Calibration of WIM systems

The objective of a system calibration is to establish the average measurement error of the system. The measurement error is the difference between the value measured/calculated by the WIM system and a static reference value. Once the average error has been noted this value can be used to make adjustments to the system. Strictly speaking, the term calibration means just the act of comparison, and does not include any subsequent adjustment. In daily WIM practice ‘calibration’ means both, calibration and justification.

For a proper calibration it is recommended to use the most common type of vehicle at the site and, if possible, to use two different types of vehicles (i.e. one 2-axle rigid truck and one 5-axle tractor + semi-trailer combination). The truck(s) should be loaded close to their maximum permissible loads, and each should make at least ten passes over the system at speeds that are typical for the site. For trade or tolling applications, the same number of passes should be made when the trucks are empty or partially loaded.

The frequency and exact procedure for the calibration (the number and type of vehicles, the number of runs and the loading of the vehicles) should be agreed between the vendor and the customer prior to the calibration itself. Examples of calibration procedures can be found in several WIM standards.

Another form of calibration is often referred to as ‘automatic calibration’ which is strictly speaking not calibration since no hard reference values are used. This type of compensation is described in the next paragraph. Sometimes the term ‘automatic calibration’ is also used for the compensation of drift in the measurements caused by external influences, like temperature variations.