A difference between a belt weigher and a weighbridge needs a fair comparison before either instrument is blamed. They can measure the same material by different methods, but unmatched batches, timing, tare or moisture changes can create a discrepancy. A belt weigher and a weighbridge rarely agree exactly, so a small gap on its own doesn’t prove either one is wrong, though a steady bias still adds up over many tonnes.
Are you comparing the same material?
A conveyor belt scale totals material crossing its weighing point. A weighbridge commonly determines a vehicle’s net load from gross and tare measurements. For a useful comparison, the totals must represent the same identifiable batch, with any material added, removed or retained between the two points accounted for.
- Timing: record the start and finish totaliser readings and allow the batch to clear the weighing point.
- Inventory: account for product left on conveyors, in bins or on stockpiles between the instruments.
- Tare: check the correct vehicle and tare method, and whether changes in the vehicle’s mass affect the net result.
- Material changes: consider moisture gain or loss, drainage, spillage and diverted material.
- Records: exclude duplicate tickets, missing loads, resets and mixed units.
How do you calculate the difference?
Difference (%) = (belt weigher total − reference net mass) ÷ reference net mass × 100. Use matching units and a positive reference mass. The sign convention here makes a positive result a belt weigher over-read relative to the reference.
Example only: 1,010 tonnes on the belt weigher against 1,000 tonnes of matched reference material gives +1.0%. This describes the comparison. It does not establish which instrument is wrong, or automatically authorise a span adjustment.
How much disagreement is acceptable?
There is no universal tolerance for an uncontrolled reconciliation. Consider the specified performance of both instruments, the quality and uncertainty of the reference, the batch size and how closely the material boundaries match. Set the comparison method and acceptance criteria before testing.
Look at trends, but do not dismiss a stable offset. A growing or sudden discrepancy helps identify when something changed; a steady discrepancy may indicate a persistent measurement bias or an accounting difference. Both can justify investigation when the tonnes affect production or commercial decisions.
How do you arrange a controlled material comparison?
Agree the method with the site and technician. Use a suitable reference scale whose performance is known, capture the same batch both ways and retain the gross, tare and net records. Select a test quantity and operating conditions suitable for the equipment and required performance. Repeat the comparison to check consistency.
A stockpile survey or plant material balance is a useful independent cross-check on which instrument has moved, though a survey carries its own density and shape assumptions. A belt cut test can help investigate sampled belt loading, but it does not replace every requirement of a complete material test. See belt weigher calibration methods.
What should be checked on the belt weigher?
- Zero: an empty-belt offset can have a large relative effect during low loading.
- Speed: inspect the measurement arrangement and compare indicated speed with a suitable independent check.
- Mechanical condition: inspect weigh idlers, alignment, clearances, build-up and belt behaviour.
- Signals and settings: check load cells, wiring, units, low-flow cut-offs, run logic and any recent configuration changes.
- Recent work: identify belt, splice, idler or sensor changes and the checks performed afterwards.
After a belt replacement or new splice, re-measure belt speed and length, re-enter them and recalibrate. On the weighbridge side, have its condition, tare practice and measurement performance assessed too.
Should I adjust span until the two totals match?
Only after the comparison and diagnosis justify it. An unexplained speed error, changing zero, unstable mechanics or mismatched batch can be hidden by a span adjustment and reappear under different conditions. A correction is model-specific and should be followed by a fresh verification test.
Accurate Industries can assess the current system, repair where practical and recommend an upgrade where persistent faults or unreliable measurement make that better value. See weightometer calibration and belt weigher servicing and common belt weigher faults.
Frequently asked questions
Can the weighbridge be wrong?
Yes. The comparison alone does not establish which measurement is correct. Check both instruments and the material records.
Can moisture change the comparison?
Yes, if the material gains or loses water between measurement points. Record the circumstances rather than using moisture as an assumed explanation.
Can I ignore a small steady difference?
Not automatically. Assess it against the comparison quality, agreed criteria and the consequences of the discrepancy over the operating period.
Need a manual for your equipment?
Talk to us about manuals for your equipment. Send the make, model, serial number and software version (a photo of the label is fine) and we’ll help you find the right document.
Contact us about a reconciliation problem with the batch records, equipment models and recent changes. Use email to attach reports or photographs.