Acoem’s tunnel sensor range includes an industry leading multiparameter tunnel visibility sensor that measures the opacity of the air to articulate precise visibility levels. It communicates with the tunnel’s operating system to engage its jet fans if the atmosphere loses clarity and may be dangerous.
In addition to measurement of particulate matter (PM) and gas emissions, Acoem tunnel visibility monitors also assist operators with visual management and control of traffic flow and road user safety which can be affected other visibility factors including:
Weather at either end of the tunnel
The tunnel’s own internal microclimate, including fog
Road surface abrasion
Tyre and brake wear
Extra pollution from stationary vehicles
Smoke and/or fires due to accidents or faulty vehicles
The principle used for measuring tunnel visibility
Because a light beam decays in intensity as it passes through air, the level of decay can be used to determine the opacity of the air, which is expressed as an extinction coefficient ‘K’ in m-1 and corresponds to the light attenuation that results from pollutants in the air. Visibility reduces due to light extinction by the scattering and absorption of radiation in the visible wave-length range. Opacity monitors use these effects to measure tunnel visibility.
Visibility criteria is used to provide data for the tunnel’s operating system, informing it of drivers’ ability to stop safely. The tunnel’s ventilation system must provide visibility levels that are greater than the minimum vehicle stopping distance at designated average speeds.
Visibility measurement using the light transmission opacity technique
Acoem tunnel visibility sensors consist of a transmitter (TX) and a receiver (RX), which are mounted facing each other on the wall or ceiling along the length of the tunnel. A single pass light transmission technique is used to measure dust, smoke and particulate concentration present in the atmosphere. The TX emits a visible (green) optical beam which is received by the RX. Any dust or smoke particles present will attenuate the light beam and cause the intensity of the light received by the RX to fall. This reduction in light intensity is used to determine visibility in the tunnel.
Acoem VICONOX is our signature tunnel visibility monitoring system that also accurately measures multiparameter gases. It is certified for use around the world, meeting all international standards and guidelines for visibility monitoring. It uses the widely accepted single pass light transmission opacity technique and displays options of opacity (%), extinction coefficient (K), Meteorological Optical Range (MOR) or transmission
These measurements can be used as part of a fully integrated air quality management system for ventilation control and/or secondary smoke detection within a traffic
tunnel, rail tunnel or other confined space or environment.
Compliance with local and international guidelines
While not all countries have legally enforceable standards for tunnel visibility monitors, most provide values for normal operation for the layout of tunnel ventilation according to the World Road Association (PIARC) international guidelines.
These lie typically at a value of Klim = 0.005 m-1 or 5mE/m (milli-extinction per metre). At a visibility of 12mE/m, the tunnel has to be closed for safety reasons.
Extinction coefficient K
Length of light beam L with I/lo=exp(-K L) =20%
Table 1: Correlation between visibility conditions, extinction coefficient and light bean length for an acceptable light level of 20% of the light source intensity.
Free flowing peak traffic 50 – 100 km/h
Normal operation *
0.003 – 0.007 m1
Daily congested traffic, stopped on all lanes
Planned maintenance work in a tunnel under traffic**
Exceptional congested traffic, stopped on all lanes
Threshold value for tunnel closure
* Intermediate set points and thresholds may be set at levels below the design values
**National workplace levels to be considered
Table 2: Design values for various traffic situations and the limits for tunnel operation according to PIARC.
Benefits of implementing Acoem tunnel visibility monitor solutions
Ensure proper visibility levels for drivers
Improve tunnel road safety
Minimise risks to drivers and tunnel workers
Reduce traffic related accidents in tunnels
Help to manage gas and particulate levels in tunnels
Trigges tunnel ventilation system to provides optimum clean air levels