What does the ASTM E74 standard specifically say about calibration class load cells?

• It is an outline of the procedure and methods for calibrating load cells. 

• Calibrations may use deadweight or hydraulic test equipment.

• Methods to determine uncertainty of the test results and local specific gravity.

• Definition of curve fitting using a second order polynomial fit.

• Calibration should be carried out in 10% intervals throughout the range.  If lower load limits are required less than 10% intervals may be used.  A total of 30 force applications are required with at least ten different forces used. 

• Good calibration practices, like temperature control and application of weights are described. Temperature should not change by more than ±1.0º C during the calibration.

• Before each calibration, the load cell should be preloaded to the maximum force at least two times to establish the hysteresis pattern. 

• A total of three calibrations are required with a one-third or 120º rotation between each of the calibrations. A total of 30 force applications must be made with at least ten different forces. 

• The uncertainty is defined as 2.4 times the standard deviation. The standard deviation uses the differences between each reading and the corresponding curve fit data point. 

• For Class AA load cells (instruments used as secondary standards), the uncertainty must not exceed 0.05% and the lower load limit is 2,000 times the uncertainty. 

• For Class A load cells (instruments used to verify test equipment), the uncertainty must not exceed 0.25% and the lower load limit is 400 times the uncertainty.

• For devices with temperature compensation built in the error due to temperature on Class AA load cells should not exceed 0.01% and 0.05% for Class A load cells. 

• Class AA load cells (secondary standards) need to be calibrated annually. Class A load cells shall be recalibrated one year after the first calibration and then every two years, if the first recalibration did not change by 0.1% from the first calibration.

• The report or calibration certificate should include the following: the reference standard used for the calibration including a statement of errors and uncertainty, the coefficients from the curve fit equations, the uncertainty with the calibration results and the limits of the Class A load range. The report is not required to state the accuracy, linearity or hysteresis of the load cell.