The Construction & Principle of Operation of Load Cells

A load cell converts a force (usually the gravitation force from an object being weighed) to a strain which can then be converted to an electrical signal by strain gauges. A load cell will typically have a local circuit as illustrated in figure 1(a) with four gauges; two in compression and two in tension and six external connections i.e. a six core cable is required.

The span and zero adjust on test (AOT) are factory set to ensure the bridge is within limits that can be further adjusted on the site.

The temperature compensation resistors compensate for changes in Young’s modulus with temperature and not changes of the gauges themselves which the bridge inherently ignores.

Coupling of the load requires care; a typical arrangement is illustrated below:

A pressure plate applies the load to the proof-ring via a knuckle and avoids error from slight misalignment. A flexible diaphragm seals against dust and weather. A small gap ensures that shock overloads will make the load cell bottom out without damage to the proof-ring or gauges. Maintenance must ensure that dust does not close this gap and cause the proof-ring to carry only part of the load. Bridging and binding are common causes of load cells reading below the load weight.

Multi cell weighing systems can be employed with readings from each cell being summed electronically. Three cell systems inherently spread the load cells across all cells. With four cell systems the support must ensure that all cells are in contact with the load at all times.

You can also read: Types of Sensors used in Measurement and Process Control

Normally the load cells are the only route to ground from the weighing platform. It is advisable to provide a flexible earth strap, not only for electrical safety but also to provide a route for any welding current which might arise from repairs or later modification.