Online Equipment Focus: Maintaining Conveyor Belt Scales
Conveyor belt scales should be calibrated periodically to ensure the accuracy of your material weight total.
by Austin Amos
It is important to note that a conveyor belt scale is no different than any other type of scale (i.e., lab balance or truck scale) in regards to the need for recalibration. Because a belt scale provides a “weight-in-motion” (unlike the static weight of a lab balance or truck scale), if the speed, zero, or span reference numbers of your belt scale have drifted, you will face a compound effect of inaccuracy over a given period of time (hours, days, or months) with a running conveyor belt. This will amplify the difference, positive or negative, between the weight shown on your belt scale integrator and the actual weight of the material you conveyed.
Recalibration of the belt scale once per month is recommended; however, some applications, at a minimum, can be lengthened to once every three months, with each particular application being somewhat different. The calibration of the belt scale can drift slightly (either positive or negative) due to wear on the belt scale, changing conditions of the conveyor belt, or deviation through the continued operation of the conveyor. Monthly calibration of the scale is preferred to keep these changing factors in check with the conveyor belt scale.
It is also important to note that the calibration procedure presented in the reference manual provided by the conveyor belt scale manufacturer should be followed in order from start to finish. Most discrepancy issues regarding accuracy are related to not following the required steps of the calibration process correctly or totally ignoring some steps completely.
Typically, the conveyor belt scale integrator will have a Setup/Calibrate mode that must be selected by the user in order to change the instrument parameters or run calibration tests. When in this mode, integration stops. The calibration process involves the completion of speed, zero, and span tests.
Some integrators feature a menu-driven display that has multiple lines of full English text for display/entry of the setup parameters for the scale; some require the user to refer to their operator’s manual for a code decipher listing of each code that is given on a strictly numeric display.
The addition of a speed sensor to the conveyor belt scale allows the integrator to process and display a “weight-in-motion.” The exact measurement of the conveyor belt speed is critical to conveyor belt scale accuracy. The speed sensor provides a constant measurement of the rate (unit of length/unit of time) of the conveyor. Any change in speed from a loaded or unloaded belt based on material flow is directly transmitted to the belt scale integrator along with the signals from the load cells. The integrator handles the processing of the “integrated” speed and weight and provides the user a “weight-in-motion.”
The user would complete a speed test by first calculating or measuring actual belt speed using a handheld tachometer. For the speed test, the conveyor is run empty (without material) up to normal speed.
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