In short
- Net weight is gross weight minus tare weight. Net is what you are billed for, so the other two have to be real readings and not assumptions.
- Billing by weighed tonne removes the two oldest arguments in the trade: how full the truck was and how many trucks came.
- Shortage disputes come from a short list: tare that was never weighed, moisture, two bridges that disagree, spillage, and slips that do not match transit passes.
- Agree in the contract which bridge gives the billing weight and what difference between two bridges is accepted, then reconcile every month, load by load.
Aggregate is ordered in tonnes, delivered in tippers and argued about in both. The supplier's bill says one figure, the site register says another, and at month end somebody has to work out which loads the difference is hiding in.
The fix is a weighbridge slip for every load and the habit of matching slips before the bill is raised. Gross weight is the loaded vehicle on the bridge. Tare is the same vehicle empty. Net is gross minus tare, and net is what you pay for. When all three figures on every slip are real readings, most billing arguments never start.
Gross, tare and net
A weighbridge weighs the vehicle, not the stone. To get the weight of the material it needs two readings, one with the load on and one without. At a dispatching plant the tipper normally weighs in empty, loads, and weighs out full. At a receiving site it is the other way round: full on arrival, empty after tipping. Both give a net figure for the same load.
Tare is where the shortcuts creep in. A tipper's empty weight is not constant. It changes with the fuel in the tank, with material stuck in the body from the last wet load, with a spare wheel added or a body repair. A tare taken once and reused for months is an estimate, and an estimate subtracted from a measured gross gives an estimated net.
What a proper slip carries
| Field on the slip | Why it matters |
|---|---|
| Serial number | Slips are numbered in sequence, so a missing or duplicated number shows up at once |
| Date and time, in and out | Ties the load to a shift and shows whether gross and tare were taken on the same visit |
| Vehicle registration number | Matches the slip to the transit pass and to the gate register at site |
| Gross weight | The loaded reading |
| Tare weight | The empty reading. It should be clear whether it was weighed on this trip or carried forward |
| Net weight | The billed quantity |
| Material and size | 20mm and WMM are billed at different rates, and the slip is the proof of which was on board |
| Operator name or ID, and signature | Somebody is answerable for the reading |
| Which bridge | Tells you which machine to check when two readings disagree |
A slip printed from the bridge's own indicator is worth more than a handwritten one. If the slip is handwritten, ask why.
Why weighed tonnes beat truck counts and body volume
Three ways of measuring aggregate are in use, and two of them invite argument. Counting trucks assumes every load is the same, which is true of no fleet. Bodies differ and loader operators differ. Measuring the volume in the body is better, but it depends on how level the load is, how far it settled on the way, and somebody climbing up with a tape for every vehicle.
Volume has a further problem. Loose volume in a tipper body is not the compacted volume in the layer, and turning one into the other, or either into tonnes, needs a density figure that both sides then argue over. Converting tonnes, cubic metres and brass shows how much disagreement can sit in that one number.
A weighed tonne has none of this. It is a reading from a machine, printed, numbered and tied to a vehicle. That is why our dispatch weight is the billed weight and why a slip goes with every load. We run ten calibrated weighbridges across our operations, because billing arguments cost more than a weighbridge does.
Where shortage arguments come from
- Tare not weighed. A stored or assumed tare is the commonest cause and the easiest to remove.
- Moisture. Wet material weighs more than dry. Crusher dust, GSB and WMM all hold water, and a load weighed in the rain includes it. The bridge is not wrong. The contract has to say whether the billed weight is as dispatched.
- Two bridges. Any two weighbridges give slightly different readings for the same vehicle. Without an agreed tolerance, every small difference becomes a claim.
- Spillage and diversion. An uncovered, overfilled body sheds material on a rough road. A load that leaves the plant and never reaches the site is a different problem, and the times on the slips at each end are how it is caught.
- Slips not matched to transit passes. The pass and the slip should carry the same vehicle, date and quantity. Where they do not, the load is hard to defend at audit, whatever it weighed. Royalty and transit passes explains what the pass is.
Calibration is a legal matter
A weighbridge used for trade comes under legal metrology. It is verified and stamped by the state Legal Metrology department at set intervals, and the operator holds a certificate saying so. That certificate is worth asking any supplier for, along with the date of the last verification. A bridge with a lapsed stamp is a weak basis for a bill, and the buyer's auditor will say so before the supplier does.
Between verifications, a careful operator checks the bridge in simple ways: the same vehicle weighed twice, and a zero reading with nothing on the platform. Mud packed under the platform and water standing in the pit are ordinary reasons a bridge drifts. The slip also records what the vehicle weighed on a public road, which is why tipper payloads and the overloading rules are worth reading alongside it.
A month-end reconciliation in five steps
- Get the supplier's dispatch statement for the month: every slip number, date, vehicle, material and net weight.
- Get the site's receiving register for the same period, and the site's own slips if there is a bridge at the road head.
- Match them load by load on slip number and vehicle number. Do not start from the totals.
- List the exceptions: slips with no receipt, receipts with no slip, differences beyond the agreed tolerance, the wrong material, and loads with no matching transit pass.
- Settle the exceptions one at a time, sign off the agreed tonnage by product, and raise the bill on that figure.
Done monthly, the exception list is short and the people who loaded and received each vehicle still remember it. Left for a quarter, it becomes a negotiation about averages.
Where an aggregate rate contract sets the weighing method and the tolerance in advance, the whole exercise is clerical. That is the purpose of delivered supply with a slip against every load: reconciliation that takes minutes instead of meetings.
Standards and sources
- Weighbridge count and dispatch practice: SM Infra's own operating record
- Verification and stamping of weighbridges used for trade is a legal-metrology matter handled by the state Legal Metrology department. No interval or tolerance is quoted here; check the current rules and your contract
Your contract and its technical schedules override anything written here. Check the clause before you build to it.