Load cells are force transducers that convert applied force into an electrical signal, used in scales, testing machines, and process control. Calibration verifies the load cell's output against traceable force standards across its rated capacity. Proper calibration ensures accurate force and weight measurements for quality and safety applications.
Inspect the load cell for physical damage, cable integrity, and connector condition. Measure the bridge resistance (input and output) with a multimeter and compare to manufacturer specifications. Check insulation resistance between the bridge and the load cell body.
Install the load cell in the calibration fixture with proper alignment to ensure the applied force is concentric and axial. Use a spherical seat or universal joint to minimize moment loads. Verify the indicator or data acquisition system is connected and functioning.
Apply the rated capacity force three times before beginning data collection to condition the load cell and seat all mechanical interfaces. Allow the load cell to return to zero between preloads.
Apply calibrated forces at a minimum of five points (20%, 40%, 60%, 80%, and 100% of rated capacity). At each point, record the load cell output (mV/V or displayed force). Repeat the ascending run three times per ASTM E74.
Reduce the applied force at the same five points from 100% to 0%. Record the output at each point. This data is used to determine hysteresis and return-to-zero performance.
Calculate the average output at each force level, the nonlinearity, hysteresis, repeatability, and creep (if measured). Fit a calibration curve (typically third-degree polynomial per ASTM E74) and calculate the lower limit factor (LLF).
Issue the calibration certificate with the calibration equation, lower limit factor, measurement uncertainty, and all supporting data. Apply the calibration label with date and due date.
Per ASTM E74, the loading range is defined from the lower limit factor (LLF) to the maximum applied force. Class AA requires LLF ≤ 0.05% of capacity. The calibration equation must fit the data within the stated uncertainty. Repeatability and hysteresis must be within the class requirements.
12 to 24 months
Technicians frequently apply preloads incorrectly or inconsistently, which affects zero stability and hysteresis measurements required by ASTM E74. Proper preloading to 10% of capacity before each measurement series is critical for accurate results. Another common error is insufficient settling time between load applications, particularly for higher capacity cells where mechanical components need time to stabilize. ASTM E74 requires minimum 30-second holds, but many technicians rush this step. Misalignment of the load cell axis with the force application direction creates lateral forces that skew calibration data and can damage the load cell. Using incorrect mounting hardware or applying torque beyond manufacturer specifications can introduce mechanical stress that affects linearity. Finally, technicians often neglect to account for the weight of mounting fixtures and adapters in their calculations, leading to systematic errors in the calibration data that violate the Class AA accuracy requirements of ±0.05% of applied force.
| Issue | Cause | Remedy |
|---|---|---|
| Excessive hysteresis (>0.05% of applied force) | Mechanical binding in load path or contamination in strain gauge elements | Inspect mounting surfaces for debris, verify proper alignment, check cable routing for interference, clean strain gauge area if accessible |
| Non-linear response across calibration range | Overloading damage to strain gauges or mechanical yielding of load cell structure | Perform visual inspection for cracks or deformation, reduce maximum test load to 90% of capacity, consider return to manufacturer for repair |
| Unstable zero reading with temperature variations | Thermal expansion mismatch between load cell and mounting hardware | Allow 2-hour temperature stabilization period, verify mounting torque specifications, use thermally matched mounting materials |
| Erratic readings during load application | Loose electrical connections or damaged cable conductors | Check all terminal connections for tightness, perform continuity test on all conductors, inspect cable for kinks or damage, measure insulation resistance |
| Calibration data fails Class AA linearity requirements | Inadequate reference standard accuracy or improper loading technique | Verify reference standard uncertainty ≤0.05%, ensure perpendicular force application, check for side loading, recalibrate using smaller load increments |
CalibrationOS streamlines load cell calibration management through automated scheduling that tracks calibration intervals based on usage patterns and drift history, ensuring compliance with ASTM E74 recommended frequencies. The platform automatically generates calibration certificates containing all required data per ISO 17025 Section 7.8, including hysteresis values, linearity calculations, and Class AA compliance verification. When load cells exceed ASTM E74 acceptance criteria, CalibrationOS triggers the out-of-tolerance investigation workflow, documenting potential causes, corrective actions, and impact assessments on previous measurements. The software maintains comprehensive measurement uncertainty budgets specific to load cell calibrations, incorporating contributions from reference standards, environmental conditions, and repeatability per ISO 17025 Section 7.6. Digital audit trails capture all calibration activities, from pre-loading procedures to final acceptance decisions, supporting metrological traceability requirements. CalibrationOS also manages load cell usage tracking to optimize calibration intervals based on actual loading cycles rather than fixed time periods, improving both cost efficiency and measurement reliability for mechanical force measurements.
The lower limit factor (LLF) per ASTM E74 is the lowest force at which the load cell can be used with its stated accuracy. It is calculated from the calibration data as 2.5 times the standard deviation of the curve fit residuals. Forces below the LLF have unacceptable relative uncertainty.
Deadweight testers apply force by stacking calibrated masses, providing direct traceability and very low uncertainty (0.005-0.02%). Load cells are more practical for high forces (above 10 kN) where deadweights become impractically large. For calibrating other load cells, a reference load cell or deadweight machine can be used.
Yes, the cable length adds resistance to the bridge circuit, which can affect the output signal. The load cell should be calibrated with the same cable length and indicator that will be used in service. If the cable is changed, the calibration should be repeated.
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