A Load Cell is a transducer that measures force and outputs an electrical signal proportional to the intensity of the force. Most load cells use strain gauges to detect force intensity, but hydraulic and pneumatic load cells are also available in the industry. Strain gauge load cells generally contain four (4) strain gauges arranged in a conventional Wheatstone bridge electric circuit configuration. When a unit force is applied to the strain gauge, the deformation of the gauge induces a change in electric signal, and the force output is calculated using the change of electric signal and empirical relations. The accuracy of a Load cell depends on the quality of strain gauges, the electronics embedded in the load cell, and the calibration frequency.
Since load cells are primarily used for measuring force, they are also called Force Transducers. Transducers are devices that generally detect or measure a particular parameter (force, in case of load cells) and provide an uninterrupted electric signal. Following are the types of Strain gauge load cells and their selection criteria:
Though strain gauge-based load cells are widely used as an industry standard for force and weight measurements, depending on application needs, safety norms, and environmental constraints, Pneumatic or Hydraulic load cells can also be selected:
Pneumatic load cells are designed to automatically regulate the balancing air pressure applied through a diaphragm into the gauge. A nozzle attached at the bottom of the load cell acts as a release vent, and a pressure gauge measures the differential pressure to indicate the intensity of force applied.
Hydraulic load cells, as the name suggests, hydraulic load cells use the classic Cylinder-Piston arrangement, placed in a thin elastic diaphragm. The load cell and the cylinder are completely filled with oil. When a unit force is applied to the load cell, the piston displacement, which is directly proportional to force intensity, is measured as a change in pressure value. The differential pressure is transmitted to a hydraulic pressure gauge, which correlates the pressure in terms of force intensity. Due to the complex construction and the high operation and maintenance costs, hydraulic load cells are often restricted to hazardous environmental applications.
In addition to the above-mentioned load cell type categories, load cells are generally available with digital or analog output options, with ATEX Zone (explosion resistant) and IP (Ingress protection) provisions.
For precise measurements, it is necessary to select load cells as specified to the load application. Post selection, installation, and mounting procedure should depend on the specified load direction (often, force direction should be perpendicular to the mounting frame). Side forces, bending, or torsional movements acting on the load cell yield erroneous results and risk reducing load cells’ life. Following are certain operational errors, often resulting in an inaccurate measurement:
Each load cell is designed for a specific application to measure various load categories or weights (static or dynamic) in an industrial environment. The design characteristics of the load cell (mainly consisting of three parameters – Combined error, Minimum verification interval, and Resolution) define the service life, measurement range, and accuracy of the load cells. However, due to operational errors, these design characteristics deviate and hence calibration is necessary.
The operational errors (refer to the previous section) hamper the performance characteristics of the load cells, ultimately leading to physical damage or garbage readings. Therefore, annual calibration should be standardized irrespective of the application and industry to restore the design characteristics.
e2b calibration offers industry-leading ISO-certified load cell calibration services (tension or compression up to 50000 lbf). Our labs are ISO/IEC 17025 accredited and operated by a team of qualified calibration experts to test and calibrate your load cells. Our verifiable services are unmatched in the industry. We are registered with ANAB. We are also ANSI/NCSL Z540-1-1994 certified. We have the NIST Traceable Wide scope of ISO/IEC 17025 accreditation. Contact e2b calibration for all your equipment calibration needs.