Air flow measurement based on difference of transition times of ultrasonic pulses

The air flow monitoring system measures the air flow based on different transmission times of ultrasonic pulses sent in one or the other direction. This cross section measurement delivers the average air speed more reliable and meaningful than methods that measure only one or two points on the tunnel wall.

Two sensors are mounted below the tunnel ceiling, cross section with an angle of 30° to 60° (usually 45°) such that a vector component of the air flow overlaps the direction of the ultrasonic sound pulses exchanged by the two sensors.

Air flow in the tunnel influences the transition time of the pulses between sender and receiver. Based on the difference in transition times of ultra-sonic pulses sent in one or the other direction the measured values are calculated. Measured values are air speed, air flow rate, direction of air flow and air temperature.

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t/FL-US Air flow monitoring system – Ultrasonic cross-section

  • Accurate ultrasonic air flow measuring system based on different transition times of ultrasonic pulses
  • Measurement of average air flow over the whole tunnel cross section
  • Sensor setup via service interface
  • • Suitable for tunnel cross sections up to 18 m (Measuring path up to 25 m)
  • Connection to tunnel control system either via analogue output and relaysor via RS-485 MODBUS RTU
  • Visible LEDs to indicate sensor status
  • Corrosion-resistant non-wearing polyamide membranes
  • Optional, remote touch operating unit to display measured values and to modify parameters


Data sheet DE (PDF)
Data sheet EN (PDF)


System setup


Air flow sensor A (Master)

The master sensor A is mounted on the tunnel wall and aligned with the slave sensor B.


Air flow sensor B (Slave)

The slave sensor B is mounted on the tunnel wall and aligned with the master sensor A.


Terminal box master

The terminal box master is used for power supply and connections to the slave sensor B and the tunnel control system.


Terminal box slave

The terminal box slave connects to the terminal box master.


Connection to tunnel control system

Connection to tunnel control system can be implemented by analogue outputs and relay contacts and/or by MODBUS RTU.


Touch operating unit (optional)

The touch operating unit displays measured values and status. It can also be used to configure the system. One operating unit can connect up to 3 air flow monitoring systems.