I just bought one on my local Amazon version by searching "CO2 meter". While true numerical accuracy is "who knows", on rest/full ventilation it shows 400PPM an

I just bought one on my local Amazon version by searching "CO2 meter". While true numerical accuracy is "who knows", on rest/full ventilation it shows 400PPM and blowing at it (even gently) shows the CO2 increases drastically, as expected, so I'm happy with it.