Visibility measuring system according to the transmission principle, specially developed for the requirements in tunnels. The measuring range for the analog signal is freely assignable.




In-situ visibility monitoring by optical transmission

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.

The monitoring system consists of a light emitting and receiving sensor and an optical reflector. The sensor determines the light transmission along a 10 m measuring path. Visibility and extinction coefficient are calculated based on the determined transmission.



  • Visibility measurement by optical transmission method
  • Reliable measurement along a path of 10 m parallel to the tunnel axis
  • Parametrisation of the sensor via Modbus RTU
  • External cable length of 1000 m to tunnel control system
  • Signal output either via analogue and relay outputs or RS-485 with MODBUS RTU protocol
  • Visible LEDs show sensor status



  • Sensor mounted on tunnel wall parallel to tunnel axis aligned with reflector
  • Reflector to reflect the light emitted by the sensor mounted on the tunnel wall in a distance of 10 m from the sensor
  • Terminal box to connect the sensor, power and to connect to the tunnel control system



  • Specifically developed for application in tunnels
  • No separate control unit required
  • Influence of contamination of optics on the measuring results is compensated
  • Easy configuration
  • Corrosion resistant against aggressive tunnel atmosphere
  • Sensor and reflector can be replaced quickly, no tools, no re-alignment required
  • No expendable parts
  • Extremely low maintenance requirements
  • Optional combination with CO sensor


Technical Specifications:

Measuring method Optical transmission of visible light passing the measuring path twice
Measured values Dust concentration in mg/m³
Opacity in %
Visibility (Extinction coefficient VIS k)
Measuring range 0 to 0,015 E/m (configurable up to 0,15 E/m)
0 to 97,5 % opacity
Response time > 2s to 180 s, configurable
Light source LED green, 530 nm
Measuring path Distance Sensor – Reflector 9,5 to 11 m
Measuring path length 19 to 22 m
Orientation Parallel to tunnel axis
Analogue outputs 1 x 4-20 mA, 400 Ω, isolated
can be assigned any measured value and output range
Output range configurable, typical: 0 to 0,015 E/m
Contacts 2 x Status (NO)
Max. 48 V / 0,5 A
Digital interface 1 x RS-485 MODBUS RTU
Model D-R 220T M
Operating voltage 24 VDC ± 10 %
Current consumption Ca. 0,4 A
Service interface USB 1.1
Temperature range -20 to 50 °C
Protection class IP 65
Dimensions 150 x 220 x 140 mm
Weight Ca. 2,7 kg
Material Polyamide, approx. RAL5017,
Flammability rating: B1 (UL 94 V0)
Model D-R 220T R3
Protection class IP 65
Dimensions 150 x 110 x 140 mm
Weight Ca. 1,6 kg
Material Polyamide, approx. RAL5017,
Flammability rating: B1 (UL 94 V0)
Model D-R 220T T
Dimensions 280 x 110 x 100 mm
Weight Ca. 3 kg
Material Stainless steel 1.4571 / 316Ti
Model D-TB 100T
Operating voltage 90 bis 264 VAC, 48 bis 62 Hz
Power consumption ca. 30 VA
Protection class IP 66
Dimensions 160 x 160 x 110 mm
Material approx. 1,5 kg
Model Polycarbonate, RAL 7035,
Flammability rating: B1 (UL 94 V0)
Electrical standards 2006/95/EC Low voltage directive (LVD)
2004/108/EC Electromagnetic compatibility (EMC)
EN 61326-1
Tunnel safety standards AT: RVS 09.02.22
DE: RABT 2006
CH: ASTRA RL 13001, Fachhandbuch BSA





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