LED bargraph changes color as ozone levels increase: green indicates a safe level, yellow is caution level, and red is the danger level. Features a heated metal oxide semiconductor (HMOS) sensor for excellent sensitivity and quick response time. Also features an audio alarm and relay output that are triggered at 0.1 ppm (danger level) and a 0 to 3 VAC recorder output. Order optional calibrator to verify that your ozone detectors are conforming to their origianl calibration.