Technology Zone
Weighing Scales and Systems
Every recyclable is bought and sold by weight. This zone covers truck scales, floor and hopper scales, belt weighers and the software that makes weighing data part of billing, inventory and plant control.
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What this zone covers
Weighing Scales and Systems: how it works, key numbers and troubleshooting
Fundamentals
Weighing is the invoice
Every tonne of recyclables is bought and sold by weight, so the weighbridge is the cash register of the plant. Truck scales (pit or surface mounted, 60–80 t capacity) weigh loads in and out; the difference is the invoice, the compliance report and the input to the mass balance. Legal-for-trade approval, verification intervals and a documented calibration routine are not optional.
Scales inside the plant
Belt scales on conveyors measure flow per stream; hopper and bunker scales on load cells measure inventory and batch weights; floor and pallet scales weigh bales, big bags and containers; on-board weighing on loaders, handlers and refuse collection vehicles records every load at the point of handling. Together they give the mass balance that proves recovery rates and finds losses.
Load cells and installation
A load cell converts force into a signal; the scale error usually comes from the installation: rigid connections, side loads, temperature, water in junction boxes, debris under the platform, and vibration from nearby machinery. Sealed stainless cells, lightning protection, flexible connections and a level, drained foundation decide whether a scale holds its accuracy through a recycling plant's winters.
Automation of the weighbridge
Unattended weighbridges with number-plate recognition, driver terminals, RFID and traffic lights cut queues and errors; the weighing software links each transaction to the customer, the material, the contract price and the permit's waste codes, and exports to invoicing and to the regulator's reporting. Cameras record the load for disputes about contamination.
Compliance and data
Permits require reporting of tonnes accepted per waste code and tonnes sent out per destination; extended producer responsibility schemes require proof of recycled tonnes; customers require certificates. The weighing system is the source of record for all of it, so its data model — material, source, destination, contract, vehicle, time — matters as much as the scale's accuracy.
Key parameters
| Parameter | Typical range | Rule of thumb |
|---|---|---|
| Truck scale capacity and length | 60–80 t; 16–24 m | Whole vehicle on the deck; longer trucks need longer decks or split weighing |
| Truck scale accuracy | Class III; verification scale interval 20 kg at 60 t | Legal-for-trade with periodic verification; keep the test-weight certificate |
| Belt scale accuracy | ±0.5–1 % | Weekly zero, monthly span; idler alignment |
| Hopper / bunker scale | ±0.1–0.5 % of capacity | Flexible connections; cells at 30–80 % utilisation |
| On-board loader weighing | ±1–2 % per load | Calibrate per attachment; good for internal accounting, not invoicing |
| Floor scale for bales | 1.5 × 1.5 m; 1–3 t; ±0.5 kg | Bale weight and count feed the sales record |
| Weighbridge throughput | Attended 20–30 trucks/h; unattended 40–60 | Number-plate recognition and terminals remove the queue |
| Load cell protection | IP68/69K stainless; lightning and surge protection | Water in the junction box is the commonest scale failure |
Troubleshooting
| Symptom | Likely causes | What to do |
|---|---|---|
| Weighbridge reads unstable or drifts | Debris under the deck; water in junction box or cables; temperature; damaged cell | Clean under deck, seal and dry junction box, check each cell's output, re-verify |
| In and out weights inconsistent | Vehicle partly off the deck; tare stored for a different trailer; fuel and driver changes | Deck length, position sensors, tare per vehicle-trailer combination, tare expiry |
| Belt scale disagrees with weighbridge | Belt scale drift; material bypassing the scale; moisture change | Zero and span, mass balance review, moisture correction |
| Queues at the weighbridge | Attended weighing; paperwork; single lane | Unattended terminals, number-plate recognition, second scale or in/out lanes |
| Disputes over contamination and weight | No photo record; no sampling procedure | Cameras on the weighbridge and tipping floor linked to the transaction, sampling protocol |
| Reporting to the regulator takes days | Weighing software not linked to waste codes and destinations | Configure the data model, automated exports |
| Scale fails legal verification | Mechanical damage; cell drift; foundation settlement | Foundation check, cell replacement, pre-verification service |
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What this zone covers
Frequently asked about weighing scales and systems
Pit or surface-mounted truck scale?
Pit-mounted scales are flush with the yard, need no ramps and take less space, but the pit collects water and debris and needs drainage and access for cleaning. Surface-mounted scales are easier to install and maintain but need ramps and length. In a recycling yard with loaders and debris, the ease of cleaning under the deck often decides.
How often must a weighbridge be verified?
Legal-for-trade scales are verified at installation and at the interval set by national metrology rules — typically every one to two years — plus after any repair that affects weighing. In between, a monthly check with a known vehicle or test weights catches drift before it costs money or a failed verification.
Can I invoice on loader or belt scale weights?
Not normally: legal-for-trade requirements apply to weights used for invoicing, and on-board and belt scales are usually not approved for that. Use them for internal accounting, inventory and the mass balance; invoice on the weighbridge or an approved hopper or floor scale.
How do I make the weighbridge unattended?
Number-plate recognition or RFID identifies the vehicle, a driver terminal confirms the material and destination, traffic lights and barriers control the flow, cameras record the load, and the software books the transaction against the contract and waste code. Throughput doubles and errors fall; an attended lane stays for exceptions.
Why does my scale read differently in winter?
Temperature affects load cells slightly, but the usual causes are ice and debris under the deck, water in junction boxes and cables, and frozen or stiff flexible connections. Heated pits, drainage, sealed cells and a cleaning routine keep the scale within tolerance; re-verify after a cold snap if readings shift.
What data should the weighing system hold?
For each transaction: vehicle and trailer, customer or supplier, material and waste code, source or destination, contract and price, gross-tare-net, time, operator, and links to photos and sampling results. That record serves invoicing, permit reporting, EPR proof and disputes; design it before choosing the software.
How do I check that my mass balance is right?
Sum the weighbridge inputs, sum all outputs (products, residues, rejects) and compare with the change in stock measured by hopper scales and stock counts. A closure within 2–5 % over a month is good; a larger gap means an unmeasured stream, a drifting scale or a moisture change between reception and dispatch.
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