Capacitance WIM Site Maintenance

The highly durable polyurethane which totally encapsulates the capacitance weigh sensor provides excellent and durable protection to the sensor. As such, capacitance pads require relatively low maintenance.

It is recommended that the bolts be regularly checked. After the first installation the bolts must be checked after three months to confirm that the sensor has settled in well under traffic (recommended torque is 55 N*m).

A minimum of two maintenance visits per year is recommended, but local conditions and prevailing QA procedures might require more regular visits.

The final sensor height should also be checked for possible settlement. Any cracks that appear around the sensor frame and the feeder slots should be filled with an appropriate low viscous sealant.

These procedures should form part of regular checks that must be included in an operational quality system.

The most likely damage that can occur is the embedding of loose stones into the polyurethane surface. However, embedded stones rarely penetrate right through the polyurethane mats and thus resultant damage is usually only superficial.

Minor testing of the output data signals from the capacitance sensors should also be carried out with each visual inspection to ensure accurate weight readings.

For capacitance WIM sites using the Mikros Systems RakTel traffic data logging equipment, the TelWinPlus program provides a comprehensive sensor and logger diagnostic function.

A diagnostic plot of connected loops and weigh sensors to the loggers can be obtained to check on the performance of all sensors (shown in the figure below).

Figure: Diagnostics interface of the TelWinPlus program showing a typical response trace as a light vehicle passes over a loop

Fig 9.2

Source: Mikros-2006

It should be noted that as the micro-electronic components are embedded within the capacitance pads they can only be repaired by Mikros Systems in Pretoria, South Africa; however, replacement changeover capacitance pads are generally held in stock in Australia.

Another point worth noting is that the Mikros RakTel traffic data logger can be interfaced also to Roadtrax piezo sensors, Kistler piezo ceramic sensors and PAT bending plates thus giving a common output data format for all commonly used sensor types.

Further Reading

Mikros 2006, ‘TelWinPlus product specification MS001-94001-40’, issue 1, Mikros Systems, Pretoria, South Africa

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