A weighbridge is a robust instrument, but it operates in a harsh environment. Sand, mud, leaves, snow and ice collect around the deck, and heavy vehicles drive over it every day. Many of the errors we find on weighbridges have a simple cause that the operator could have spotted. A fixed routine prevents a weighbridge from weighing incorrectly for weeks or months.
This article gives a practical approach to daily and weekly checks, working with a reference vehicle, the warning signs of load cell problems and the moment at which a calibration or re-verification of the weighbridge is needed.
Why a weighbridge is sensitive to dirt and obstructions
The deck of a weighbridge rests on load cells. All of the force must pass through those load cells to the foundation. If the deck finds support anywhere else, for example on a layer of sand against the end, a stone in the gap beside the deck or a ridge of ice, part of the weight bypasses the load cells. The weighbridge then reads low, and often not by the same amount everywhere.
The insidious part is that the indication looks normal. There is no error message and the zero may be perfectly correct. Only under load does the result deviate. Temperature also plays a role: a deck expands in the sun and may then just touch an edge that was still clear in the morning.
Daily checks
These checks take little time and can be done by the weighbridge operator, for example at the start of the day:
- Zero: with the weighbridge empty, check that the indication reads zero and is stable. A zero that drifts or does not settle is a reason to look further.
- Clearance beside the deck: look along the gaps at the sides and the ends. Nothing may be wedged between the deck and the foundation or approach ramp.
- Visible dirt: remove mud, sand and loose material on and around the deck, especially where soil, waste or agricultural products are handled.
- Snow and ice: clear the gaps and edges before weighing. Do not use agents that can damage cables or load cells, and follow the supplier's instructions.
- Weighing procedure: the vehicle is fully on the deck, stationary, and nobody gets in or out during the weighing.
Weekly and periodic checks
- Space under the deck: on a weighbridge in a pit or on a raised foundation, dirt accumulates under the deck. Have that space cleaned periodically, more often in autumn and in wet, sandy conditions.
- Drainage: check that water drains properly from the pit or around the foundation. Standing water promotes corrosion and can freeze in winter.
- Stops and bearings: check that the movement limiters are not binding and that the load cell mountings look normal, without misalignment or visible damage.
- Cables and junction box: check the cables for damage and the junction box for moisture. Leave the opening of sealed components to an authorised technician.
- Approach and exit ramps: holes and subsidence at the ramps cause impacts when vehicles drive on, which puts extra strain on the load cells.
Working with a reference vehicle
One of the most practical checks is regularly weighing a reference vehicle: a vehicle with a known weight that is as constant as possible, such as your own loader, forklift or empty lorry.
- Weigh the vehicle several times immediately after a calibration and record the average as the reference value.
- Then weigh it at fixed moments, for example weekly, under comparable conditions: roughly the same fuel level, without load and without extra passengers.
- Preferably also weigh it in different positions: at the front of the deck, in the middle and at the rear. Large differences between positions point to an obstruction or a problem with one of the load cells.
- Record the results and agree in advance on a limit at which you take action.
A reference vehicle is not a calibrated weight. Fuel, dirt and rainwater change its mass. So choose the limit realistically: the aim is to detect a shift, not to calibrate the weighbridge. The same idea lies behind interim checks on other scales, see checking your scale between calibrations.
Signs of load cell problems
Load cells and their cabling are the sensitive parts of a weighbridge. Damage is caused by, among other things, moisture, corrosion, overloading, heavy impacts and surges after a lightning strike. Watch for these signs:
- a zero that keeps drifting or does not become stable;
- an indication that varies for the same vehicle without a clear reason;
- clear differences depending on the position of the vehicle on the deck;
- a weighbridge that does not return to zero after the vehicle drives off;
- error messages on the indicator, or abnormal behaviour after a thunderstorm or flooding.
When you see these signs, first check that nothing is wedged and that the deck is clean. If the problem persists, have a technician test the load cells and connections. On a verified weighbridge, work on the load cells or indicator is a repair to the weighing system, which makes re-verification mandatory.
When to calibrate or re-verify?
A periodic calibration according to EURAMET cg-18 provides the evidence that the weighbridge weighs accurately. Annually is a common interval. With intensive use, or if you invoice by weight, many companies choose a shorter interval. How to justify the interval is explained in how often to calibrate a scale.
In addition, have the weighbridge calibrated earlier if the reference vehicle shows a persistent deviation, after a collision or subsidence, after work on the foundation and after replacing components. If the weighbridge is verified, re-verification is mandatory after a repair to the weighing system or a broken seal; see re-verification after repair. In the Netherlands there is no fixed re-verification interval for weighbridges, but as the user you are responsible for ensuring that the weighbridge weighs within the legal tolerances.
How MASSLAB helps
MASSLAB calibrates weighbridges on site with calibrated mass pieces from our own laboratory, using our own transport and equipment. During the visit we also inspect the deck and the space around it, and report anything that affects the weighing. As a Recognised Verifier (Erkend Keurder) under the Dutch Metrology Act, we carry out the re-verification of verified weighbridges after repair or adjustment, once we have checked that we are permitted to verify that instrument. If you want to carry out a test with heavy weights yourself, you can hire mass pieces. More information can be found at weighbridge calibration.