Tunnel Monitoring – How do you manage fog?

Fog is a weather condition which most drivers have experienced. The result of fog is reduced visibility and an increased chance of road traffic accidents because drivers have less time to react. The emergence of fog is typically influenced by a nearby body of water, or from moist ground combined with the right topography and wind conditions. Fog appears when water vapour (water in its gaseous form) condenses. During condensation, molecules of water vapour combine producing liquid water droplets that hang in the air.

Road tunnels are not immune from fog hampering driver visibility. Due to the wide variety of locations where tunnels are constructed, they can be susceptible to the presence of fog near to, and sometimes inside the tunnel. As part of a tunnel’s ventilation system, powerful Jet fans are installed to expel pollutants in order to maintain a safe atmosphere. However, when operating, tunnel jet fans have the potential to draw fog into a tunnel via the portals.

The Tunnel Sensors VICONOX, a combined gas and visibility monitor, has an internal relative humidity sensor. The sensor has a range of 0-100%, and continuously operates whilst the VICONOX measures the visibility and toxic gas concentrations within the tunnel. If a VICONOX, located close to a tunnel portal, measures poor visibility and simultaneously a high relative humidity level (above 90%), this is a strong indication that fog is present in the tunnel. The VICONOX’s communications channels, Modbus RTU via RS485, 4-20mA outputs and digital relays can be used to report the relative humidity level to the tunnel’s control systems. Allowing the operator to make a more informed decision about activating the jet fans and the possible consequence of drawing fog into the tunnel.

Alternative tunnel atmosphere monitors, (such as those using forward scatter technology to calculate visibility) can evaporate moisture (fog) as part of the measurement system. The evaporation of fog means tunnel operators may not realise it is present and unknowingly draw fog into the tunnel bore when operating the tunnel’s jet fans to remove pollutants.

The Viconox’s open path visibility system provides an direct measurement of opacity, which means dust, particulate and fog are measured and combined to provide a total visibility reading.

Please contact us if you would like more information about the VICONOX tunnel air quality monitor or any of our accessories for your next tunnel project.

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