When we calibrate a belt weigher (also called a weightometer, belt scale or conveyor belt scale), we are really checking two numbers: the zero, which is what it reads on an empty belt, and the span, which is how accurately it counts material as it passes. Zero is a fixed error that matters most at low flow. Span is a proportional error that scales with tonnage, so even a small span error can hide or invent a lot of tonnes over a shift. "Out of tolerance as found" simply means the weigher had drifted past our service band before we touched it, which is common and is exactly what a routine service exists to catch. After we adjust it, a repeatability check tells us whether the new calibration held.
What is zero (tare) on a belt weigher, and why does it drift?
Zero, also called tare, is what the weigher reads when the belt is running empty. In theory an empty belt reads zero tonnes an hour. In practice it rarely does, because the scale cannot tell the weight of a clean belt apart from a belt carrying a thin, even coating of something.
Zero is a fixed error. Whatever offset the empty belt shows gets added to (or taken off) every reading, regardless of how much is on the belt. That is why it matters most at low flow, when the offset is large next to the real tonnage. Run a nearly empty belt and a zero error can be most of what the totaliser counts.
Zero drifts for ordinary physical reasons we see on site every visit:
- Build-up: material packs onto the belt, the weigh frame or the idlers in the weigh area, so the "empty" belt is heavier than the scale was told to expect.
- Rollers: worn, dented or seized idlers in the weigh span change how the load sits on the scale.
- Tracking: a belt that wanders off centre shifts the load across the weigh frame and moves the zero.
None of this is a fault as such. It is the normal life of a conveyor in a dusty, wet, high-tonnage environment, and it is the main reason a weigher needs regular servicing rather than a set-and-forget install.
What is span, and why do span errors cost real tonnes?
Span is the gain, or sensitivity: how much the reading rises for a given weight of material on the belt. If zero is where the scale starts from, span is how steep the line climbs above that.
A span error is proportional. It is a percentage of whatever is actually moving, so it grows with throughput. That is why we say a span error is straight tonnage money.
Here is a worked example, with the numbers set as assumptions. Say a belt runs 2,000 tonnes an hour across a 10-hour shift, so about 20,000 tonnes cross it. A span error of 0.5% (the point where we call the span out of tolerance) is 0.5% of 20,000 tonnes, which is around 100 tonnes a shift counted that never moved, or moved and never got counted. Over a week of shifts that is real product, and depending on which way the error runs it is either tonnes you are paying for or tonnes you are giving away.
That is the core reason span gets a tighter band than zero.
What is "as found" versus "as left"?
Every calibration records two states. As found is what we measured on arrival, before changing anything: the honest picture of how the weigher has been reading since its last service, drift and all. As left is what it reads after we have adjusted it. When we correct a calibration we retune the integrator (the unit that turns belt loading and speed into tonnes) so it reads the true value again.
The gap between as found and as left is the drift we removed on the day. A report that shows a large as-found figure and a clean as-left is not a bad result. It means the service did its job.
One thing we do not adjust: belt speed and belt length. We measure those and feed them straight into the tonnage calculation, but they are not calibration settings we turn. If a speed or belt-length figure looks wrong, that points to a sensor or a measurement to fix, not a number to dial in.
What tolerance bands do we use?
We work to two levels for each parameter: a lower band where we take note, and an upper band where we call the weigher out of tolerance and correct it.
| Parameter | Take note beyond | Out of tolerance beyond |
|---|---|---|
| Zero (tare) | +/-0.5% | +/-1.0% |
| Span | +/-0.25% | +/-0.5% |
Span carries the tighter band for the reason above: at working throughput it moves more tonnes. Zero gets a wider band because its effect shrinks as flow rises.
These are our service criteria: practical maintenance bands we use to decide when a routine calibration needs correcting. They are not trade-measurement limits, and legal-for-trade verification of a weigher is a separate process with its own rules. If you need that, tell us and we can point you in the right direction.
What is repeatability, and why does 0.25% matter?
Tolerance and repeatability answer two different questions, and it helps to keep them apart. The first is: how far had it drifted? That is the as-found tolerance check above. The second is: did the fix hold?
After we adjust a weigher, we run the same test load across it more than once and see how closely the readings agree. If they land within 0.25% of each other, we treat the new calibration as good. That is our repeatability criterion.
Repeatability is the more telling of the two. A weigher can drift out of tolerance and still be perfectly healthy, because drift is expected between services. But if we adjust it and it will not repeat, that is a sign something mechanical is unstable: a loose weigh frame, a bad idler, belt tension moving around, or a mount that flexes. A repeatability figure above 0.25% is the one we flag for a follow-up, because recalibrating an unstable scale only moves the problem, it does not fix it.
Why is "out of tolerance as found" normal and not a crisis?
Because catching drift is the whole point of a routine service. A belt weigher lives on a moving conveyor in dust, water, vibration and changing loads. It will drift. The question a service answers is not "did it drift" (it did), but "how far, which way, and does it hold once corrected". A report showing the zero was out of tolerance as found, was recalibrated, and then repeated inside 0.25% is a good outcome, not a bad one.
The results worth a closer look are the other ones: a span a long way out and climbing service on service, or an adjustment that will not repeat. Those are the cases where we usually recommend more than a calibration, such as replacing idlers, sorting out belt tracking, or checking the weigh frame.
Frequently asked questions
Does "out of tolerance" mean the weigher is broken?
No. It means it had drifted past our service band before we adjusted it, which is normal between services. A weigher worth worrying about is usually one that will not repeat after adjustment, not one that was simply out of tolerance on arrival.
Is zero or span more important?
They matter at different ends. Zero dominates at low flow, span dominates at high flow. For a busy conveyor moving a lot of tonnes, a span error costs the most, which is why it gets the tighter band.
How often should a belt weigher be calibrated?
It depends on tonnage, material and how much the reading matters to you. Many sites settle on a regular cycle and tighten it if the as-found drift keeps growing. A weigher that drifts further every visit is a reason to service more often, or to fix the mechanical cause of the drift. See how often should a belt weigher be calibrated?
Do these tolerances mean our weigher is approved for trade?
No. These are practical maintenance criteria for keeping a weigher reading true day to day. Legal-for-trade verification is a separate process.
What does repeatability tell me that tolerance does not?
Tolerance tells you how far the reading had drifted. Repeatability tells you whether the fix held. A scale that repeats within 0.25% after adjustment is stable; one that does not has a mechanical problem worth chasing.