Extech CO250 Digital Air Quality CO2 Meter with Datalogging is a handheld meter for reading Carbon Dioxide (C02), Temperature, Humidity, Dew Point, and Wet Bulb. The Extech CO250 also calculates statistical weighted averages of TWA (8 hour time weighted average) and STEL (15 minute short term exposure limit), features easy-to-use datalogging, and has a backlight for working in poorly lit areas.