Basic Metrology Concepts

The aim of this article is to redefine various metrology concepts for those who still have some doubts on that matter.

Any measurement result has an error, called measurement error, that cannot be known due to its nature. Indeed, a measurement is a comparison with a gauge reknown by all. Various factors have an influence during a measurement process:

• The object to measure
• The gauge with which it is compared
• The operator in charge of the measurement
• The measuring instrument
• The method
• The atmosphere in which the measurement is made
• etc.

The imperfection of each and every one of these elements creates the measurement error. Consequently, it is understandable that it is impossible knowing and correcting totally the measurement error, and that is why a measurement result must always be associated to a measurement uncertainty.

Here are some definitions of terms used in the measurement process of parts:

- Measurement error: a measurement result, less a value ensured by the measuring person.
- Measurement uncertainty: parameter associated to the measurement result, designating the dispersion of the values that could be attributed to the measuring person.
- Precision error: the systematic error of the measuring instrument.
- Repeatability: tight relation between the results of various measurements carried out with the same gauge. (with measurements made in the same measurement conditions).
- Reproducibility: tight relation between the results of various measurements carried out with the same gauge (with measurements made in different measurement conditions, by varying factors such as the measurement method, the observer, the measuring instrument, the reference gauge, the conditions of use, the weather…)

These definitions are simple and understandable to quickly catch their meaning.