The tunnel flow velocity measuring device for long-term operation The FLOWSIC200 is used for the contact-free and accurate measurement of flow velocity and flow direction inside tunnels or exhaust ventilation ducts. Ultrasonic technology provides the cross-sectional mean average value of the flow velocity. This device is used for measurement necessary for optimal ventilation control and reducing running costs to a minimum when air flow is dictated by climatic conditions or by traffic.In case of fires in tunnels the accurate, reliable and representative measurement of air velocity and direction across the tunnel is mandatory. This is the only way to check reliably the smoke propagation and to achieve the necessary information for optimal control of the ventilation system.