E004 - Repeatability
Repeatability is the agreement between the results of successive measurements of the loadcell output for repeated applications of a given force applied in the same direction. Good repeatability is essential for accurate measurements as the other important performance parameters such as non-linearity and hysteresis cannot be determined accurately if the measurements are not repeatable.
Repeatability is normally measured by applying the full rated load to the loadcell three times under carefully controlled conditions. The zero and load outputs from the loadcell are recorded for each load application cycle. Environmental conditions must remain constant during the test and care must be taken to ensure that the load axis does not vary between load applications. The timing of the load application cycles is fixed to reduce errors caused by creep.
The full range outputs are calculated from the three sets of zero and load outputs, then the biggest difference between the outputs is determined. This difference expressed as a percentage of the full rated load of the loadcell is its repeatability. The measurements are carried out at the full rated load of the loadcell so the rated output is the average of the three output values obtained from the test. Output values can be expressed as millivolts per volt (mV/V) or millivolts (mV) providing they are all in the same units.
As the full rated load of the loadcell and its rated output are equivalent the repeatability value, as a percentage, can be converted to a force or millivolt output as required. See Engineering Sheet E003 for a worked example of this that also demonstrates the practical implications of repeatability on measurement accuracy.