Load cells have various applications across several fields, from verifying the correct amount of stamping press force to testing rocket thrust stands. They are so common that almost everyone owns several in some shape or form, whether they realize it or not. Load cells are found in your car, bathroom scale, and almost any digital weighing device. They are used in the test and measurement, automation, material testing, auto, aerospace, nuclear, and medical industries, to name a few.
Important considerations for proper load cell calibration
For a load cell to read correctly, it must be calibrated at some point in its life, either after installation in a consumer product or on a more frequent basis if it is a standard to disseminate the unit of force through several applications and industries. When maintained with regular calibration, a load cell can accurately measure tension or compression forces, also known as weight or strain. The key to properly calibrating a load cell starts with keeping the line of force pure and free from eccentric loads (Figure 1).
Figure 1: Morehouse Load Cell Showing the Importance of Keeping the Line of Force free from Eccentric Forces
Another factor for proper calibration involves using the appropriate loading frames, or masses. The technician performing the calibration must understand the mechanical properties of the load cell and use the proper adapters to keep the line of force pure, which provides a consistent measurement of strain. When measuring weight, typically a mass calibration is required. For this measurement, the load cell should be calibrated by a mass that was compared with reference mass standards traceable to the International System of Units (SI). When measuring force, the load cell should be calibrated using measurement standards traceable to the SI through a National Metrology Institute (NMI) recognized under the CIPM MRA. This recognition is necessary as some NMI's cannot provide traceability. A good example is NIST which is not recognized by the CIPM MRA for torque, thus claiming traceability to SI units for torque through NIST is not possible.
Lastly, a load cell should derive its traceability through deadweight primary standards (Figure 2). A deadweight primary standard is defined by ASTM as “a deadweight force applied directly without intervening mechanisms such as levers, hydraulic multipliers, or the like, whose mass has been determined by comparison with reference standards traceable to the International System of Units (SI) of mass.”1 The weights are corrected for the effects of local gravity where the machine is used, air buoyancy, and material density. Both mass and force applications use strain gauge load cells, hydraulic load cells, pneumatic load cells, or a transducer, which converts one type of energy to another.
Figure 2: Morehouse Deadweight Machine Calibrating a Load Cell in Tension
Metrological traceability and its effect on load cell calibration
The International Vocabulary of Metrology (VIM) defines metrological traceability as the “property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.”2 The International System of Units (SI) is at the top of the measurement hierarchy pyramid (Figure 3). The next tier in the pyramid is a National Metrology Institute (NMI) such as the National Institute of Standards and Technology (NIST), which is recognized by the International Committee for Weights and Measures (CIPM) and has derived its capability from these base SI units of better than 0.0005 %. Per the NIST website, "Metrological traceability  requires the establishment of an unbroken chain of calibrations to specified reference standards: typically national or international standards, in particular realizations of the measurement units of the International System of Units (SI). NIST assures the traceability to the SI..."3
Figure 3: Load Cell Calibration Traceability Chain from Morehouse Force Training
The next tier in the pyramid is a primary reference laboratory, like Morehouse Instrument Company. Traceability to SI units is validated by NIST calibrations on our several deadweight force machines. NIST provides metrologically traceable force transducer calibrations to SI units by applying a relatively known force using deadweight primary standards. NIST has the capability to perform compression or tension calibration from 44.5 N to 4.448 222 MN (10 lbf to 1 000 000 lbf) in one of their six deadweight machines with a standard uncertainty of the applied force from 0.0004% to 0.0005 %. NIST uses deadweight machines and derives its traceability to SI units. Figure 4 shows that force is derived from the base units of mass, distance, and time. Using these base units, the CIPM/BIPM defines 1N as the force required to accelerate one kg to one meter per second per second in a vacuum.