LOAD CELLS

A load cell is a transducer that converts an applied force into a measurable electrical signal. Most precision load cells use strain gauges bonded to a carefully designed elastic spring element. As force is applied, the element deforms by a very small amount, changing the electrical resistance of the strain gauges.

The gauges are typically wired into a Wheatstone bridge, producing a differential output proportional to the applied load. Raw output is a millivolt per volt (mV/V) signal, although load cells can also incorporate signal conditioning to provide amplified outputs such as 0-5 V, 0-10 V, 0.5-4.5 V, 4-20 mA or digital signals (Modbus RS-485, CAN Bus, EtherNet etc.).

Load cells are available in many configurations to suit different forces, mounting arrangements, accuracy requirements and space constraints. Common types include compression load cells, tension load cells, bi-directional load cells, pancake or low-profile load cells, S-beam load cells, load buttons, miniature load cells, annular or through-hole load cells, and bending or shear beam load cells.

PCM also designs and manufactures custom load cells where a standard load cell cannot meet the required dimensions, capacity, mounting arrangement, environmental conditions or electrical output.

Load cell accuracy depends on the sensor design, capacity, installation, instrumentation, temperature and calibration. Important performance characteristics include non-linearity, hysteresis, repeatability, creep, zero return and temperature effects.

Rather than relying on a single headline accuracy figure, the complete measurement system should be considered. Mechanical installation, load introduction, off-axis forces, signal conditioning and calibration can all influence the final measurement uncertainty.

For high accuracy applications, where full traceability and adherence to recognised British Standards is key, PCM can calibrate load cells through its UKAS ISO/IEC 17025 accredited force calibration laboratory in accordance with BS 8422.

The correct load cell capacity should exceed the maximum force expected in service, but not by greater than 10x, in order to retain sufficient measurement accuracy over the normal operating range. Dynamic loads, shock loads, fatigue, accidental overload and off-axis forces should also be considered.

Selecting a load cell purely on the nominal working load can result in inadequate overload protection, while choosing an unnecessarily large capacity can give rise to a poor signal-to-noise ratio.

PCM can review the application's working load, maximum expected load, load cases and required factor of safety to help specify an appropriate load cell capacity.

Yes. Calibration establishes the relationship between the force applied to a load cell and its electrical or indicated output. It also provides traceability, allowing important performance metrics such as repeatability, interpolation error and, where applicable, hysteresis to be assessed over time.

Calibration should be performed using recognised procedures and equipment traceable to National Standards, with Uncertainty of Measurement clearly reported.

PCM operates a UKAS-accredited ISO/IEC 17025 force calibration laboratory and can calibrate load cells in accordance with BS 8422, including both new sensors and customers' existing equipment, irrespective of model type or manufacturer.

A conventional strain-gauge load cell requires a stable excitation voltage and instrumentation capable of accurately measuring its small differential bridge output, typically expressed in mV/V.

Depending on the application, this may be a load cell indicator, signal conditioner, amplifier, data acquisition system or PLC interface. Instrumentation can convert the raw bridge output into standard analogue or digital signals such as 0-10 V, 4-20 mA, Modbus, CAN or other industrial communications.

PCM can supply the load cell and instrumentation calibrated as a complete measurement system.

Yes, provided the load cell is designed and protected for the intended environment. Factors can include water and dust ingress, corrosion, temperature, vibration, shock, chemicals, nuclear radiation, hydrostatic pressure and electromagnetic interference.

Load cells can be manufactured from corrosion-resistant materials and provided with appropriate environmental sealing, cable systems and connectors. For demanding applications, the environmental requirements should be considered at the design stage rather than treating sealing as an afterthought.

A load cell should be installed so that force is introduced along its intended measurement axis without unwanted bending, eccentric off-axis loading, side loading or torsion. Mounting surfaces should be suitable for the sensor design, and fasteners should be correctly specified and tightened.

Cable routing, grounding, environmental protection and mechanical overload protection should also be considered.

Poor installation can significantly degrade the performance of even a highly accurate load cell, so PCM can provide installation drawings and application-specific guidance where required.

There is no single recalibration interval suitable for every load cell. The appropriate interval depends on usage, required measurement uncertainty, environmental conditions, loading history, stability and the consequences of an incorrect measurement.

A 12-month calibration interval is commonly used as a starting point, but heavily used or safety-critical equipment may require more frequent calibration. Conversely, a highly stable sensor with documented calibration history may justify a different interval.

A load cell should also be checked or recalibrated following an overload, repair, significant mechanical event or whenever its performance is in doubt.

Yes. Load cells can be integrated with PLCs, data acquisition systems and industrial control systems using suitable signal conditioning.

A conventional mV/V load cell normally requires an amplifier or signal conditioner before connection to a standard PLC analogue input. Alternatively, electronics can be incorporated into the load cell to provide outputs such as 0-10 V, 4-20 mA or digital communications.

For industrial installations, 4-20 mA is particularly useful where signals must travel over longer cable distances or operate in electrically noisy environments.

A compression load cell measures forces that push or compress the sensor, while a tension load cell measures forces that pull it apart. A bi-directional load cell is designed and calibrated to measure force in both directions.

The appropriate configuration depends on how the load is introduced into the structure. Correct mounting is particularly important because bending moments, side loads and misalignment can introduce measurement errors or mechanically overload the sensor.

A custom load cell is designed specifically around the mechanical, electrical and environmental requirements of an application rather than selected from an existing catalogue range.

Parameters can include overall dimensions, mounting interfaces, rated capacity, overload capability, material, temperature range, environmental sealing, connector or cable arrangement and electrical output.

PCM designs and manufactures custom strain-gauge load cells in-house, allowing the sensing element, strain gauge installation, temperature compensation techniques, conditioning electronics, machining and calibration to be fully optimised together for superior performance.