Extech CO200 Desktop Indoor Air Quality CO2 Monitor Checks for Carbon Dioxide (CO2) concentrations with Max/Min value recall function Maintenance free NDIR (non-dispersive infrared) CO2 sensor Indoor Air Quality displayed in ppm with Good (380 to 420ppm), Normal (1000ppm) indication Displays Year, Month, Date, and Time Automatic Baseline Calibration (minimum CO2 level over 7.5 days) or manual calibration in fresh air