Extractive visibility and gas monitoring

Visibility and gas monitoring are used to control the tunnel ventilation at normal operation. If and with how much power artificial ventilation by jet fans is operated depends on the measured visibility.

The extractive monitoring system extracts air from the tunnel through an intake system and feeds it into the visibility sensor and gas analyser.

The electrochemical CO gas analyser measures the concentration of CO gas in the inducted air and periodically performs an automatic calibration with zero and reference gas to ensure long term stability and accuracy.

The visibility sensor measures the intensity of light scattered at an angle of 30° by the inducted air and sets it into relation with the light passing through. The scattered light intensity is then multiplied by a factor to obtain the extinction coefficient.

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t/EXT Extractive visibility and gas monitoring system

  • Extraction of air to be analysed from the tunnel’s driving area through an intake system
  • Sensors located in an accessible room outside the driving area, e.g. in a niche, a crosscut, an operating room inside or outside the tunnel
  • Control and evaluation unit to display measured values and with signal outputs to the tunnel control system
  • Electrochemical gas analyser with automatic calibration of zero and reference value
  • Visibility measurement using scattered light method with integrated fog compensation
  • Intake tube length up to 400 m
  • Monitoring of up to 4 measuring points through one monitoring system
  • Delivered in an enclosure with all wiring, pumps and tubes ready for operation incl. zero and reference gas


Product info DE (PDF, 316 kB)
Product info EN (PDF, 390 kB)


System setup


Intake point

The intake point is mounted on the tunnel wall in a height of approximately 1.5 m above the carriageway. It protects the intake system against foreign objects, spray and splash water.


Extractive monitoring system in enclosure

The enclosure located in a niche or operating accomodates the main system components. It is accessible at all times without the need to close the tunnel for traffic.


Connection to tunnel control system

The connection to the tunnel control system can be established via analogue outputs and relay contacts or via field busses. Interfaces are implemented project specific.


Several intake points

An extractive system can analyse air from up to 4 intake points.


Intake tube

The intake tube connects the intake point with the monitoring system accomodated in an enclosure outside the tunnels driving area.