Belt weigher calibration checks how the instrument responds to a reference under stated conditions. Zero, span and repeatability describe different parts of that performance. “Out of tolerance” means a result falls outside the limits agreed for that installation and test; there is no single maintenance percentage that suits every belt weigher, weightometer or conveyor belt scale.
Accurate Industries assesses the equipment, operating requirements and test results together. We investigate the cause of poor performance, make appropriate repairs or adjustments, and record what the completed checks demonstrate.
What is zero (tare) on a belt weigher, and why does it drift?
The zero setting accounts for the unloaded belt and the mechanical load seen by the weighing system. An empty-belt check looks at the residual indication under the specified running conditions, usually over a defined number of complete belt revolutions so variation around the belt can be considered.
A zero error in belt loading is an offset, often expressed in kilograms per metre. Its effect on indicated flow depends on belt speed. At a fixed speed, the same offset becomes a larger percentage of the reading as material flow decreases. Low-flow operation can therefore be particularly sensitive to an unstable zero.
Possible causes include build-up, idler condition, belt tracking or tension changes, mechanical restraint, load cell problems, moisture in wiring and electrical noise. These deserve investigation rather than being dismissed as inevitable drift. See why a belt weigher zero keeps drifting.
What is span, and how can an error affect recorded tonnes?
Span describes the response to a change in load after zero has been established. A gain error can produce a proportional measurement error, but an observed difference in totals is not automatically a span fault. Speed scaling, loading conditions, mechanical effects and the comparison reference also need checking.
Illustration only: if 20,000 tonnes pass over a scale and it has a consistent measurement bias of 0.5% throughout that run, the recorded total differs by 100 tonnes. This is an example of the calculation, not an Accurate service limit or proof that 100 tonnes of product were physically lost. The operational or financial effect depends on how the data is used.
What is “as found” versus “as left”?
As found describes the condition and results recorded before relevant work or adjustment. As left describes the results after the recorded work. The report should make clear whether cleaning, repairs or setting changes occurred between those checks and whether the same reference and conditions were used.
An as-found check is a snapshot at the time of testing. It does not establish exactly when an error began or prove that the same error affected every tonne since the previous visit. If historical totals matter, review the service history and available operating evidence before applying a retrospective correction.
Calibration and adjustment are different activities: establishing the relationship to a reference is the calibration; changing a setting to alter the response is an adjustment. A service visit can include both, followed by checks of the resulting performance. This distinction follows the international metrology definition of calibration.
Do belt speed and belt length need checking?
Yes. Speed is part of the mass-flow calculation, and belt-length or belt-revolution information may be used by the instrument’s calibration routines. The correct values and scaling depend on the equipment. Measure and confirm them using the applicable procedure; do not assume they are fixed values that can never require correction.
A slipping sensor drive or an incorrectly scaled speed input can affect recorded tonnes even when a static load check looks satisfactory. See our belt weigher speed sensor guide.
What tolerance should apply to your belt weigher?
Every installation is different. Agree the required performance and acceptance criteria for the intended use, operating range, equipment and test method. A general rule of thumb should not become a hard pass/fail limit on a service report.
- Identify the quantity: zero indication, span deviation, accumulated mass error or repeatability.
- Define the percentage basis: reference mass, test load, rated capacity or another explicitly stated quantity.
- State the conditions: belt speed, flow range, material, test duration and reference method where relevant.
- Consider the reference: its suitability, resolution and uncertainty limit what the comparison can establish.
- Record the agreed criteria: including any manufacturer, contractual or separately applicable trade requirements.
A percentage without its basis can be misleading. For example, a zero value expressed against rated capacity cannot be compared directly with a span error expressed against an applied test load.
What is repeatability, and does it prove accuracy?
Repeatability describes how closely repeated results agree under the same specified conditions. It is a check of short-term consistency, not proof that the result matches the reference or will remain unchanged for months. A scale can repeat closely and still have a consistent bias. See the metrology definition of repeatability.
Repeat checks without altering the setup between those runs. Where an adjustment is made, clearly separate the before-and-after series. If repeatability is poor, investigate the mechanics, electrical signals, operating conditions and test method before relying on a correction.
As a rule of thumb, repeat span runs should agree within about 0.1%; the exact target depends on the installation and test.
What should you do about an out-of-tolerance result?
It does not necessarily mean the whole belt weigher needs replacing, but it should not be dismissed as harmless or normal. Consider the size and direction of the error, confidence in the test, the use of the measurement and whether results are changing over time.
Accurate Industries explains the findings, repairs where practical and recommends further work where reliable performance cannot be restored. Repeated faults or unreliable totals may justify comparing an upgrade with ongoing repairs and downtime: see repair, upgrade or replace an existing belt weigher.
Frequently asked questions
Is zero or span more important?
Both matter. Their effect depends on loading, speed and the nature of the error. A low-flow application may be particularly sensitive to zero offset, while a proportional error affects totals throughout the run.
How often should a belt weigher be calibrated?
Use the equipment requirements, operating conditions, consequences of error and service history to set the interval. Significant conveyor or component changes may warrant additional checks. Increasing the frequency does not replace fixing the cause of unstable results.
Does a good calibration result mean the scale is approved for trade?
No. Routine maintenance checks do not establish trade approval or verification. Tell us if the weighing result is used for trade; the National Measurement Institute describes the separate requirements.
Can you explain our report?
Yes. Send the report and equipment details to admin@accurateindustries.com.au, or contact our service team. We can explain the recorded method, results, limitations and recommended action.
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.