The Extech CO220 air quality CO2 meter measures carbon dioxide, air temperature and humidity. In addition, the CO220 calculates humidity and dew point. For use in conference rooms, schools, greenhouses, factories, hotels, hospitals and anywhere high levels of carbon dioxide are generated. Calculates statistically weighted average values for TWA (time weighted average over 8 hours) and STEL (Short Term Exposure Limit - limit value for short-term exposure over 15 minutes). Maintenance-free NDIR (Non-Dispersive InfraRed) CO2 sensor. Manual storage/recall of 99 measured values.