Why two weighbridges give different weights

The load left you at 32,140 kg. It arrived, and your customer's scale said 31,900. A gap of 240 kg, and both sides are certain their scale is right. This happens at practically every weighing site, and it usually ends in a grudging discount — because nobody has anything to settle it with.

The fact that should reassure both parties: a difference is normal. A scale is a measuring instrument, not a counter, and every measurement has limits. The right question is not “why is there a gap?” but “is this gap inside the permitted range or outside it?” — and that question has a documented answer.

First: how much difference is acceptable at all?

Every scale has a division — the smallest step it displays, typically 10 or 20 kg on a weighbridge — and an accuracy class recorded on its calibration certificate. The error permitted in service is expressed as multiples of that division, not as a percentage of the load.

Take your calibration certificate and read the figure written on it — that is your reference, not your estimate. And if your scale divides in 20 kg while your customer's divides in 10, you already have a rounding difference before any real cause enters the picture.

Real changes in the load — with neither scale at fault

This is the part most often overlooked: sometimes both scales are perfectly correct, and it is the load itself that changed between the two measurements.

• Fuel burned. A truck burns tens of kilos over a hundred kilometres. You weighed it with a full tank; it arrived with a lower one. That difference is real and measurable.
• Moisture. Sand, grain and organic waste lose or gain water in transit. A load that left after rain and arrived after hours of sun genuinely weighs less.
• Mud and dust. What clings to the tyres and chassis at loading can fall off on the road — it leaves inside your weight and does not arrive inside theirs.
• Spillage in transit. A loose cover over bulk material means a genuine loss that shows up only on the second scale.

Differences caused by how you weigh — these are fixable

Zero was never checked

By far the most common cause. The scale does not quite return to zero after a vehicle leaves — dirt, standing water, or the start of real drift — and that offset then enters every ticket after it. A daily glance at zero catches it; what follows is in the calibration article.

The vehicle is not fully on the platform

One axle off the deck, or a wheel on its edge, is enough to move the figure by hundreds of kilos. It happens by accident as readily as on purpose — covered in the article on stopping fraud.

Slope around the deck

If the approach or exit is on a gradient, load distributes across the axles differently than on a scale with level ground. The difference is small but consistent in direction — so it shows up on every load with the same customer, and starts to look deliberate.

Recording before the reading settles

A figure recorded while the indicator is still moving is a random value inside a range. The other scale recorded after stability, so the two disagree — and the cause is not the scale but the moment of capture.

A stored tare instead of an actual one

You used the vehicle's saved tare; your customer weighed it empty for real. Tare changes with fuel, equipment and mud, so the difference here is in where the number came from, not in either scale's accuracy.

How to settle it in practice

1. Compare the tickets, not the numbers. Times, gross, tare, net, and each scale's division. Many disputes end right here: one side used a stored tare, or six hours separate the two weighings.
2. Re-weigh immediately, both ways. Same vehicle, same load, both scales, as close together in time as possible. A gap that repeats at the same size and direction means one scale has drifted. A gap that varies randomly is a stability or capture problem.
3. Check zero before and after each weighing in that test, and write it down. An unstable zero explains the whole dispute on its own.
4. Use a neutral reference. A sealed load of known weight, calibration weights, or an accredited third scale. Only this tells you which scale has drifted, rather than merely that they disagree.
5. Call a qualified calibration technician if the repeated difference exceeds what the calibration certificate allows.

The real fix: agree in writing before the dispute

Most of these arguments are settled by a contract clause, not by technology. Agree with your customer in writing — before the first load — on four things:

• Which scale governs when the two disagree, and that its calibration is current.
• The tolerance below which no dispute is opened at all.
• The window for objections — say 24 hours from delivery, after which transit losses cannot be separated from weighing differences.
• The resolution procedure once the tolerance is exceeded: re-weigh, then a neutral reference.

That clause will save you more repeated argument than any improvement to the scale itself.

What makes your ticket the one that is believed

In any dispute, the side with a record wins — not the side with a number. A figure written on paper proves nothing. A ticket carrying the time to the second, who recorded it, whether the reading was stable, which scale it came from, and a photo of the vehicle at the gate is not something anyone argues with.

That is the practical difference between a system that displays a weight and one that protects your position: the first hands you a number, the second hands you what proves it a month later.