BW Technologies GA24XT-X GasAlertClip Extreme 2-Year Detector, O2, 0-30.0 Percent Measuring Range, 19.5 to 23.5 Percent Alarm Setpoint Single-gas detector for surveying levels of oxygen (O2&lt;/SUB) in natural and industrial environments, and in other personal safety, training, research, and ambient air monitoring applications Electrochemical cell sensor monitors ambient oxygen levels continuously for up to two years without requiring unit calibration, recharging, or battery replacement Audio, visual, and vibration alarms alert simultaneously when oxygen gas reaches factory-set low or high levels LCD screen shows self-test status, high and low alarm set-points, maximum exposure, gas type, alarm level (low or high), detector life remaining, and time since last maximum exposure Back-mounted alligator clip allows wearing the unit on a belt, and a concussion-resistant boot helps protect against damage