Span test

The purpose of the span test is to set the upper limit of the weighing capacity of the conveyor belt scale to match the maximum capacity of the weighing application. The difference with the span test (versus the zero test) is the attachment of calibration test weight(s) to the weigh bridge(s) during the span test.

In the event that the design rate of your process changes, you can achieve the best accuracy by completing a new span test with more or less calibration test weights.

During the span test, the conveyor should be run to speed without material being loaded onto the belt with the calibration test weights attached.

Calibration test load methods

Typical calibration test weight designs include hand calibration test weights and calibration test chains, in addition to material tests. Hand calibration test weights are attached to the scale only during the brief completion of the span test and then stored until the next span test is performed. Hand test weights are universal, and one set of multiple hand test weights can be used across all belt scale(s) at the given site or regionally by all sites of a company.

Close-up detail of a fabricated calibration test chain used during the span test during the calibration of the conveyor belt scale.

Another option is to purchase an engineered calibration test chain for the belt scale. A calibration test chain looks like an oversized “bicycle chain.” Installation of a calibration test chain commonly includes a storage trough mounted above the conveyor and either a hand or electronic winch. The winch is used to lower the calibration test chain onto the conveyor belt to hold it stationary during the span test. The winch is then used to retrieve and store the calibration test chain in the trough until the next span test is performed.

A material test of the conveyor belt scale can also be completed where the weight produced by the conveyor belt scale is factored to another known weight. This known weight can be either a pre-weight, such as bagged material, or the unloading of a rail car or truck load of material across the conveyor scale.

Material that has been conveyed can also be collected in a truck or rail car and then weighed out on either a truck or rail scale. The conveyor belt scale can be factored to match the known weight of either the pre-weight or collected material to achieve the best accuracy.

Routine maintenance

Once the conveyor belt scale is properly installed and calibrated, proper routine maintenance will ensure proper operation. Since the conveyor belt scale will likely be located in a rather hostile environment, it is recommended that the user perform periodic checks whenever inspecting the conveyor or servicing other components in the area of the belt scale.

Two hand test weights are shown attached to the weigh bridge in typical fashion as done during the span test of the conveyor belt scale calibration.

The conveyor belt scale should be kept clean of debris or spilled material. Heavy buildup of static material will register as weight on the scale and be added to the total on the integrator. This will produce a compound effect and can produce a high error in the accuracy of the belt scale over a long period of time. The scale idler(s) attached to the weigh bridge(s) and speed sensor should also be kept properly lubricated.