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The Virginia Engineer's Feature Articles

Precision Load Cells for Weighing and Batching Solutions

June, 2004

by Todd A. Rae

The use of load cells in product manufacturing has seen a change in emphasis from unit cost considerations alone back towards weighing performance. This recent change has been brought about by several factors including precision batch process automation requirements, material accountability/process yield reporting as well as ISO9000/FDA certification requirements.

Properly evaluating a weighing system’s performance starts with the design of system’s fundamental components, the stress measurement member, normally referred to as the ‘Load Cell’, and the appropriate mounting hardware.

All industrial load cells share some common design-principles when using strain gauges to measure the deflection or deformation of a steel or aluminum structure. By using this kind of technology, the load cell functions basically as a force-measuring device. Taking this into account means that any force which is transferred to the load cell will influence the strain gauge.

The expectation of a load cell is to get a signal that is truly proportional to the applied load (force = mass x gravity) under dynamic process conditions. The following measurable factors (errors) are influencing to all load cells and ultimately to the absolute in-process measurement output accuracy. It is important to bear in mind that a load cell measures applied force.

To accurately compare load cell performance, all of these factors must be taken into consideration. The combination of all of these possible errors can be minimized by using the right alloys, proper design of strain gauges to fit the mechanical structure, precision machining practices, special material heat treatment and hardening processes, having a highly reliable manufacturing process as well as testing and adjusting the sensor while in manufacturing. All too often, most manufacturers of shear beam and compression load cells calculate a COMBINED ERROR based on only two of these errors, HYSTERESIS and NON-LINAERITY. A really true indication about load cell accuracy is only possible by taking all of the above errors into account when calculating the overall ACCURACY CLASS.

The difficulty arises in that these terms are not standardized and only the very fine print shows what is actually used in the calculated accuracy statement. Even then the verbiage can be confusing regarding performance