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
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.
CalibrationOS tracks due dates, stores certificates, and generates audit-ready reports.
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