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 SCM4 to ambient quickly with the zero button, audio and visual alarms with mute, bright blue, backlit dual-display with max and real-time measurements displayed, magnetic hanger for hands-free operation, auto power-off (APO).