Review on Klein Tools ET110 CO Meter: Carbon Monoxide Tester, Detector & Exposure Limit Alarm - Includes 4 x AAA Batteries & Carry Pouch by Riley Skeet

I used this tool to test a ventless gas fireplace to make sure it burned clean. It provided real-time CO readings down to 1ppm and updated instantly to show me the fireplace was mostly burning clean with a random 40ppm CO surge. You should not trust carbon monoxide detectors or detectors. You need a meter like this to be sure. Carbon monoxide alarms do not have certified ratings for what level of carbon monoxide they will detect or sound at. Most need to be exposed to 40ppm CO for more than 10 hours before an alarm goes off and see nothing below. Even at 200ppm, they typically require an hour of constant exposure to activate. They can save you from a lethal dose of carbon monoxide, but they won't protect you from prolonged exposure to low levels of carbon monoxide, which are still very dangerous! Many manufacturers of carbon monoxide detectors do not know or specify what levels their device will detect. Instead, I recommend using a gauge like this one.