Belt scale calibration | Mätarkontroll

How it works

Measuring mass flow in motion

One or more load cells measure the material load on a weigh bridge (kg/m). A speed sensor measures the belt speed (m/s). The weighing electronics multiply the signals, compute the instantaneous flow rate (t/h) and totalize the quantity passed.

Milltronics belt scale bridge with load cells — Milltronics belt scale bridge: load cells beneath the running belt, a speed sensor and weighing electronics that integrate to t/h.

Process control

How much right now

The instantaneous flow rate drives dosing, blending and energy efficiency in real time.

Inventory management

How much in total

Totalizing for inventory and reconciliation of material flows.

Load-out

Truck, train & ship

Loading and continuous monitoring — including for custody transfer applications.

Industry

From mine to power plant

Aggregates, cement, coal and biofuel, recycling, food and feed.

News · Weighing terminal

SIWAREX WT500 — a new generation of weighing electronics

Siemens' latest stand-alone terminal for belt scales, weighfeeders and solids flowmeters. We follow it closely — here is what makes it interesting for anyone who wants more accurate, more reliable and simpler weighing.

SIWAREX WT500 weighing terminal from Siemens

Accuracy 0,1 % of full scale, resolution 0,02 % — precise measured values
Handles 1, 2, 4 or 6 load cells — from simple to advanced scales
Moisture and incline compensation keeps the reading stable despite material and conveyor variations
Weight and chain factoring — new or unknown test references are aligned to the current span

Easy commissioning

Guided setup

Commissioning and zero/span calibration wizards make setup simple, with manual fine adjustment for critical applications.

Control & alarms

PID and diagnostics

PID control of process values and advanced alarms for flow, load, speed and diagnostic faults.

Connectivity

All common fieldbuses

Modbus RTU and TCP/IP, PROFIBUS DP as well as PROFINET and EtherNet/IP (upcoming). Built-in webserver via Ethernet for remote monitoring and configuration; Mini-USB service port.

Data & traceability

8 GB Micro-SD

Logs changes to parameters and firmware as well as an alarm history — easy to follow up and restore.

Build & operation

Rugged and energy-efficient

IP65/Type 4X polycarbonate enclosure, removable terminal blocks for easier wiring and an EcoTech Profile that reduces energy consumption.

Operation

Clear HMI

Six configurable graphic screens with intuitive menu navigation, four keys and 21 languages.

Environment & life cycle

Energy-efficient and sustainable

12.1 W at full load and a 10-year reference service life. Eco-design per IEC 62430, RoHS/REACH/WEEE and high recyclability — with an environmental product declaration (EPD Type II per ISO 14021).

Where they are used

Two examples from the real world

Belt scales sit where material moves — at receiving, transfer and load-out. Two typical flows:

From train to ship: receiving hopper, belt conveyor with belt scale and load-out to a ship — Example 1 — From train to ship: bulk material is tipped from rail and a belt scale on the transfer conveyor totalizes the quantity loaded on board — the basis for stock balances, invoicing and load planning (custody transfer).

Example 2 — Quarry with wheel loader: crushed rock is fed by conveyor and a belt scale measures the flow to stockpile and load-out — for production monitoring, fraction dosing and truck load-out.

Read also about general weighing — hopper and silo weighing, dosing and process protection.

Why calibration

Only as accurate as the reference

A conveyor belt scale is only as accurate as the reference it is calibrated against — and the error carries straight through: a 2 % error on the speed gives a 2 % error on the flow. That is why both load and speed are set against a known reference.

Zero and span: the average of the signal over one belt revolution — Zero = the mean weight of the empty, warmed-up running belt. Span = the reference with test weights. Zeroing must always be done on an empty running belt.

Four calibration methods

Static test weights

Weights are placed on the scale and simulate the material load 1:1 thanks to direct force transfer — on an empty running belt.

Test chains

High-precision roller chains apply a uniform, known load (kg/m) onto the belt itself — calibration under real loading and tension conditions.

Electronic calibration

The span is set from load cell data (capacity, sensitivity, excitation) when weights or chains are not possible. Always supplemented with zero calibration.

Material test

The most accurate method — real material is compared against a trusted reference scale. The deviation is corrected in the integrator.

Linearization

Correct across the whole measuring range — not just at one point

A single-point adjustment may hit the mark at one flow but miss the others. With multi-point linearization the measurement error is straightened out across the entire flow range.

Measurement error across the flow before and after linearization

Belt tension determines linearity

Too loose tension makes the belt sag and material can spill over — non-linear load cell loading. Too tight tension makes the belt and material form a bridge over the scale — also non-linear.

Ideally the belt sags at most 2 % of the idler spacing at nominal load
Linearity is checked by comparing the load cells' mV signal on an empty belt and with test weights
Constant belt tension — preferably with a gravity take-up station

Verification

Accurate — and repeatable

Verification is done with a material test against a reference scale that is more accurate than the belt scale (for example a vehicle scale). It is not enough to hit the mark once — the result must be repeatable.

Accuracy and repeatability — three outcomes

ReferenceMust be more accurate than the belt scale

FlowAt least 50 % of design capacity

Test timeAt least 3 belt revolutions or 10 minutes

NumberAt least three tests — the mean deviation becomes the correction factor

What the outcome means

Repeatable but wrong = a systematic error that is corrected in the integrator. Neither correct nor repeatable = a mechanical problem — then installation, alignment and belt are checked before the scale is re-adjusted. Correct and repeatable = an approved scale.

Installation & accuracy

Accuracy is built in during installation

A belt scale never becomes more accurate than its installation. The right location, a rigid frame and exact alignment are the prerequisites for the calibration to hold.

Alignment

Tolerances in millimeters

The weigh idlers are aligned to the neighboring idlers (at least 2–3 on each side): ±0,8 mm for a 0,5 % scale, ±0,4 mm for a 0,25 % scale. The belt must not lift.

Accuracy classes

From ±0,5 % to ±0,125 %

A single-idler scale reaches ±0,5 %, a two-idler scale ±0,25 % and a three-idler scale ±0,125 % — higher requirements call for more weigh idlers and better installation.

Load cells

Direct force transfer

Parallelogram-type load cells compensate for horizontal forces, have no moving parts and feature built-in overload protection — high reliability, low maintenance.

Speed sensor

Slip-free measurement

The sensor is mounted against the return belt or on an end/bend pulley so that it is driven without slip — otherwise pulses are lost and the speed (and thus the flow) is measured too low.

Service & maintenance

The calibration does not hold on its own

Temperature drift, material build-up and mechanical interventions change the scale over time. Regular service keeps the accuracy up.

Regular zeroing

Temperature drift and material build-up affect belt tension and zero point. Zeroing is done on an empty running, warmed-up belt (15–30 min warm-up) and over one or more complete belt revolutions. Auto-Zero can handle zero tracking automatically within a defined range.

Cleaning and mechanical inspection

Clean the scale regularly — the idler station must not bind. Check that all idlers turn, that the belt tracks properly, and that no covers or skirts press against the idler stations in the weigh area.

Recalibration after changes

If the belt or support idlers are replaced, or other mechanical work is carried out, the scale must be re-aligned and recalibrated. When welding near load cells, these must be disconnected and grounded so that the welding current is not conducted through the cells. Use transport locks during mechanical work.

Diagnostics and monitoring

Modern weighing electronics monitor each load cell and the speed sensor for cable breaks, measure the load cells' impedance and record limit-value exceedances with a time stamp. When integrated into SIMATIC, all measured values and operating states are visible remotely.

Factors that affect accuracy

Installation and alignment (idlers aligned within approx. ±1 mm), the belt's tension and tracking, the quality of the idler stations, material build-up and roll-back on steep belts, as well as correct parameterization (measuring range, pulse constant, test load).

Calibration, linearization and verification of belt scales