Measures Air Velocity, Air Flow, Relative Humidity, Wet Bulb, Dew Point and Carbon Dioxide The Extech AN320-NISTL 4" (10cm) Large Vane Thermo-Anemometer series allows for more precise readings on larger size ducts. Each meter simultaneously displays Airflow or Air Velocity and Temperature. Choose models that also measure Air Volume, Humidity & Carbon Dioxide (CO2).