Codel International Ltd

- Model 402 - Extractive NO2, CO and Visibility Air Quality Monitor



The CODEL TunnelTech 402 NO2 CO and Visibility Air Quality Monitor is a compact extractive analyser configured for the continuous measurement of three key parameters in road tunnels. This extremely stable and reliable monitor incorporates an LED to utilise the capacity of Nitrogen Dioxide to absorb UV and blue light to provide accurate readings in parts per billion. It also uses the same sampling system to monitor Visibility using a unique double sensor design. An electrochemical cell is also added to the sampling cell to measure CO, NO2 , CO and particulate are produced naturally by the combustion processes within the internal combustion engine and are emitted from the exhausts of all types of vehicles.


NO2 is also particularly toxic and prolonged exposure to levels as low as a few hundred parts per billion will have a detrimental effect on human health. There is a growing international requirement to measure and limit the levels of NO2 within road tunnels to reduce the exposure of tunnel users to this toxic gas.

CO also presents a hazard to health but at much higher levels than NO2 and is now more of a nuisance to tunnel users. Visibility monitoring has two functions, to ensure that drivers can sufficiently see the road ahead to drive safely and to protect users from ingesting fine articles (
The TunnelTech 402 can also play an important role in reducing energy usage within road tunnels. When employed to control tunnel ventilation systems, they can significantly reduce the use of jet fans etc. and thereby reduce energy use and its significant costs.

The TunnelTech 402 NO2 monitoring system utilises a very accurate measurement technique. NO2 absorbs UV and blue light very strongly; the TunnelTech 402 uses a precision transmissometer which measures the attenuation of UV and blue light by NO2 in the tunnel atmosphere. The light source is a near-infrared LED and the interfering effects of particulate in the tunnel atmosphere are eliminated by making the measurement within a metre long diffusion cell within the main sampling tube. NO2 diffuses through sintered stainless steel filters to remove the particulate.

The TunnelTech 402 Visibility monitor uses the same cell and fan but has a separate LED source and detector system to monitor changes in Light Transmission through the sample cell. The CO monitor is an Electrochemical unit which uses a long life cell. This is mounted on a boss on the side of the sampling cell to enable a constantly changing sample to be available to the sensor. TunnelTech 402 in Mont Blanc Tunnel The result of this integration is a very accurate and stable analyser that has very low maintenance requirements. Its key feature is that it is an extractive system that can be mounted outside the tunnel bores in an alcove or machine room. This means that maintenance tasks can be performed whilst the tunnel is in operation with no danger to maintenance staff from passing vehicles.

Fully configurable analogue and alarm outputs are generated inside the Station Control Unit (SCU) which are fully configurable via the door mounted PC, In addition, there is a choice of either RS232 or RS485 outputs which can be utilised to deliver MODBUS protocol to a SCADA system located in the tunnel control centre. There is an Ethernet output for Internet connection, usually by CODEL technicians for diagnostic purposes. A USB port is added for data collection.

  • Continuous measurement of NO2, CO and Visibility in road tunnels
  • High accuracy determination of NO2 down to low ppb levels ( 0-1000ppb)
  • Extractive analyser for easy maintenance and calibration
  • Minimal maintenance requirements, low cost of ownership
  • Panel mount PC for commissioning, control and diagnostics
  • RS485 (MODBUS) and Ethernet output

  • Measurements: NO2, CO and Visibility
  • Measuring units: NO2 ppm / CO ppm / Visibility K factor(M-1) or metres
  • Measurement Range: NO2 0 – 1ppm standard, configurable up to 0 – 5ppm / CO 0 – 500ppm / Visibility 0 – 0.015m-1
  • Ambient Temperature: -20°C to +50°C
  • Power supply: 12V DC @1.5A
  • Construction: Corrosion resistant epoxy coated aluminium housing sealed to IP65
  • Analogue outputs: 4 x 4-20mA, 200V common mode isolation, max. load 500Ω
  • Relay Outputs: 4 x volt-free contacts SPCO, 0.25A @ 125V AC, 1A @30V DC, 0.25A @ 100V DC
  • Communication Ports: RS 485 MODBUS, Ethernet RJ45, USB

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