Metrological traceability

A core concept in metrology is metrological traceability, defuv^d by the BIPM as "the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons, all having stated uncertainties." The level of traceability establishes the level of comparability of the measurement: whether the result of a measurement can be compared to the previous one, a measurement result a year ago, or to the result of a measurement performed anywhere else in the world.

Traceability is most often obtained by calibration, establishing the relation between the indication of a measuring instrument and the value of a measurement standard. These standards are usually coordinated by national metrological institutes: National Institute of Standards and Technology, National Physical Laboratory, UK, Physikalisch-Technisc'he Bundesanstalt, etc.

Traceability is used to extend measurement from a method that works in one regime to a different method that works in a different regime, by calibrating the two using an overlapping range where both work. An example would be the measurement of the spacing of atomic planes in the same crystal specimen using both X-rays and an electron beam. Traceability also refers to the methodology used to calibrate various instruments by relating them back to a primary standard.

Tracebility, accuracy, precision, systematic bias, evaluation of measurement uncertainty are critical parts of a quality management system.

Basics Mistakes can make measurements and counts incorrect. Even if there are no mistakes,

nearly all measurements are still inexact. The term 'error' is reserved for that

inexactness, also called measurement uncertainty. Among the few exact measurements

are:

The absence of the quantity being measured, such as a voltmeter with its leads shorted

together: the meter should read zero exactly.

Measurement of an accepted constant under qualifying conditions, such as the triple

point of pure water: the thermometer should read 273.16 kelvin (0.01 degrees Celsius,

32,018 degrees Fahrenheit) when qualified equipment is used correctly.

Self-checking ratio metric measurements, such as a potentiomecer: the ratio in between

steps is independently adjusted and verified to be beyond influential inexactness

All other measurements either have to be checked to be sufficiently correct or left to chance. Metrology is the science that establishes the correctness of specific measurement situations. This is done by anticipating and allowing for both mistakes and error. The precise distinction between measurement error and mistakes is not settled and varies by country. Repeatability and reproducibility studies help quantify the precision: one common method is an ANOVA gauge R&R study.

Calibration is the process where metrology is applied to measurement equipment and processes to ensure conformity with a known standard of measurement, usually traceable to a national standards board.