A belt scale can only provide reproducible flow-rate and totalized quantity values when the weighbridge, conveyor belt, speed measurement and evaluation electronics are tested and matched to one another as a complete dynamic measuring system.
Unlike a static scale, the measurement result is generated while the conveyor belt is moving. The belt scale measures the material load acting over a defined weighing section, while a speed sensor simultaneously measures the current belt speed.
The instantaneous mass flow rate is calculated from these two quantities.
Mass flow rate = belt loading × belt speed
Even small errors in the zero point, speed or test load therefore have a direct effect on the calculated conveying rate and the totalized material quantity.
Reliable calibration therefore does not begin with applying a test weight, but with a mechanical inspection of the conveyor, verification of the belt speed and a reproducible zero point with the empty belt running.
Suitable systems from the ICS portfolio include, for example, the Milltronics MSI precision belt scale, the Milltronics RBSS speed sensor and the Milltronics BW500 integrator. Test chains and the Milltronics MWL calibration system are also available for recurring adjustment work.
Further solutions can be found under belt scales and under force, weighing, speed and torque sensors at ICS Schneider.
Table of Contents
- How does a belt scale measure?
- Distinguishing calibration, adjustment and alignment
- Why the mechanics must be checked before calibration
- Determining the zero point with the empty belt running
- Why the zero point should be checked several times
- Measuring belt speed correctly
- Avoiding slip in speed measurement
- Span calibration
- Calibration using test weights
- Calibration using a test chain
- Material test as a process verification
- Influence of belt tension and belt stiffness
- Checking weighing idlers and alignment
- Considering uniform material loading
- What happens when belt speed changes?
- Considering temperature and operating condition
- When should the system be checked again?
- Practical example: belt scale consistently reads too low
- Typical fault patterns
- Recommended calibration procedure
- Suitable weighing technology from ICS Schneider
- Conclusion
- FAQ
How does a belt scale measure?
A belt scale essentially combines two measured quantities:
- material loading on the conveyor belt,
- belt speed.
The weighbridge or weighing idler station measures the vertical force of the conveyed material over a defined weighing section.
This results in a belt loading, for example in:
kg/m
At the same time, the speed sensor measures the movement of the conveyor belt, for example in:
m/s
The mass flow rate is calculated in simplified form as:
q = m' × v
where:
q= mass flow rate,m'= material mass per unit belt length,v= belt speed.
Example
The measured belt loading is:
25 kg/m
and the belt speed is:
1.5 m/s
This results in:
25 kg/m × 1.5 m/s = 37.5 kg/s
or:
135 t/h
The total quantity is then calculated by integrating the flow rate over time.
An error of, for example, 2% in the speed measurement can therefore directly cause a corresponding systematic error in the calculated material flow rate.
Distinguishing calibration, adjustment and alignment
In everyday industrial practice, the term calibration is often used generally for belt scales, even when the device settings are also corrected as part of the procedure.
From a metrological point of view
Calibration initially means determining the deviation of a measuring system from a known reference.
Adjustment or alignment, on the other hand, means setting the measuring system so that the identified deviation is reduced.
A typical maintenance procedure for a belt scale therefore consists of:
- checking the current condition,
- determining the deviation,
- correcting the zero point or span if necessary,
- checking the result again.
For this article, the term “calibration”, as commonly used in practice, refers to the entire checking and adjustment procedure.
Why the mechanics must be checked before calibration
Electronic correction cannot reliably compensate for a mechanically defective conveyor system.
Before each calibration, it should therefore be checked whether the conveyor is in a reproducible mechanical condition.
The following should be checked in particular
- belt tracking,
- belt tension,
- weighing idlers and adjacent carrying idlers,
- alignment of the idler stations,
- material deposits,
- mechanical stress,
- contamination of the weighbridge,
- loose or moving structural components,
- contact between the belt and external components.
Typical example
Several kilograms of material have accumulated beneath the weighbridge.
The operator simply performs a new zero adjustment.
This may compensate for the current condition, but the actual mechanical cause remains.
If the deposited material later falls away, the zero point shifts again.
A contaminated or mechanically stressed belt scale should therefore first be repaired and only then recalibrated.
Determining the zero point with the empty belt running
The zero point of a belt scale is not simply determined with the conveyor belt stationary.
The belt should be empty and running under normal operating conditions.
Why must the belt be running?
Even an unloaded conveyor belt generates dynamic influences due to:
- the belt’s own weight,
- belt splices,
- variations in belt stiffness,
- idler rotation,
- belt tension,
- mechanical irregularities.
These effects only occur fully while the belt is actually running.
Before zero calibration
the belt should therefore run for a sufficient period of time so that:
- the mechanical condition is stable,
- the belt tension corresponds to normal operation,
- no residual material remains on the belt.
The integrator then determines the zero signal over a defined period of time or a specific number of belt revolutions.
Why the zero point should be checked several times
A single successful zero adjustment provides only limited information about the repeatability of the system.
The zero point should therefore be determined several times in succession.
Example
| Zero test | Deviation |
|---|---|
| 1 | +0.08% |
| 2 | +0.06% |
| 3 | +0.07% |
These values indicate good repeatability.
Another example
| Zero test | Deviation |
|---|---|
| 1 | +0.1% |
| 2 | -0.8% |
| 3 | +1.2% |
In this case, it would make little sense simply to accept the last value.
The cause of the poor repeatability should first be identified.
Possible causes
- unstable belt tracking,
- pronounced belt splice,
- contaminated idlers,
- fluctuating belt tension,
- mechanical contact with the weighbridge,
- electrical signal problem.
For Siemens belt scale systems, it is recommended during routine testing to repeat zero calibration several times in order to confirm repeatability.
Measuring belt speed correctly
Alongside belt load, speed is the second key measured quantity of a belt scale.
Before calibration, it should therefore be checked whether the belt speed indicated by the integrator corresponds to the actual speed.
An independent check can be carried out, for example, using
- a suitable tachometer,
- a known belt length and time measurement,
- comparison with an independent reference sensor.
Measurement over a known distance
If a defined length is marked on the belt, the speed can be approximated using:
v = s / t
Example:
s = 20 m
and:
t = 10 s
results in:
v = 2.0 m/s
If the integrator simultaneously indicates only:
1.94 m/s
the speed measurement should be investigated before the actual belt scale adjustment is carried out.
Avoiding slip in speed measurement
A speed sensor should ideally measure the actual movement of the conveyor belt rather than only the rotational speed of the drive motor.
Why?
Differences can occur due to slip or mechanical changes between the:
- drive motor,
- gearbox,
- drive pulley,
- conveyor belt.
A sensor whose measuring wheel runs directly on the return belt measures the actual belt movement directly.
Milltronics RBSS
The Milltronics RBSS operates with a wheel running on the return belt and generates a pulse signal proportional to the wheel rotation.
It provides:
60 pulses per revolution
and is therefore suitable for high-resolution measurement of belt speed.
Contact with the belt is essential
The measuring wheel must:
- remain in constant contact with the belt,
- rotate freely,
- not be blocked by material deposits,
- be mechanically aligned correctly.
A slipping or bouncing measuring wheel directly results in incorrect speed measurement.
Span calibration
After establishing a stable zero point, the sensitivity of the scale is checked using a known test load.
This procedure is commonly referred to in belt scale systems as:
Span Calibration
.
A known mechanical load is applied to the weighbridge for this purpose.
The integrator knows the corresponding reference value and compares it with the measured signal.
Here too, repeatability is more important than a single value
The span calibration should be repeated several times where possible.
If the deviation changes significantly with each repetition, the mechanical system should first be investigated.
Calibration using test weights
Static test weights are a simple and reproducible way of applying a defined load to the weighbridge.
A known mass is applied to the weighing mechanism through the mounting points specified by the manufacturer.
Advantages
- known and reproducible test load,
- fast testing procedure,
- no conveyed material required,
- well suited for regular routine checks.
Limitations
A static weight does not reproduce all real operating conditions of a loaded conveyor belt.
In particular:
- belt deformation,
- material distribution,
- dynamic loading,
- belt tension under product load
are only simulated to a limited extent.
Milltronics MWL
The Milltronics MWL calibration system makes it possible to apply reference weights mechanically to the belt scale and raise them again afterwards.
It is designed for the following belt scale types:
- MCS,
- MSI,
- MMI,
- MUS
and enables the handling of weights up to:
340 kg
.
This is particularly advantageous for recurring tests because the weights can remain permanently on the calibration system.
Calibration using a test chain
A test chain reproduces material loading more dynamically than a purely static test weight.
The chain is placed on the running conveyor belt and restrained at both ends so that it remains within the intended weighing section.
The belt runs underneath the chain while the rollers or chain links roll on the conveyor belt.
The known mass per unit length creates a defined belt loading
This means that the force is not applied only at individual calibration points; instead, the conveyor belt itself is loaded within the weighing section.
Advantages compared with individual test weights
- more dynamic loading of the belt,
- loading over a longer weighing section,
- high test loads possible,
- better simulation of real material loading.
ICS offers special test chains for belt scales for this purpose.
A test chain does not automatically replace a material test
Even a test chain only mechanically simulates the actual conveyed material.
Material distribution, product characteristics and real operating conditions may still differ.
Material test as a process verification
During a material test, a known or independently determined quantity of material is actually conveyed across the belt scale.
The quantity totalized by the belt scale is then compared with the reference quantity.
Example
Reference quantity:
20.000 t
Belt scale:
19.760 t
The deviation is:
-0.240 t
or relative to the reference quantity:
-1.2%
Why the material test is particularly meaningful
It simultaneously takes into account:
- actual material loading,
- belt deformation,
- belt tension,
- speed measurement,
- weighbridge,
- integrator,
- totalization.
The material test therefore checks the entire measuring chain under actual process conditions.
For reliable comparison, the reference quantity should be determined using a suitable independent weighing system.
At different conveying rates
material tests at several load points may be useful.
This makes it possible to determine whether the belt scale operates with sufficient linearity over the actual operating range.
For the BW500, for example, Siemens describes material tests at different percentages of the design load for evaluating and, where necessary, linearizing the system.
Influence of belt tension and belt stiffness
The belt scale should ideally measure only the vertical gravitational force of the material.
In practice, however, the conveyor belt itself also influences the weighing idler station.
High or changing belt tension can
- transfer additional forces to the weighing station,
- change the zero point,
- influence sensitivity.
Particularly critical factors include
- strongly fluctuating tensioning systems,
- changed belt tension after maintenance work,
- new belts compared with heavily worn belts,
- temperature-related changes in the belt.
After replacing a belt or significantly changing the belt tension, the calibration should therefore be checked again.
Checking weighing idlers and alignment
The height of the weighing idler station relative to the adjacent carrying idlers is crucial to the measurement.
If an idler is set too high or too low, the mechanical force transmission into the weighbridge changes.
The following should be checked
- idler alignment,
- uniform idler spacing,
- free rotation,
- wear,
- material deposits,
- deformation of the supporting structure.
Even perfectly calibrated weighing electronics cannot reliably compensate for an incorrectly aligned idler station.
Considering uniform material loading
A belt scale can operate with varying material loading, but the material should remain as stable as possible on the conveyor belt within the weighing section.
Potential problems include:
- material being fed directly before the weighbridge,
- strongly bouncing bulk material,
- uneven lateral material distribution,
- strong acceleration or deflection zones.
The weighing station requires a stable belt section
There should be sufficient distance between the material feed point and the weighbridge to allow the conveyed material to settle on the belt.
Immediately adjacent deflection or drive pulleys can also influence belt forces.
What happens when belt speed changes?
A belt scale can generally measure correctly even at variable speed, provided that the actual belt speed is measured continuously and with sufficient accuracy.
Example
Belt loading:
20 kg/m
At:
1 m/s
the flow rate is:
72 t/h
At:
2 m/s
with the same belt loading, the flow rate is:
144 t/h
If the electronics continued calculating with 1 m/s, a major measurement error would result.
Always consider the speed signal as part of the calibration
Particularly with:
- variable-frequency drive operation,
- weighfeeders,
- changing process capacities
the speed measurement must function correctly throughout the entire relevant operating range.
Considering temperature and operating condition
A conveyor system may behave mechanically differently when cold and stationary than after several hours of operation.
As operating time increases, the following may change:
- belt temperature,
- belt tension,
- belt stiffness,
- bearing behaviour,
- mechanical expansion.
Routine calibration should therefore be carried out, where possible, under conditions representative of normal plant operation.
Seasonal changes may also be relevant
Outdoor conveyor systems can show significant differences between summer and winter operation.
Temperature, humidity, contamination and weather conditions can influence the mechanics and therefore also the zero point.
When should the system be checked again?
A belt scale should not be checked only immediately after commissioning.
A new zero check and, where necessary, span check is particularly advisable after:
- belt replacement,
- changes to belt tension,
- replacement of carrying or weighing idlers,
- work on the weighbridge,
- replacement of a load cell,
- work on the speed sensor,
- major material buildup,
- noticeable changes in measurement results,
- long shutdown periods.
Regular periodic testing is also advisable even when no obvious fault is present.
Siemens explicitly points out that temperature and weather changes, maintenance work and material buildup on the weighbridge, belt or idlers can be reasons for recalibration.
Practical example: belt scale consistently reads too low
A belt scale regularly determines a lower total quantity during loading than a downstream reference weighing system.
Deviation
Reference:
100.0 t
Belt scale:
96.8 t
The belt scale therefore reads approximately:
3.2%
too low.
1. Mechanical inspection
The weighbridge is clean and moves freely.
The idlers are correctly aligned.
2. Zero test
Three zero runs produce almost identical results.
The zero point is therefore reproducible.
3. Speed test
The integrator indicates:
1.50 m/s
However, an independent measurement gives:
1.455 m/s
The deviation is approximately:
3.1%
Cause
The measuring wheel of the speed sensor has an incorrect effective circumference or the scaling of the speed input is no longer correct.
The scale itself and its load cells are not the cause.
After correction
The speed measurement is recalibrated and another material test is then carried out.
Only after this correction do the totalized quantities agree within the required tolerance.
This example shows why a deviation in conveyed quantity should not automatically be corrected by performing a new load-cell or span adjustment. It must first be determined which part of the dynamic measuring system is causing the deviation.
Typical fault patterns
| Observation | Possible cause | Recommended check |
|---|---|---|
| Zero point changes constantly | Belt tracking, contamination or mechanical stress | Perform several zero tests and inspect the mechanical system |
| Zero point is correct, but material quantity is not | Incorrect span or speed | Check speed and test load |
| Deviation is proportional across the entire range | Scaling or span error | Perform span calibration and material test |
| Deviation changes with conveying rate | Non-linearity or mechanical influence | Perform material tests at several load points |
| Reading fluctuates strongly | Unstable material loading or mechanical vibration | Check installation location and conveyor mechanics |
| Speed reading jumps | Measuring wheel loses contact with the belt | Check speed sensor and installation |
| Measurement is correct at only one belt speed | Incorrect speed scaling | Compare several speed values |
| Significant deviation after belt replacement | Changed belt tension or stiffness | Repeat zero and span calibration |
| Different zero point after maintenance | Changed idler position or belt tension | Check mechanical alignment |
| Test weight is correct, but material test is not | Real belt or material influences are not represented | Analyze mechanics and material test |
| Winter and summer values differ | Temperature and belt-tension influence | Check seasonal zero-point stability |
| Totalized quantity drifts over time | Small systematic error in load or speed | Perform reference material test |
Recommended calibration procedure
- Secure the system and perform a visual inspection: Check for contamination, damage and mechanical abnormalities.
- Clean the weighbridge: Completely remove material deposits.
- Check the idlers: Verify free rotation and correct alignment.
- Check belt tracking: Exclude lateral wandering and unusual movement.
- Check belt tension: Establish normal operating conditions.
- Check the speed sensor: Verify mechanical installation and secure contact with the belt.
- Measure belt speed independently: Compare the indicated value with a reference.
- Run the conveyor belt empty: Establish a stable mechanical operating condition.
- Perform zero calibration: Run the empty belt for the specified period.
- Repeat zero calibration: Check repeatability.
- If the zero point is unstable, find the cause: Do not simply accept the last value.
- Apply a known test load: Use test weights, MWL or a test chain as appropriate for the system.
- Perform span calibration: Check the sensitivity of the scale.
- Repeat the span test: Evaluate reproducibility.
- Completely remove the test load: Check the zero point again.
- Perform a material test: If possible, convey an independently determined quantity of material.
- Compare totalized values: Compare the belt scale result with the reference quantity.
- Check several load points if necessary: Evaluate linearity throughout the operating range.
- Make corrections only in a traceable manner: Document zero, span or linearity changes.
- Perform a final verification: After adjustment, verify again using a test load or material.
- Document the results: Record date, zero deviation, span deviation, belt speed, test load and material test results.
Suitable weighing technology from ICS Schneider
Milltronics MSI precision belt scale
The Milltronics MSI is a heavy-duty single-idler precision belt scale for continuous in-line weighing.
Its technical features include:
- parallelogram load-cell design,
- fast response to vertical product loading,
- high repeatability,
- suitable for high belt speeds,
- maximum belt speed up to 5.0 m/s,
- conveying rates up to 12,000 t/h,
- accuracy of ±0.5% or better, depending on application and configuration.
In combination with a suitable integrator, parameters including the following are measured or calculated:
- flow rate,
- totalized quantity,
- belt load,
- belt speed.
Milltronics RBSS speed sensor
The Milltronics RBSS is a high-resolution, wheel-driven speed sensor designed for installation on the return belt.
It offers:
- direct measurement of actual belt movement,
- 60 pulses per revolution,
- robust construction,
- simple installation near the belt scale.
The speed signal is transmitted to the belt scale integrator and processed together with the weighing signal to calculate the material flow rate.
Milltronics BW500
The Milltronics BW500 is an integrator for belt scales and weighfeeders.
Functions particularly relevant to calibration include:
- zero calibration,
- span calibration,
- evaluation of belt load and belt speed,
- flow-rate and totalization calculation,
- alarm functions for load, speed and diagnostics,
- material-test and linearization capabilities.
Test chains for dynamic test loads
The test chains simulate a defined material load on the running conveyor belt.
They are particularly suitable:
- for high-capacity belt scales,
- when large test loads are required,
- when a material test cannot be carried out practically.
Milltronics MWL calibration system
The Milltronics MWL simplifies regular calibration using static reference weights.
The weights can be mechanically lowered onto the belt scale and then raised again without requiring the operator to work directly inside the conveyor system.
Further solutions can be found under belt scales at ICS Schneider.
Conclusion
A belt scale is not a static weighing system, but combines mechanical loading, conveyor belt movement and electronic integration to produce a dynamic measurement result.
The zero point must be determined with the empty belt running
Only in this way are the actual influences of the belt, idlers, belt tension and belt splice taken into account.
Repeatability is crucial
Strongly fluctuating zero or span values should not simply be compensated for by changing the settings, as they may indicate a mechanical or electrical problem.
Belt speed is part of the calibration
A correct weighing signal will still produce an incorrect flow rate if the speed is measured incorrectly.
Test weights enable fast routine checks
They provide a known and reproducible load on the weighbridge.
Test chains provide a better simulation of dynamic operation
They load the running belt over a defined section and allow high test loads to be applied.
A material test checks the complete system under real operating conditions
This allows the weighbridge, belt, speed sensor, integrator and totalization to be evaluated together.
The mechanical system remains crucial
Belt tension, idler alignment, material deposits and belt tracking can influence the measurement more strongly than the electronic accuracy of individual components.
For practical applications
Check the mechanical system → clean the weighbridge → check belt tracking and belt tension → independently verify speed → run the empty belt until stable → check the zero point several times → apply a known test load → check span several times → confirm the zero point again → perform a material test where possible → check several load points if necessary → document all changes and results.
FAQ: Calibrating Belt Scales Correctly
How does a belt scale work?
It measures the material loading per unit belt length and the belt speed at the same time. Mass flow rate and totalized quantity are calculated from these two values.
Why must belt speed be checked during calibration?
Because speed is directly included in the flow-rate calculation. A systematic speed error therefore directly causes a systematic error in the calculated material flow rate.
How is the zero point of a belt scale determined?
The conveyor belt is operated empty under normal operating conditions. The integrator determines the zero signal over a defined period of time or several belt revolutions.
Why should the zero point not simply be set with the belt stationary?
Because belt movement, idler rotation, belt tension and the belt splice generate dynamic forces that are not fully present when the belt is stationary.
How often should the zero point be checked?
For a meaningful test, the zero run should be repeated several times. Good repeatability of the results is the decisive factor.
What does span calibration mean?
During span calibration, a known test load is applied to the weighbridge and the sensitivity of the weighing system is checked accordingly.
Can I calibrate a belt scale using test weights?
Yes. Test weights are a widely used method for routine checks and span calibration of a belt scale.
What is a test chain?
A test chain has a known mass per unit length and runs or rolls on the conveyor belt during calibration. This simulates a defined dynamic belt loading.
What advantages does a test chain have compared with a single test weight?
It loads the running conveyor belt over a longer section and therefore reproduces certain mechanical influences of normal conveyor operation more realistically.
What is a material test?
During a material test, an independently determined quantity of material is conveyed across the belt scale and then compared with the quantity totalized by the belt scale.
Is a material test useful after calibration?
Yes. It is particularly meaningful because it checks the complete measuring system under real conveying conditions.
Why can the test weight be correct while the material test is incorrect?
A static test weight does not reproduce all influences of real conveyor operation. Belt deformation, material distribution, belt tension or speed can cause additional deviations.
How can belt speed be checked independently?
For example, using a suitable tachometer or by timing a known belt length.
Why is a speed sensor on the return belt useful?
It directly measures the actual movement of the conveyor belt and is therefore less affected by differences between motor speed and actual belt speed.
What happens if the measuring wheel of the speed sensor slips?
The measured belt speed becomes too low or unstable. This also causes an error in the calculated material flow rate.
Does belt tension influence the scale?
Yes. Changes in belt tension can transfer additional forces to the weighing idler station and influence both zero point and sensitivity.
Does the scale need to be recalibrated after replacing the belt?
A new check is strongly recommended because belt stiffness, dead weight and belt tension may have changed.
Can material buildup change the zero point?
Yes. Deposits on the weighbridge, idlers or belt can generate additional forces and shift the zero point.
Why must the weighing idlers be aligned correctly?
The height and position of the weighing idlers determine the mechanical force transmission into the weighbridge. Misalignment can cause systematic measurement errors.
Can a belt scale measure at variable speed?
Yes, provided that the actual belt speed is continuously measured with sufficient accuracy and included in the flow-rate calculation.
Why should several material loads be tested?
This makes it possible to determine whether the system operates linearly across the actual operating range or whether the deviation depends on conveying rate.
When should a belt scale be checked again?
In particular after belt replacement, work on idlers or the weighbridge, changes to belt tension, replacement of sensors or noticeable deviations in material quantities.
Which belt scale is suitable for high accuracy and high conveying rates?
The Milltronics MSI is a heavy-duty precision belt scale for continuous in-line weighing and is also suitable for high belt speeds and conveying rates.
Which speed sensor is suitable for a belt scale?
The Milltronics RBSS measures belt speed using a wheel running on the return belt and provides 60 pulses per revolution.
Which integrator is suitable for Milltronics belt scales?
The Milltronics BW500 processes the signals from the belt scale and speed sensor and calculates parameters including flow rate, totalized quantity, belt load and belt speed.
Does ICS offer test chains for belt scales?
Yes. ICS offers test chains for simulating a defined dynamic material load.
How can heavy test weights be applied safely?
With the Milltronics MWL calibration system, static reference weights can be mechanically lowered onto the belt scale and raised again afterwards.
Where can I find further belt scales and accessories?
Further systems can be found under belt scales at ICS Schneider.