The Triplett GSM400 Indoor Air Quality CO2 Meter measures CO2, Temperature, Humidity, Wet Bulb, and Dew Point and can be used to measure air quality in office buildings, schools, exhibition halls, shopping malls, meeting rooms, fitness centers, restaurants and other public places. The meter incorporates NDIR (non-dispersive infrared) waveguide technology and simultaneously displays CO2, Temperature and Relative Humidity on its backlit triple LCD display. It features TWA (Time Weighted Average - 8 hours) and STEL (Short Term Exposure Limit - 15 minute weighted average) measurement with a user-adjustable CO2 alarm level. Additional features include Min/Max to record the highest and lowest readings, Data Hold to freeze the reading on the display, and Auto Power Off with disable to conserve battery life. °F/°C selectable. Includes (4) AA batteries and carrying case..