The SCM4 Carbon Monoxide (CO) Detector has a very fast reaction time, and measures CO in parts per million (ppm). Its electrochemical sensor is fast enough for walk-around tests. Features and benefits include: Field-replaceable CO sensor; Advanced three-pin sensor measures 0 to 1000 ppm CO; Calibrate / Zero SCM3 to ambient quickly with the ZERO button; Audio and visual alarms w/mute; Bright-blue, backlit dual-display with MAX and real-time measurements displayed; Magnetic hanger for hands-free operation; Auto Power-Off (APO); Use optional pump (AOXP2) for remote / flue gas testing.