The short answer
A weighbridge slip records three weights. Gross is the truck with its load, tare is the truck empty, and net is gross minus tare, which is the weight paid for. A slip that will stand up in a dispute also carries a serial number, the vehicle number, date and time in and out, the material and a signature at each end. Reconcile slips against receipts daily, not at month end.
On a package moving a thousand tonnes a day, a discrepancy of one per cent is ten tonnes a day and three hundred a month. Nobody steals that in one go. It leaks through small habits at the bridge, and a routine is what stops it.
01Reading the slip
| Field | What it records | Why it matters |
|---|---|---|
| Serial number | A running number from the bridge software | A missing number in the sequence is a missing load |
| Vehicle number | Registration of the truck | Ties the load to a vehicle and its known tare |
| Gross weight | Truck plus load, in kg | First weighing when loaded |
| Tare weight | Truck empty, in kg | Should be weighed, not typed in |
| Net weight | Gross minus tare | The quantity invoiced |
| Date and time, in and out | When each weighing was taken | Shows a tare taken days earlier, or an impossible trip time |
| Material and source | Product and plant | Stops 10 mm being billed as 20 mm |
| Signatures | Bridge operator, driver, receiver | Without the receiver's, the load has only left |
02Tare is not a constant
An empty tipper does not weigh the same on Tuesday as it did on Monday. A full diesel tank against a nearly empty one is over 200 kg. Mud packed under the body after rain, a spare wheel added or removed, water in the body and a driver standing on or off the platform all move it. A bridge that uses a stored tare from last month is billing an estimate.
The sound practice is to weigh the truck empty on every trip. Where that is not practical, weigh the tare at least once a shift and never accept one keyed in by hand.
03Where the difference comes from
| Cause | Typical size | How to tell |
|---|---|---|
| Two bridges, each within tolerance | A few tens of kg a load | Small, and in both directions |
| Moisture drying out on a long haul | Up to 1 or 2% on fines | Only on dust and GSB, only in dry weather |
| Spillage from a heaped load | Small | Stone along the route |
| Stored or manipulated tare | 100 to 500 kg a load | Always in one direction |
| Truck not fully on the platform | Large | Odd gross weights for a known truck |
| Load diverted | A whole load | Slip issued, no receipt, long trip time |
Differences that go both ways are measurement. Differences that only ever go one way are behaviour.
04A daily routine
- At the plant, the bridge prints the slip in triplicate: one stays, two travel with the driver.
- At the road head, the receiver weighs or checks the load, signs both copies and keeps one.
- Each evening, the dispatch list by serial number is matched against the signed receipts. Every serial should have a partner.
- Any load with no receipt is chased that evening while the driver and the receiver both remember the day.
- Trip times are scanned. A 20 km haul that took three hours needs an explanation.
- Once a week the totals are agreed between the two sides and initialled. Month end then adds four numbers.
05Keeping the bridge honest
A weighbridge used for trade must be verified and stamped by the Legal Metrology department, and re-verified every year. It also has to be checked again after it is moved, which matters for a bridge that follows a crusher plant from site to site. Ask to see the current verification certificate. Between verifications, a quick check is to weigh the same loaded truck on two bridges on the same day.
Agree in the contract which bridge governs and what difference between the two will be accepted without query. Once that tolerance is written down, the conversation about a discrepancy is short.