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
  • Electrochemical gas analyser with automatic calibration
  • Visibility sensor based on scattered light method
  • Operating unit
  • Zero and reference gas
  • Intake fan
  • All components above installed in an enclosure ready for operation
  • Intake system consisting of intake point to be mounted in the tunnel and intake tube system to connect with the monitoring system in the enclosure
  • Specifically designed for application in tunnels which require high availability
  • Highest availability of tunnel as all important components of the monitoring system are located in accessible rooms
    (no need to close the tunnel for traffic)
  • No electronic components in the tunnel’s driving area
  • Robust, corrosion-free intake cowls with protection against ingress of water
  • Automatic calibration and high accuracy of measurement
  • Flexible integration into the tunnel control system
Technical Specifications
CO measurement
Measuring method Electrochemical gas analysis
Measured value Gas concentration in ppm CO
Measuring range e.g. 0 .. 200 ppm CO (according RVS 09.02.22)
Other gases and measuring ranges on request
Accuracy ± 2 % of end of measuring range
T90 response time 45 s
Visibility measurement
Measuring method 30° scattered light intensity
Measured value 0 .. 100 PLA
Measuring range 0 .. 0,015 E/m
Resolution < ± 0,25 % of end of measuring range
Air flow 25 to 30 l/min
Temperature -20 °C to +50 °C
Power supply
Operating voltage 230 VAC, 50 Hz
Power consumption Depends on individual system
(Number of measuring points, length of intake system)
Analogue outputs 2 x 4 – 20 mA (CO and visibility) per measuring point
Digital interface RS-485 MODBUS RTU
Isolated contacts 1 x Operation/Fault indication
1 x Calibration active
Electrical standards 2006/95/EC Low Voltage Directive (LVD)
2004/108/EC Electromagnetic compatibility(EMC)
IEC 61326-1:2012
IEC 61010-1:2010
Road safety standards AT: RVS 09.02.22
DE: RABT 2006
CH: ASTRA RL 13001, Fachhandbuch BSA
Gas measurement AT: ÖNORM M9418, ÖNORM M9419
DE: VDI 2053


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.