Robust and reliable air velocity sensor is available in the market and there are several sensing methods that used for measuring air velocity in tunnels. Functionality and reliability in normal situation and in emergency cases are important, and the overall need for maintenance and calibration are also important in choosing the sensing method and technology. Up-to-date signal processing techniques have also an impact on the selection of sensor. There are several types of airflow sensors used in road tunnels; propellers, heat transport sensors, ultrasonic wave sensors and pressure tubes are example of technologies.
The sensors have to measure the airflow velocity and the direction of the airflow, and have to be reliable and robust in harsh tunnel conditions. There are also requirements to keep the functionality during a fire.
A robust sensor mechanism without moving parts that is practically maintenance-free are delivered by the Swiss manufacturer, Schiltknecht Messtechnik. The sensor has been used in long tunnels like St.Gotthard Tunnel in Switzerland. Measurement is based on high-precision differential pressure measurement (Pitot tubes) which results in accurate flow measurement of the air through the tunnel. The data from the separately mounted field transmitters are transferred digitally into a evaluation unit. Pitot tubes are designed in such a way that infiltrating soiling or water cannot interfere with the measurement. Even in an extreme case of fire the measurement is not disturbed and resists hot gases with the temperature of 1200 °C . This can give a safe and continuous control of the fan almost under any operating conditions.