More mining and quarry sites now have a laser or radar profile scanner sitting over a conveyor as well as, or instead of, a belt weigher, and it raises a fair question: if the scanner already tells you how much material is on the belt, do you still need the weigher? The short answer is that the two measure different things. A volumetric scanner reports the volume of material passing a point. A belt weigher reports the mass, and totalises it into tonnes. When money, stock or a stockpile balance rides on the number, mass is what you are accountable for, and volume is not a substitute for it.
We calibrate and service belt weighers across Australian mining and quarrying, and we are often called in when a site's scanner and weigher disagree and nobody can say which one to believe. Here is how the two actually differ.
What does a volumetric scanner measure?
A volumetric scanner sits above the belt and profiles the material without touching it. A laser or radar sweep measures the shape of the load: how the material heaps across the belt, measured against a zero reference for the empty belt. The two set that zero differently. A laser unit scans the bare belt itself, so the belt has to be running empty and clean when the zero is captured. A radar unit cannot see a bare belt at all, so it is zeroed against a reflective target laid on the belt instead. It is worth knowing at commissioning, because on a radar unit there is no empty belt zero to go looking for. The sweep gives the cross-sectional area of the burden at that instant. Multiply that area by the belt speed and you get a volume flow rate, in cubic metres per hour, and a running total of volume.
Because it is non-contact and mounted above the belt, a scanner brings some things a weigher cannot. It sees the profile, so it can flag an off-centre load, a chute that is choking, a surge, or a belt running emptier or fuller than it should. Some units also report where the load and the belt are sitting: the centre of the load across the belt, the clear space to each belt edge, the belt edges it detects and the belt width it measures, with an alarm if that width drifts outside what the belt should be. That tells you when the load is running to one side, or when the belt is not sitting where it should. What it does not tell you is the condition of the belt itself, so it is no stand-in for a belt inspection. There is no weigh frame under the belt to keep clean and level. The scanner still has to know how far the belt has moved, though, because volume only comes from the cross-sectional area combined with belt travel. That means there is still a belt speed sensor on the conveyor to look after, and on the common types it runs in contact with the belt. Build-up on it makes the reading run low, so it wants the same regular check as anything else on the conveyor. What it is really good at is watching the shape of what the belt is carrying.
What does a belt weigher measure?
A belt weigher weighs the loaded belt directly. The material presses down through the idlers onto load cells, the integrator reads that load against the belt speed, and it works out a mass flow rate and totalises it into tonnes. For the parts and the principle behind that, see how a belt weigher works.
The output is the number the rest of the business runs on. Production accounting, stockpile reconciliation, blend ratios and any pay-by-weight arrangement all deal in mass, not volume. A weigher gives you tonnes measured, not tonnes inferred. What it does not give you is any picture of profile: it cannot tell you the load is sitting to one side, or that a chute is about to block. It weighs, and that is the job it does well.
What is the density gap between a scanner and a belt weigher?
This is the heart of it. A scanner measures volume. To turn volume into tonnes on its own it has to work from an assumed bulk density, a figure for how many tonnes sit in each cubic metre. The trouble is that bulk density is not a constant. It shifts with moisture, with how coarse or fine the material is broken, with the blend, and with how much the load has compacted. A wet, fine, well-packed load and a dry, lumpy, freshly-tipped load can fill the same cross-section on the belt and weigh quite differently.
So volume multiplied by an assumed density is an estimate, and it is only as good as the density figure on the day. When the material changes and the assumed density does not, the scanner's tonnes drift away from the truth, while nobody sees the assumption slip. A belt weigher sidesteps that entirely, because it never assumes a density. It feels the actual weight. That is why, when tonnes are accountable, weighing beats calculating from volume. The reverse is also true: a weigher tells you nothing about where the load sits on the belt, its shape, or spillage risk, so neither instrument replaces the other.
When is a scanner the right tool?
A scanner earns its place where profile and continuity are what you care about. It is the better tool for watching load shape and belt loading, catching an off-centre or surging feed, spotting a blocked or choking chute early, and keeping an eye on where the load and the belt are running. It is non-contact and sits above the belt, with no weigh frame underneath, which can suit an awkward or hard-to-reach conveyor. It is not maintenance free, though. It still depends on a belt speed sensor down on the conveyor, and it needs a clear view of the load. A laser unit works off the light that bounces back to it, so it will raise a poor signal warning when it stops getting a clean return, and mist or fog in the air is enough to give it trouble on its own. Worth thinking about before you site one over a wet or dusty transfer point, which is often exactly where the awkward conveyor is. As a fast, continuous indication of how full the belt is and how the material is presenting, it does a job a weigher was never built for. Where its volume reading only ever guides an operator, and no one is billed on it, an assumed density is perfectly honest.
When is a belt weigher the right tool?
A belt weigher is the right tool the moment the tonnage is accountable. If the number feeds production reporting, reconciles against a stockpile survey, sets a blend, or settles a payment, you need measured mass, and you need it calibrated. This is also the instrument you can prove: it is calibrated against known reference weights and checked against material comparisons, so its number can be defended. When "how many tonnes went across this belt" has to be a figure someone signs off, the weigher is what answers it. Choosing the right weigher for that duty is its own decision, covered in choosing a belt weigher.
Why do many sites run both?
Plenty of well-run sites fit both, and it is a sensible layout rather than a duplication. They answer different questions, and each one covers the other's blind spot. The scanner watches the profile and the belt in real time; the weigher measures the accountable tonnes. The real bonus is that the two cross-check each other. If the scanner's volume trend and the weigher's mass trend move together, both are behaving. When they start to diverge, that gap is a signal worth chasing: it can flag material build-up on the weigh frame, a belt tracking or tension change, or a genuine shift in bulk density, which bites whenever the scanner is running on a density figure someone entered rather than one worked out from the weigher. Two instruments that usually agree make it obvious when one of them stops telling the truth. If yours have started to disagree, common belt weigher faults is a good place to start on the weigher side.
How does this affect calibration?
A belt weigher is calibrated against known weights and material comparisons, so its tonnes can be trusted and proven, and every weigher we look after carries its calibration history and next due date in AIMM, our asset management system. A scanner working on its own turns volume into tonnes using a density figure someone has typed in, and its tonnes are only ever as good as that figure. Paired with a belt weigher it does not have to guess. The system divides the weighed mass by the scanned volume to get the density from live readings, and keeps a running average from the two totalisers, which beats any assumed figure. Whichever way it is set up, the scanner has its own zero and alignment to keep right: the zero is captured off an empty belt, and any change to its angle or distance settings means capturing it again. So it is not an instrument that looks after itself. Either way, this is routine maintenance work to keep the numbers honest, not a trade certification of either instrument.
Frequently asked questions
Can a volumetric scanner replace my belt weigher?
Not where tonnes are accountable. A scanner measures volume and only estimates mass by assuming a bulk density, and that assumption drifts as moisture, fragmentation and blend change. For production accounting, stockpile reconciliation or pay-by-weight, you want measured mass from a weigher, not tonnes calculated from volume.
Why do my scanner and weigher disagree?
Usually because the density the scanner is working from no longer matches the real material, which is the common case when it runs on a figure someone entered, or because something has changed on the weigher: build-up on the frame, a belt tension or tracking change, or a zero that has drifted. A steady disagreement is worth investigating, because it often points at a real problem in one of the two.
Which one is more accurate?
It is the wrong comparison, because they measure different quantities. For volume and profile, the scanner is the right instrument. For accountable mass and totalised tonnes, a calibrated belt weigher is, because it never has to guess a density.
Do I still need to calibrate the weigher if I have a scanner cross-checking it?
Yes. The cross-check tells you when something has changed, not by how much or which instrument is wrong. Only a proper calibration against known weights re-establishes the weigher's true reading. A scanner is a useful early warning, not a calibration.
Can a scanner tell me if my belt weigher has a problem?
Often, yes, as an early flag. If the two normally track together and then diverge, that is a strong hint to check the weigher (or the scanner's density figure). It narrows down where to look, but the actual fault-finding and calibration still happen on the weigher itself.