Belt scale calibration is different from the adjustment of a static platform scale. The measurement result depends not only on the load cell signal, but also on the belt speed, the geometry of the weighing bridge, material flow, belt tension, and zero stability. Therefore, calibration should include both electronic and mechanical checks.
How does a belt scale measure?
The weighing bridge measures the instantaneous material load on the belt. The speed sensor indicates the belt speed. The electronic integrator combines these two values to calculate the instantaneous flow rate and total amount transported. A small error in the load or speed channel continuously affects the total tonnage.
Mechanical check before calibration
- Check that the rollers in the weighing section are aligned and rotate freely.
- Bring the belt tensioner and its mark to normal working condition.
- Clean material accumulation on the weighing bridge.
- Verify that there is no jamming or side load in the load cell assemblies.
- Check for slippage, dirt, and mechanical clearance on the speed sensor pulley.
- Ensure that the feed flow is centered and regular.
Zero calibration
A zero test is performed for a sufficient full rotation while the belt is empty but running at normal operating speed. A single short sample is not sufficient due to belt splice, thickness variation, and mechanical oscillation. Consecutive zero tests should be repeated within the acceptance limit. If there is a large deviation, the mechanical cause should be found before adjusting the span.
Span calibration methods
Test weight
A known weight is applied to the point determined by the manufacturer. It provides fast and repeatable electronic/mechanical control; however, it may not fully represent the actual material behavior.
Chain calibration
The calibrated chain simulates a specific line load on the belt. It loads the weighing area more broadly, but the value of the chain, its placement, and the belt speed must be correct.
Material test
The material passing on the belt within a certain period is weighed on a reliable reference scale and compared with the total of the belt scale. It is the method that best represents the actual working conditions. Collecting the entire sample, moisture loss, and the traceability of the reference scale are important.
How is the calibration coefficient evaluated?
The ratio between the reference quantity and the belt scale reading is calculated, and the span factor is adjusted according to the manufacturer’s procedure. Instead of relying on a single test result, successive tests should be conducted; if repeatability is insufficient, the process and mechanical structure should be examined before changing the coefficient.
Speed sensor errors
If the sensor measures the drive motor speed instead of the actual belt speed, an error may occur due to slippage. The measuring element must have reliable contact with the belt and be protected from impacts and material buildup. Pulse/revolution and environmental parameters must be entered correctly.
Why does the zero constantly change?
Belt tension, temperature, adhering material, bearing friction, belt splice, and frame vibration affect the zero. If automatic zero tracking is used, it should only be activated under empty belt conditions; it should not mistakenly accept a low flow as zero.
How is accuracy maintained?
Daily zero check should be applied together with regular mechanical inspection and planned material testing. Recalibration is required after maintenance or belt replacement. Results should be recorded with date, product, speed, flow, reference amount, and deviation.
Common mistakes
- Making zero adjustment while the belt is stopped
- Correcting span with misaligned rollers
- Accepting the result with a single and short test
- Not checking the speed channel
- Selecting a calibration flow that is very different from the normal process flow
- Incomplete collection in material testing
Frequently asked questions
How often is the belt scale calibrated?
The periodic process risk is determined according to material, working time, and past deviations. Intermediate control is required when changing the belt, roll, load cell, or speed sensor.
Is the test weight sufficient?
It is useful for routine control; whenever possible, system performance should also be verified with an actual material test.
Does accuracy deteriorate when the belt speed changes?
With correct speed measurement and within the appropriate calibration range, the system can operate at variable speeds. Slippage, low flow, and dynamic effects should be checked.
Selection of load cell and weighing cell in the belt scale
The sensor on the weighing frame of the belt scale is the main component that continuously measures the material load. The selected load cell should have a capacity suitable for belt tare, maximum flow rate, roll geometry, vibration, and environmental conditions. A load cell with excessively high capacity can reduce signal resolution, while a low-capacity load cell may be overstressed under impact loads.
Proper load cell installation prevents the transmission of lateral forces and mechanical binding onto the load cell. Before calibration, every load cell connection, cable, and frame movement should be checked.
The working principle of the belt scale can be examined; for system selection and commissioning you can contact Kobastar.