Air flow measurement by different 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 ultrasonic 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 measuring system – ultrasonic

  • Precise ultrasonic air flow measuring system based on different transition times of ultrasonic pulses
  • Measurement of average airflow 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 relays or via RS‑485 MODBUS RTU
  • Visible LEDs to indicate sensor status
  • Optional, remote touch operation unit to display measured values and to modify parameters
  • Ultrasound membranes are no expendables
  • Two sensors (master & slave) with mounting clamps mounted below the tunnel ceiling in an angle of30° to 60° to the tunnel axis
  • Terminal box for master to connect the power supply and the connections to the tunnel control system and the slave
  • Terminal box for slave
  • Optional remote touch operating unit
  • Specifically developed for application in tunnels
  • Relay outputs freely configurable
  • No control unit required
  • Easy configuration
  • Corrosion resistant against aggressive tunnel atmosphere
  • Sensors can be replaced quickly with no tools and no re-alignment required
  • Tested in railway tunnels up to a speed of 350 km/h
  • No expendable parts (e.g. ultrasound membrane)
  • Extremely low maintenance requirements
Technische Daten
Air flow measurement
Measuring method Determination of direction dependant differential transition times of ultrasonic pulses
Measured values Air speed
Air flow rate
Direction of air flow
Air temperature
Measuring range -40 to 40 m/s
Resolution 0.1 m/s, accuracy dependent upon measuring distance, flow profile, installation, typically <±0.2 m/s at 3 m/s
Response time > 1s … 180 s, configurable
Measuring distance 1.2 – 25 m
Alignment 30 – 60° to tunnel longitudinal axis, typically 45°
Analogue outputs 1 x 4-20 mA, 400 Ω, isolated(can be assigned any measured value and output range)
Output range configurable, typically: -20 to 20 m/s
Contacts 1 x fault indication (NC)
1 x direction of air flow (NC)
max. 48 V / 0,5 A
Digital interface 1x RS 485 MODBUS RTU to control system, bidirectional
1x Intercom RS 485 master-slave
Model D-FL 220T M
Operating voltage 24 VDC ± 10 %
Current consumption approx. 1 A
Service interface USB 1.1
Temperature range -25 to 55 °C
Protection class IP 67
Dimensions 270 x 130 x 95 mm
Weight approx. 2,2 kg
Material Stainless steel 1.4571/316Ti, Polyamide, ca. RAL5017, Flammability rating: B1 (UL 94 V0)
Terminal Box
Model D-TB 100T
Operating voltage 90 to 264 VAC, 48 to 62 Hz
Power consumption approx. 30 VA (for master, for slave only optionally)
Protection class IP 66
Dimensions 160 x 160 x 110 mm
Material Polycarbonate, RAL 7035, Flammability rating: B1 (UL 94 V0)
Electrical standards 2006/95/EC Low voltage directive (LVD)
2004/108/EC Electromagnetic compatibility (EMC)
IEC 61326-1:2012
IEC 61010-1:2010
Tunnel safety standards AT: RVS 09.02.22 2010 & ASFiNAG PlaPB
DE: RABT 2006


Product info DE (PDF, 340 kB)
Product info EN (PDF, 340 kB)


System setup


Air flow sensor A (master)

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


Air flow sensor B (slave)
The slave sensor is mounted on the tunnel wall and aligned with the master sensor.


Terminal box master

The terminal box for the master connects the system to the power supply, the slave and the tunnel control system.


Terminal box slave

The terminal box of the slave connects the slave to the master sensor.


Connection to tunnel control system

The tunnel control system can be connected via the analogue outputs and relay contacts or via the RS-485 MODBUS RTU interface.


Touch operating unit (optional)

A touch operating unit display the measured values and status information. The unit can also be used to configure the sensors. It can connect up to 3 systems.