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Appendix 3 - Guideline for the Installation of Sensors
As the RTM300 is typically installed inside a duct, this Appendix provides
supporting information on considerations for the duct design and installation.
This guideline should be read in conjunction with local and national
instructions, specifications and guidelines and are not a substitute for
them.
Duct Minimum Bend Radius
Ideally the inside of the duct should be smooth and the duct should be rigid and
straight over its entire length. If the site conditions are such that this is not
practical and a flexible duct has to be installed, consideration should be given to
the minimum bend radius to allow the RTM300 sensor to pass easily.
Duct Depth
The ideal depth of the duct to allow for resurfacing of the carriageway without
damage and for maximum flexibility of sensitivity adjustment of the RTM300
sensor, is 150mm below the surface. However subject to the nature of the
carriageway material the RTM300 sensor can operate down to 600mm below the
surface. Most traffic signals ducts are buried at 600mm cover, which places them
above other services.
Duct Diameter
The RTM300 sensor is approximately 18mm diameter, and we therefore
recommend an absolute minimum internal diameter of a 20mm duct for easy
passage of the sensor. If smaller diameter ducts are used, then care should be
taken that re-instatement of the carriageway does not distort the duct as this may
prevent easy passage of the sensor.
If more than one sensor is to be fitted inside a cross carriageway duct, allow
adequate clearance for the cable and draw cord for further sensors to pass by
the nearer sensor.
Choice of Duct Materials
The RTM300 sensor is less likely to work reliably if mounted within a
ferromagnetic duct, for example a duct with a steel liner or an old cast iron pipe.
Sealing The Duct
Where blind ducts are used, the far end must be sealed to prevent the ingress of
mud or other materials from the backfill which may clog the duct.
After sensors are installed, the chamber end of the duct may be loosely plugged
to minimise water and contamination ingress. Suitable sized pieces of bubble
wrap or foam are suitable for this purpose.
Alternative to Ducts
The RTM300 sensor does not have to be installed in a duct, and due
consideration should be given to alternative installation locations as follows:
the degree of protection offered to the sensor and cable.
the mounting of the sensor to prevent movement.
the loss of sensitivity with increasing distance from the vehicle to be
detected.
the savings in installation cost.
