When does a weighbridge need calibration?

A weighbridge rarely fails outright. It drifts — a kilo here, a kilo there — until the gap is wide enough to show at stocktake, by which time you have been buying and selling on wrong numbers for months.

The good news is that drift signals itself long before that, and most of those signals you can spot yourself without any equipment.

Signs that your scale needs checking now

Zero does not return to zero

After a vehicle leaves, the reading should settle at exactly zero. If it rests at 20 or 40 kg, or if zero differs between morning and evening, something is worth looking at: dirt under the deck, standing water, or a load cell whose characteristics have begun to change.

The figure changes with position

Same vehicle, same load, but the weight differs depending on whether it stops at the front, the back, or to one side. This is the classic symptom of uneven load distribution across the cells — and it is also what makes positioning worth manipulating.

The reading will not settle

Constant fluctuation of tens of kilos with the vehicle completely still. The cause is usually electrical — poor earthing, a damaged cable, interference from nearby equipment — rather than the scale itself. But it turns every reading into a matter of when the button was pressed.

A consistent difference with someone else

The strongest signal of all: your weight is lower (or higher) than the customer's or the supplier's consistently, in the same direction. Random differences are normal; a systematic one means one of the two scales has drifted — not necessarily yours, but you need to know which.

Two checks you can run yourself

Neither replaces a formal calibration, but they catch the problem early and answer the question “do I call the technician?”

1. Repeatability: one loaded vehicle, weighed five times, driving off and back on each time. The differences should be very small. If results vary by hundreds of kilos, you have a problem regardless of anything else.
2. Position check: the same vehicle weighed at the front, then the middle, then the back. A large difference between positions points to a cell or a mounting that needs adjustment.

Record both with the date. The value is not in a single reading but in the comparison: is the drift growing?

When you need a calibration technician

Real calibration needs traceable test weights and a qualified body — it cannot be substituted with an estimate or a comparison against another scale. Call one when:

• Either check above fails, or complaints about differences with a third party recur.
• After civil work near the deck, a replaced load cell, or a new indicator.
• After an overload incident or a heavy impact on the deck.
• When the periodic calibration falls due.

The interval itself is set by your local authority's requirements, the manufacturer's guidance and how hard you use the bridge — a scale running a hundred times a day is not one running five. Check the rules that apply to you rather than a general figure: non-compliance here has legal consequences, not just operational ones.

What protects you between calibrations

A calibration certifies that the scale was accurate on one day. Between them, the only thing that tells you something has changed is your own record of readings: average load per vehicle, how a given vehicle's tare moves across weeks, and any systematic difference with a particular counterparty.

That requires readings captured automatically and timestamped — not written in a notebook. Then spotting drift is a glance at a chart rather than a surprise at year end.

In short

• Watch the return to zero daily — the cheapest check and the most telling.
• Run the repeatability and position checks monthly, and record the result.
• Treat a consistent difference with a counterparty as an alarm, not a commercial dispute.
• Calibrate per your local requirements, and after any physical work on the scale.
• Keep a time series of your readings — it is what exposes slow drift.