The VNOX tunnel monitor uses differential optical absorption to measure nitrogen dioxide (NO2) in tunnel atmospheres and makes a visible opacity measurement to determine the visibility within the tunnel. These measurements can be used as part of an air quality management system for ventilation control and/or secondary smoke detection within a traffic tunnel, rail tunnel or other confined space.


  • Direct optical measurement of nitrogen dioxide (NO2) using differential absorption
  • Visibility measurement using the widely accepted light transmission opacity technique
  • Temperature and humidity compensated measurements, with data output available
  • IP65 rated external enclosure supplied with quick release dust protection tubes and wall mounting brackets
  • Intelligent analyser with optional TSCU operator interface
  • Choice of interface options enabling easy integration into tunnel control system


  • Designed specifically for in-situ monitoring in tunnels
  • Rugged design to withstand corrosive atmosphere and regular tunnel washing
  • Plug and socket cable connection enabling simple installation
  • No moving parts and low maintenance requirements
  • Suitable for right or left hand hanging to enable compliance with regulations governing tunnel light emissions facing on-coming traffic