Repeatability

REPEATABILITY

Definition: “The ability of a measurement system to indicate the same measured value each time it is given the same quantity to measure”.

Here’s a “real” calibration where the forces have been applied, in turn, 3 times to the instrument under test. The scatter, or spread of the readings then provides a measure of repeatability. (Even better, the device could be removed and replaced in the rig between the second and third runs to give a measure of reproducibility as well). The scatter may be a percentage of the signal, as shown below, or it may be a constant source (next graph down). Either way, the errors will be random and the readings at each point will be normally distributed so you can quantify it by sigma, the standard deviation for the data.

Here’s a constant noise source, (electronic mains pick-up is an obvious example):

At lower forces, the noise is now a bigger percentage of the signal, so you would need more readings to average it out.

Repeatability may be quoted as:

  • a percentage of reading,
  • a percentage of reading down to a certain percentage of range (worrying if you are operating at the bottom of the range) 
  • a percentage of full scale reading (more worrying)

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