As the equipment manufacturers, Acoem’ technical sales team are often asked where our visibility, gas and airflow monitors should be installed inside a road traffic tunnel.
Unfortunately this is a question we are unable to directly answer with any certainty. Accessibility, traffic clearance, beam direction, cable routes, communication system, jet fan locations and environmental factors such as local wind speed and direction all need to be taken into consideration.
The location of air quality monitoring points along the tunnel bore will usually be specified by the tunnel’s ventilation design engineers. Such plans will typically consist of monitoring points adjacent to both the entrance, and the exit of the tunnel bore (at least 100m inside), and a number of equally spaced monitoring points in-between. The frequency of the intermediate measurement points will usually be between 200m and 400m, and may be dictated by local or national regulations.
Human access will be required occasionally, so consideration to means and ease of access for future maintenance should be included in the decision making process. The VICONOX heads have been designed to be mounted on the tunnel wall, the ideal height is about 3.5m, this is high enough to be clear of road spray, direct exhaust gas, and cars, but still low enough to be accessed by a service engineer with an ordinary ladder. If possible, mounting the heads at excessive height, on the roof of the tunnel, or directly over traffic should be avoided.
Any device mounted to a tunnel wall will be a protrusion into the bore which will inevitably reduce the clearance between them and the traffic. It may be that the VICONOX heads reduce the lateral clearance, headroom, or both, care must be taken to ensure that clearance between the heads and the traffic flow have been calculated carefully before settling on an installation position.
Sunlight may seem like an unusual consideration in a tunnel, but instruments located near the entrance and exit of the tunnel can become exposed to direct sunlight. It is essential that the instrument is installed in such a way that sunlight cannot, at any time of the year, shine directly into the sight tube of the VICONOX’s receiver head and fall onto its optics. If the alignment of the tunnel means that direct sunlight could enter the tunnel at sunrise or sunset, then arrange the instrument so that the TX head is pointing towards the portal. If this is not possible then the instrument should be positioned at least 150m from the portal.
We hope this article goes some way to explain why we’re unable to give precise monitor location advice and that the information here provides a guide and points for consideration when selecting monitoring locations. If you would like to discuss your next road tunnel atmosphere monitoring project please do not hesitate to contact Graham Jennings or Charles Mawle