The Extech CO220 air quality CO2 monitor measures carbon dioxide, air temperature, and humidity. The CO220 also calculates dew point and wet bulb. Use this meter in conference rooms, schools, greenhouses, factories, hotels, hospitals, and any place where high levels of carbon dioxide are generated. It calculates statistical weighted averages of TWA (8 hour Time Weighted Average) and STEL (15 minute Short Term Exposure Limit). This meter uses a maintenance free NDIR (non-dispersive infrared) CO2 sensor. It can manually store/recall 99 readings.