Lecture:* Trueness – calibration and traceability
The famous story of the retired sea-captain on the island who takes his time from the watchmaker in town only to find out that the watchmaker uses the sea-captain’s cannon shots at 12 noon each day to set his own clocks! [Attributed to Harrison (MIT) by Petley.]
Examples of this kind of circular traceability in measurement are more common than one would hope.
This is the essence of the concept of trueness.
We have said much about ’quality-assured’ measurement: but what does it mean exactly? Basically, two components:
- Metrological traceability
- Measurement uncertainty
One key component of quality-assured measurement is traceability: this concept means and has the following consequences:
Traceability leads to measurement results that can be compared:
- Under both repeatability and reproducibility
- Even different measurement quantities
This gives processes and products which have :
- Improved communication
- Can be traded, are safe and lie within specifications
Calibration – gives trueness!
Calibration is an:
“operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties providedby measurement standards and corresponding indications with associated measurement uncertaintiesand, in a second step, uses this information to establish a relation for obtaining a measurement resultfrom an indication (2.39 VIM)”
- European Association of National Metrology Institutes (EURAMET) and Swedish metrology
- Bureau International des Poids et Mesures (BIPM)
- IUPAP Red Book 1987 “Symbols, Units, Nomenclature and Fundamental Constants in Physics”
Another key component of quality-assured measurement is measurement uncertainty:
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