Load Cell Maintenance & Calibration

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.

 

Load Cells – Types and Selection Criteria:

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:

 

  1. Single Point load cells: Also called Platform load cells, these are used for small to medium range platform sizes (200 sq mm to 1200 sq mm), with a fast response. Applications – Sensitive and dynamic weight measurements.
  2. Beam load cells: These are also called bending load cells and are typically used for medium to heavyweight measurements (500kg – 1000kg). Beam load cells can be designed following ATEX or PESO norms. Applications include Manufacturing lines, rugged applications.
  3. Planar Beam load cells: Similar to Bending beam load cells, these are used in low profile solutions where space is limited. Applications – Medical scales and retail scales.
  4. Tension and Compression load cells: Often used for precise weight measurement or applications where dynamic forces are applied. These load cells have superior fatigue strength and are configurable with higher divisions for accuracy and ingress protection. Typically compression load cells are used for weight measurements, and tensile load cells are used for suspended weights. Applications – Weighbridges and hopper scales.
  5. Ring Load or Canister load cells: These are designed like a cylinder with very high force/weight measurement capacities (up to 400 tons). Applications – Weighbridges for road or rail vehicles and Silos

 

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.

 

Load Cells – Operational issues and Maintenance Guidelines:

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:

 

  1. Mounting errors: As mentioned above, load cells should be mounted so that all the net load force shall go through the part of the load cell, where deformation is sensed.
  2. Installation errors: Friction between the mounting platform and the application of force induce hysteresis and result in a wrong measurement.
  3. Overloading: Each load cell has an elastic coefficient, within which the load cell regains its original shape. Exceeding the maximum rating will result in permanent deformation and lead to signal offset, loss of linearity, and mechanical damage.
  4. Wiring errors: Due to moisture in the measuring environment, load cell wires may corrode and develop high resistance. This results in a loss of accuracy and measurement linearity.
  5. Electrical damage due to external factors: Load cells can be damaged due to induced or conducted current without proper earthing. External factors such as arc welding performed close to the cells can overstress the fine resistors within the strain gauge and cause damage.
  6. Nonlinearity: Strain gauge type load cells tend to be nonlinear at the end of their scale. This error is magnified for large-range applications, mainly where shock loading occurs.
  7. Zero balance: The load cell’s output is measured at the no-load condition. Usually quoted as a percentage of full scale or zero offset, this error depicts whether the load cell has undergone a physical distortion due to overload, shock load, or metal fatigue.

 

Load Cells Calibration – Why is it essential?:

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.

 

  1. Combined error: It is measured as a deviation from the linear (standard) line, drawn between no-load and maximum (rated) capacity measurement. Combined error is often measured while gradually increasing or decreasing the loads.
  2. Minimum verification interval: To ensure Repeatability (measured as the maximum deviation between readings under identical load conditions), a minimum verification interval is defined for a load cell. Operating load cells less than this interval will permanently damage the strain gauges.
  3. Resolution: It is defined as the smallest change in the input that causes a change in output. In other words, it is the smallest increment that the system can weigh. For a load cell, the resolution is defined by the OEM.

 

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.

 

THE TRUE COST OF POOR QUALITY CALIBRATION

Poor calibration services could be more than just an inconvenience.