Air flow measurement by differential pressure

The air flow monitoring system measures the longitudinal air flow at 2 points in a tunnel cross section. The method allows to optionally remove all electronic components from the tunnels driving area to maintain a measurement also under high temperatures.
Two differential pressure pitot tubes are mounted on opposite tunnel walls. Measuring the air flow at 2 points allows to calculate a representative average air flow over the tunnel cross section.
An air flow in the tunnel generates a dynamic pressure on the side of the pitot tube facing the flow of air while on the other side only the static pressure is present. The measured differential pressure is used to calculate the air velocity under consideration of the also measured temperature. The values of the 2 measuring points are then averaged.
Following outputs are available: air velocity, direction of air flow, air tem-perature.

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t/FL-DP Air flow monitoring system – Differential pressure

  • Robust air flow monitoring system based on high-precision differential pressure measurement
  • 2-point mesh
  • Automatic zeroing of differential pressure transducers
  • Touch evaluation and control unit to display measured values, status and set parameters
  • Connection to tunnel control system by RS-485 MODBUS RTU, optionally by analogue outputs and relay contacts
  • Two differential pressure pitot tubes mounted on opposite tunnel walls; pneumatically connected to the terminal boxes
  • Terminal boxes with differential pressure transducers to be mounted in the tunnel or remotely outside of the driving area (high temperature variant); either
    • 1 terminal box: pneumatic connection to opposite side
    • 2 terminal boxes: electric connection to opposite side
  • Touch evaluation and control unit to be mounted in closet outside the tunnel’s driving area (niche, cross-cut, etc.); Connection to terminal box by RS-485 MODBUS RTU; Connection to tunnel control system by RS-485 MODBUS RTU
  • Optional analogue outputs and relay contacts by DIN rail mounted devices
  • Specifically designed for tunnels
  • Easy installation and setup
  • Long term stability by automatic zeroing
  • Measurement possible at high temperatures
  • Corrosion resistant against aggressive tunnel atmosphere
  • Minimised spare requirements
  • Extremely low maintenance requirements
Technical Specifications
Air flow measurement
Measuring method Determination of average air flow through 2 point mesh measurement by differential pressure pitot tubes
Measured values Air velocity
Air flow direction
Measuring range -20 .. 20 m/s
Resolution 0.1 m/s, Accuracy depending on: installation, flow profile, typ. <±0.2 m/s at 3 m/s
Response time 1 .. 180 s,configurable
Alignment Differential pressure pitot tubes mounted on opposite tunnel walls
Digital interface 1 x RS-485 MODBUS RTU to tunnel control system
(other interfaces on request)
Analogue outputs (optional) up to 4 x 4-20 mA, 500 Ω
Relay contacts (optional) 1 x fault indication (NC)
1 x direction of air flow (NC)
max. 2 A @ 30 VDC or 2 A @ 250 VAC
Pitot tube
Temperature range -30 .. 400 °C
Dimensions 609 x h x 400 mm (w x h x d)
Weight 5.5 kg
Material Stainless steel 1.4571/316Ti or 1.4404/316L
Terminal box
Operating voltage 85 .. 264 VAC, 47 .. 63 Hz
IP rating IP65
Temperature range -30 .. 80 °C
Dimensions 360 x 250 x 173 mm
Weight 11 kg
Material Solid rubber self-extinguishing, halogen and silicon free
or Stainless steel 1.4404 or 1.4571 (at surcharge)
Electrical standards 2006/95/EC Low voltage directive (LVD)
2004/108/EC Electromagnetic compatibility (EMC)
Tunnel safety standards AT: RVS 09.02.22 2010 & ASFiNAG PlaPB
DE: RABT 2006


Produktinfo DE (PDF, 579 kB)
Produktinfo EN (PDF, 579 kB)


System setup


Pitot tube A

Pitot tube A is mounted on the right lane in positive direction of flow.


Pitot tube B

Pitot tube B is mounted on the left lane in positive direction of flow.


Terminal box

The pitot tube are pneumatically connected to the terminal boxes by hoses. The terminal box accomodates the differential pressure transducers and a temperature measurement. It is connected to the evaluation and control unit by a RS-485 bus cable.


Evaluation and control unit

The evaluation and control unit collects the measured values of the differential pressure transmitters and the temperature measurement and processes the. The measured values are provided via RS-485 MODBUS RTU interface or optionally via analogue outputs and relay contacts.