CALIBRATION FEATURE
Weighing up the benefits of load cell calibration
Used as part of weighing systems, load cells may be subject to issues including deterioration, ageing and drift. But in industries where measurement accuracy is essential, these load cells need to be calibrated regularly. Steve Sargeant, managing director of LCM Systems, comments
B
ecause they offer non-intrusive, highly accurate load measurement
data, load cells are frequently used as part of a weighing system. Properly installed and calibrated, these will routinely achieve accuracies of around 0.03 to 1% (depending on load cell type). Such systems are of fundamental
importance to many industrial sectors – including aerospace, offshore, marine, heavy lifting and automotive. Here, product liability and safety issues demand that force measurements are demonstrably accurate, and this is usually achieved by calibration to National Standards. 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 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. Furthermore, failure to inspect or clean load cells is another significant factor that can lead to operational issues, as particulate matter can build up 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.
CALIBRATION INTERVALS ISO9000 and many other standards specify the maximum period between recalibration as once every two years, but more frequently if the instrument deterioration is significant during that
period (typically more than 1%.) Many load cell users therefore 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. Very few, however, 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 even be more frequently than one year if the load cell is being used in a particularly harsh environment or arduous operational conditions where it faces high vibration levels, excessive cyclic loading, etc. A standard calibration will test load cell
repeatability and linearity to determine the accuracy. A ‘Five 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.
UKAS SERVICES To meet customers’ needs, LCM Systems
/ INSTRUMENTATION
offers a wide range of UKAS accredited calibration services using equipment testing in accordance with ISO 7500-1. The company has also invested
significantly in improving its calibration capabilities, with the recent installation of a custom-designed 1500 tonne calibration machine at its load cell manufacturing facility in Newport, on the Isle of Wight. The addition of this machine 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 customers of large capacity load cells improved delivery times as well as cost reductions. 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. The ability to carry out calibrations up to
System for calibrating load pins
1500 tonnes is also a benefit 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.
LCM Systems T: 01983 249264
www.lcmsystems.com Enter 659
Thanks to the new 1500T machine at LCM’s Isle of Wight facility, all load cells with capacities up to 150 tonnes in tension, and 1500 tonnes in compression, can be calibrated in-house
INSTRUMENTATION | JUNE 2014 13
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44