Accuracy & Repeatability

Accuracy

The accuracy specified by a device is the amount of error that may occur when measurements are taken. It determines how precise or correct the measurements are to the actual value and is used to determine the suitability of the measuring equipment. Accuracy can be expressed as any of the following:

- error in units of the measured value

- percent of span

- percent of upper range value

- percent of scale length

- percent of actual output value

Repeatability

Repeatability defines how close a second measurement is to the first under the same operating conditions, and for the same input. Repeatability is generally within the accuracy range of a device and is different from hysteresis in that the operating direction and conditions must be the same. Continuous control applications can be affected by variations due to repeatability. When a control system sees a change in the parameter it is controlling, it will adjust its output accordingly. However if the change is due to the repeatability of the measuring device, then the controller will over-control. This problem can be overcome by using the deadband in the controller; however repeatability becomes a problem when an accuracy of say, 0.1% is required, and a repeatability of 0.5% is present.