In-situ visibility monitoring by scattered light

Visibility monitoring is 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.

Visibility is stated in the form of an extinction coefficient that corresponds to the light attenuation caused by air pollution.

Air is extracted from the tunnel by the integrated suction pump and fed into the visibility sensor. The sensor then detects the intensity of light scattered at an angle of 30° by this sample 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/VIS-SL Visibility monitoring system – scattered light, in-situ

  • Mounted directly in the tunnel’s driving area (in-situ)
  • Extraction of air to be analysed by integrated suction pump
  • Connection either to
    • Control unit for 1 sensor
    • Multichannel control unit for up to 8 sensors
    • Field bus system via bus coupler
  • Visibility measurement using scattered light method
  • Optional heater to compensate fog
  • Operating unit to display measured values and to provide signal outputs to the tunnel control system
  • Sensor and operating unit IP65
  • Sensor housing in stainless steel 1.4435
  • Delivered ready for operation
  • Sensor to be mounted directly in the tunnel’s driving area (either on wall or ceiling)
  • Terminal box in the tunnel
  • Connection to control unit (power supply through system cable from control unit) or to field bus system via bus coupler (separate power supply of sensor necessary)
  • Optional t/ECS gas sensor connected to visibility sensor
  • Specifically designed for application in tunnels
  • Single sensor instead of transmitter/receiver pair requiring exact alignment
  • Effective fog compensation by optional heater
  • Easy recalibration by recalibration plug
  • Special air flow design keeps the optics clean and prevents drift
  • Low maintenance requirements, stable, accurate
  • Flexible integration into tunnel control system
  • Optional combined monitoring for toxic gases
Technical Specifications
Visibility measurement
Measuring method 30° scattered light intensity
Measured value 0 to 100 PLA
Measuring range 0 to 0,015 E/m
Resolution < ± 0,25 % of end of measuring range
Air flow 5 l/min
Temperature range – 20 °C bis + 50 °C
Model VisGuard in-situ
Power supply 24 VDC via system cable to control unit
Material Stainless steel 1.4435
Protection class IP 65
Dimensions Ø 209 x 367 mm
Weight 6,5 kg
Heating (optional fog compensation) 230 VAC; 25 W
Digital interface RS-485 MODBUS RTU
Control unit for 1 sensor
Power supply 85 to 264 V / 47 to 440 Hz
Power consumption 20 W
Material Plastic (ABS)
Protection class IP 65
Dimensions 200 x 157 x 96 mm
220 x 142 x 91 mm (Variant „robust“)
Weight 1,5 kg
Analogue output 0 / 4 – 20 mA
Digital interface RS-485 MODBUS RTU
Contacts 2 configurable isolated contacts (250 V, 4 A)
Electrical standards 2006/95/EC Low Voltage Directive (LVD)
2004/108/EC Electromagnetic compatibility (EMC)
IEC 61000-6:2001
IEC 61010-1:2001
Tunnel safety standards AT: RVS 09.02.22
DE: RABT 2006
CH: ASTRA RL 13001, Fachhandbuch BSA


Product info DE (PDF, 343 kB)
Product info EN (PDF, 343 kB)


System setup


Visibility sensor VisGuard

The visibility sensor is typically mounted on the tunnel wall in a height of 3.5 m above the carriageway.


Terminal box

The terminal box is used to connect the sensor to the power supply and the operating unit.


Operating unit SIREL

The operating unit SIREL supplies the visibiliy sensor VisGuard with power, displays the measured value(s) and provides analogue output(s) and relay contacts.


Niche or operating room
The operating unit can be installed in a niche or an operating room. An installation in the tunnel’s driving area is possible by using a different operating unit.


Connection to tunnel control system

The tunnel control system can be connected via the analogue output(s) and relay contacts or via RS-485 MODBUS RTU.


Optional gas sensor

An optional t/ECS gas sensor can be directly connected to the visibility sensor VisGuard.