The Importance of Regular Load Cell Calibration

Load cells are frequently used as part of a weighing system because they offer non-intrusive, highly accurate load measurement data, with properly installed and calibrated load cells routinely achieve accuracies of around 0.03 to 1% (depending on load cell type). These systems are of fundamental importance to many industrial sectors including aerospace, offshore, marine, heavy lifting and automotive. Product liability and safety issues demand that force measurements are demonstrably accurate, which is usually achieved by calibration to National Standards. This traceability to a National Standard is also often a requirement for compliance to ISO9000, resulting in company procedures that specify calibration schedules and the maintenance of appropriate records as part of their Quality Management System.

 
As all load cells are subject to deterioration due to use, mistreatment, drift or ageing, calibration at regular intervals should to be carried out to establish how the load cell is currently performing, irrespective of whether the company has a Quality Management System in place. Load cells can also become less reliable due to electrical influence, mechanical effects, instrumentation faults and loose cables etc. Failure to inspect or clean load cells is another significant factor that can lead to operational issues, as particulate matter can build up around load cells even in clean environments. Unless calibrations are routinely carried out, load measurement readings can become less and less accurate, with the user potentially being unaware that they are using flawed data.
 

ISO9000 and many other standards specify the maximum period between recalibration as once every two years and more frequently if the instrument deterioration is significant during that period (typically more than 1%.) Many load cell users adopt an annual calibration as the standard interval to ensure that measurements are always as accurate as possible, which is particularly important if they are being used for safety critical applications. However, although many load cell users choose annual calibration, very few do a comparison between the current calibration and the previous calibration to determine the degree of accuracy drop-off. This is a highly useful exercise, as the results can be used to determine a more suitable re-calibration time period, which may be even be more frequently than one year if the load cell is being used in a particularly harsh environment or arduous operational conditions (high vibration levels, excessive cyclic loading).
 

A standard calibration will test load cell repeatability and linearity, which are both used to determine the accuracy. A ‘5 Step’ method is most commonly used, where a known load is applied to the load cell incrementally, and output readings are taken at each step. For example, a 100 tonne load cell would have readings taken when subjected to loads of 20, 40, 60, 80 and 100 tonnes. This process is repeated twice, and the difference in the results used to determine its repeatability/accuracy. As most load cells are used with some form of read-out/display to form a weighing system, the load cell and instrumentation should always be calibrated together whenever possible.
 

To meet customers’ needs, LCM Systems offers a wide range of UKAS accredited calibration services using calibration equipment testing in accordance with ISO 7500-1. They have also invested significantly in improving their calibration capabilities, with the recent installation of a custom designed 1500 tonne calibration machine at their load cell manufacturing facility in Newport, on the Isle of Wight. The addition of this machine to their calibration facilities means that all load cells with capacities up to 150 tonnes in tension and 1500 tonnes in compression, can be calibrated in-house.
 

The new calibration test machine was delivered after 18 months of designing and planning, and has enabled LCM Systems to offer their customers of large capacity load cells improved delivery times as well as cost reductions, making them even more competitive. Prior to its arrival, all load cells measuring loads over 200 tonnes had to be shipped to The National Physical Laboratory in Teddington for calibration, adding to the production time and also incurring significant additional costs. Having seen a large increase in the requirement for larger capacity load cells over the last few years, it became increasingly evident that investing in a bespoke calibration machine would not only dramatically speed up the production process, but would also pay for itself in just two years.
 

Having the ability to carry out calibrations up to 1500 tonnes is also rather unique, as apart from The National Physical Laboratory, there is only one other calibration machine in the UK capable of testing load cells with such high capacities.
 

Contact details:
LCM Systems Ltd
www.lcmsystems.com
sales@lcmsystems.com
Tel: 01983 249264
 

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