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E002 - Calibration Uncertainty

The uncertainty of the forces used to calibrate a loadcell set the ultimate limit on the accuracy of the measurements that will be achieved when the loadcell is used in its intended application. This means that calibration uncertainty must be considered at the start of any error analysis for a force measurement application.

Loadcells are usually calibrated using one of three basic methods. These are by weights applied directly, often called dead weight calibration; weights applied via a lever system or hydraulically applied forces. The hydraulic test rigs often measure the applied force using a reference loadcell. The typical ranges of uncertainties that can be achieved using the different methods are tabulated below.

Calibration Method

Typical Uncertainties

Dead weight

±0.001 to ±0.05% of applied force

Lever system

±0.01 to ±0.05% of applied force

Hydraulic system

±0.02 to ±1.0% of applied force

The best uncertainties listed in the table are only available from a small number of calibration laboratories. These are often National Standards Laboratories like the National Physical Laboratory.

Calibration machines using weights are limited to a maximum force of approximately 1MN (100tonnef). Hydraulic machines are available for forces up to 30MN (3000tonnef). The maximum forces given here are only available at a very small number of calibration laboratories. Not all calibration machines can carry out both tension and compression calibrations.

In many applications either the calibration uncertainties are small compared with other errors or relative measurements are more important than absolute values. If either or both of these apply the calibration uncertainty is not a problem. A realistic assessment of calibration uncertainty needed for an application is important as requiring the very best available uncertainty particularly at high force levels will have a substantial effect on the cost of a loadcell measuring system.