Position: Industrial weighing equipment for bulk handling
Industrial Weighing plays a vital role in plant and equipment across a wide range of applications in the mining, quarrying, recycling, mobile plant and bulk handling industries
Relevant weighing systems and equipment include weighbridges, bulk weighers, belt weighers, weigh feeders, hopper weighers and onboard vehicle weighers.
It is crucial for weighing equipment to be well designed, installed and calibrated to ensure it provides repeatable, reliable and accurate weight data.
The UK Weighing Federation (UKWF) is the definitive weighing authority within the UK and its members are bound by strict codes of practice to ensure they promote best practice within the weighing industry for the benefit of their customers.
Weighing Accuracy System reliability and accuracy are important factors, especially where weight data is used for commercial transactions (legal for trade) or critical process operations.
Weighing accuracy is a widely misunderstood term and is often confused with repeatability, non-linearity or resolution (readability).
Weight readings may be highly repeatable but also inaccurate, whilst displayed weight readings may well be several times smaller than the actual accuracy figure.
The implications of this will depend on the specific type of equipment and application.
Weighing accuracy is defined as 'the closeness of agreement between the results of a measured load and the true value of the load' and is usually quoted as a predetermined plus/minus variation or error from the true weight value (for instance 1050 kg +/- 0.5 kg).
It is also sometimes known as the industrial weighing scale interval 'd'.
It should be remembered that this fixed error applies over the full weighing range (full scale) so its impact at lower weights is significantly greater than at higher weights.
Accuracy may also be quoted as a percentage error, again relative to full scale.
To understand more about weighing system accuracy and how it impacts on results, the user needs to understand how the specific equipment operates.
For example, a weighbridge is used for the relatively short term weighing of vehicles.
The scale usually has a zero tracking device, which maintains the weight output from the unloaded weighbridge at zero within clearly defined limits.
(This can for example automatically compensate for any build up of material on the deck).
With the system at zero, the vehicle drives onto the bridge and the weight is recorded.
The vehicle then drives off and the scale zero can be checked, allowing any discrepancies to be identified and corrected.
Typical weighbridge accuracies are +/- 1 part in 3000 (0.03%).
By contrast a tank weighing system may be infrequently emptied, allowing no chance to continually monitor the zero load condition.
If this varies with time, the subsequent error change in indicated contents weight may go unnoticed.
Accuracies for such systems are typically 1 part in 500 (0.2%).
However if the same tank system is used to dispense material over a short period, the achievable accuracy for this may well be five or ten times better (system can be zeroed out prior to weighing).
Industrial belt weighers are designed to measure the amount of material passing along a known length of belt supported on weighed rollers.
By knowing the belt speed, the weight passing per unit time can be calculated to give throughput of material, typically in tonnes per hour.
Weighing accuracy is dependent on a number of factors, including the type of material and the length of belt carriage being weighed.
Typical accuracies range from 0.1% for special multi-idler systems up to 2% for single idlers.
Belt weighers are used extensively on both permanent and mobile plant ranging from smaller crushers and screening machines up to huge stackers and reclaimers designed for use in stockyards at ports and terminals.
Where such weighing systems are used on mobile plant, it is important to recognise the affect of varying operational angles on accuracy.
Some form of inbuilt compensation method must be included to allow accurate weighing under different angles of operation.
These belt weighers are expected to work under arduous conditions with minimal maintenance and as a result long term accuracy is very important.
* CALIBRATION: Effective calibration is vital to ensure weighing equipment provides accurate weight data in a consistent way.
The calibration process establishes the relationship between the actual load applied and the resultant output.
How well weighing equipment maintains its calibration and hence the period between calibration checks, depends on a number of factors and periodic checking is essential to ensure weight data integrity is maintained.
Factors affecting scale calibration include equipment operational hours, shock loading, overloading, vibration, material build up, and environmental conditions.
There are a number of recognised calibration methods and the UKWF have published a comprehensive Code of Practice for use by its members.
It is important that equipment is designed so that calibration can be carried out once it is in situ.
* Preventative Maintenance.
Such programmes need to take into account seasonal fluctuations in operational and environmental conditions.
Simple checking routines and preventative maintenance programmes can significantly increase industrial weighing equipment effectiveness and minimise downtime resulting from critical component failure.
