We can supply weighing modules, process weighing systems (batching/mixing/blending/filling) or complete installations.
Our products include process controllers, load cell assemblies, weight indicators, digital junction boxes, weighing scales and software. We offer a full range of ATEX certified weight controllers and systems for use in designated hazardous areas. Visit the Technical Weighing Document Section of our download section for further information on process weighing systems.
Process weighing can be broadly subdivided into the following categories, each with their own specific requirements:
- Basic level control
- Stock/inventory control
- Batching and blending
- Recipe formulation
- Loss in weight
- Bulk discharge weighing
- Filling and decanting
For optimum performance and reliability, modern process weighing systems should be based on a blend of sound mechanical design principles, versatile instrumentation. Flexible user-friendly software with inbuilt future proof capabilities will ensure comprehensive weight data management.
Key issues in the design and installation process mandate the use of qualified engineering expertise, especially for hazardous area applications and where structural integrity is an issue. Overall performance will always be affected by a range of environmental and operational factors. Cutting corners through the use of lower quality weighing components and instrumentation is false economy and will result in costly and time consuming ongoing poor performance and reliability.
Weighing system performance
Accuracy is an important factor in process weighing systems. Unfortunately the term is often misunderstood, mixed up or confused with other measurement parameters, especially repeatability and resolution.
It should also be noted that accuracy levels are often mixed up with error (or more correctly ‘uncertainty of measurement’). In common parlance it is not uncommon for levels of accuracy to be quoted, for example, as 1 part in 1000 or 0.1%. What is really meant is that the system has an ‘uncertainty of measurement’ or error of 0.1% and in fact the accuracy is 99.9%.
Accuracy versus repeatability and resolution
The definitions (courtesy of Institute of Measurement and Control document WGC0496) are as follows:
Accuracy: the closeness of agreement between the result of a load measurement and the true value of the load.
Repeatability: the measure of agreement between the results of successive measurements of a weighing system output for repeated applications of a given load
Resolution: the smallest change in weighing system output that can be meaningfully distinguished
The time frame over which the weighing process takes place has a major impact on overall weighing performance. Three key factors need to be taken into account:
- The overall long term zero stability of the weighing system
- The linearity of the weighing system between zero and full load
- Differences between increasing and decreasing load outputs (hysteresis)
ATEX weighing systems
It is of paramount importance that any ATEX weighing system must be designed and installed by fully qualified, competent engineers. The weighing system must also be made up of fully ATEX certified components.
System and structural integrity
The installation of a large percentage of process weighing systems involves mechanical changes to the structure of the tank, hopper or vessel being weighed. It is therefore important that this does not adversely affect the structural integrity of the system.
Pipe work and other force shunts
Very few process weighing systems stand in isolation without external attachments such as pipe work, gantries, ladders and other force shunts. Anything connected to the weighing systems can, to a greater or lesser degree, have an adverse effect on performance.
Selecting the correct type and capacity of load cell
Load cells form the basis of over 99% of process weighing systems. Selecting the correct type and capacity is a critical issue that needs careful consideration from the outset. Well designed hardware not only plays an important role in terms of load introduction, vessel restraint, lift-off protection and overload protection, but also facilitates load cell fitting and removal.
Calibration is a critical process in ensuring initial verification and ongoing weighing system performance. Essentially calibration involves the checking of weighing system performance by applying known loads (these may be physical or electrical) over as wide a part of the weighing range as practical.