FWIW I have had this cheap CO2meter for a few years and it seems to work just fine. https://www.co2meter.com/products/co2mini-co2-indoor-air-quality-monitor htt

FWIW I have had this cheap CO2meter for a few years and it seems to work just fine. https://www.co2meter.com/products/co2mini-co2-indoor-air-quality-monitor https://www.co2meter.com/products/co2mini-co2-indoor-air-qua... Windows open? Drops into 600ppm range. GF and I at home with closed windows and HVAC off, usually climbs into 1200pm range. Gas oven on? Easy to get over 3000ppm if there's no ventilation on. > But outside of hotels, they're rare. I haven't made it a point to look in all of them, but I've seen quite a few in microbreweries in the room they do their brewing. Not sure if required by law, but it's generally a good idea.