the lowest level seen in 24 hours is 400ppm.

You should be using the newer version, MH-Z19B, not MH-Z19.

The protocol is compatible though.

Also note they come in up-to-3000ppm and up-to-5000ppm variants. Be sure to get the latter.

In case it helps anyone: a very fast way to get it working and get graphs is to connect it to a Raspberry Pi running HomeAssistant. Takes a few clicks and 3 lines of config.

For portability, you can also hook up to an ESP8266/ESP32 and configure to talk to HomeAssistant via ESPhome: https://esphome.io/components/sensor/mhz19.html