Metrology a Scientific Discipline

Although metrology is a scientific discipline known and used across many industries, terminology and meanings will often differ. These differences can lead to wrong interpretation of test results or actions to perform, affecting the quality or the efficacy of calibration processes and measurement instruments.


Some of the most common terminology mistakes are using the word “precision” while referencing the accuracy of a measurement instrument or using the term “calibrate” for adjusting a measurement instrument. Trivial as it may seem, using the correct word for describing technical results and actions to conduct, and ensuring that all words are understood the same way is paramount in a scientific environment. Hence the reason behind creating worldwide committees and review boards to establish frameworks and standards across each nation.

In 1875, the Meter Convention international treaty was put together to provide an organizational structure for countries registered under the treaty and act in common accord on all matters relating to metrology. Since then, 62 countries including the US, Canada, the UK, Russia, and China, have worked relentlessly to improve the worldwide measurement conventions, the international System of Units, and the traceability of measurements under the metric system. This convention also led to the creation of the International Bureau of Weight and Measure (BIPM). The BIPM, based in Paris, is mandated to lead scientific and technical activities in metrology, manage communication between the countries registered under the Meter Convention, increase effectiveness of knowledge transfer, and maintain and publish international metrology content. Among all the publications, two metrological documents that are known as “Pillar Documents” were issued by the BIPM: the Guide to the Expression of Uncertainty in Measurement (GUM) and the International Vocabulary of Metrology (VIM).



Just like the GUM, the VIM is an extensive document that was written by the Joint Committee for Guides in Metrology (JCGM). It includes most, if not all the terms used in metrology along with definitions, notes, and examples. This document was put together to establish a standard and common language across all the countries registered under the Metre Convention and beyond. It is a guide for the expression of metrological results and metrological processes, and offers clarity when the definition of words such as “Calibrate” of “Precision” are debated.


Here are some of the most common terms and their official definitions:


Measurement Accuracy

  • Official Definition (VIM, 2.13): closeness of agreement between a measured quantity value and a true quantity value of a measurand.

  • VIM NOTE 3: ‘Measurement accuracy’ is sometimes understood as closeness of agreement between measured quantity values that are being attributed to the measurand.

Measurement Precision

  • Official Definition (VIM, 2.15): closeness of agreement between indications or measured quantity values obtained by replicate measurements on the same or similar objects under specified conditions.

  • VIM NOTE 4: Sometimes “measurement precision” is erroneously used to mean measurement accuracy.

Resolution

  • Official Definition (VIM, 4.14): smallest change in a quantity being measured that causes a perceptible change in the corresponding indication.

  • VIM NOTE: Resolution can depend on, for example, noise (internal or external) or friction. It may also depend on the value of a quantity being measured.

Calibration

  • Official Definition (VIM, 2.39): operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication.

  • VIM NOTE 2: Calibration should not be confused with adjustment of a measuring system, often mistakenly called “self-calibration”, nor with verification of calibration.

Measuring System

  • Official Definition (VIM, 3.2): set of one or more measuring instruments and often other devices, including any reagent and supply, assembled and adapted to give information used to generate measured quantity values within specified intervals for quantities of specified kinds.

  • VIM NOTE: A measuring system may consist of only one measuring instrument.

Adjustment

  • Official Definition (VIM, 3.11): set of operations carried out on a measuring system so that it provides prescribed indications corresponding to given values of a quantity to be measured.

  • VIM NOTE 1: Types of adjustment of a measuring system include zero adjustment of a measuring system, offset adjustment, and span adjustment (sometimes called gain adjustment).

Working Measurement Standard

  • Official Definition (VIM, 5.7): measurement standard that is used routinely to calibrate or verify measuring instruments or measuring systems.

  • VIM NOTE 1: A working measurement standard is usually calibrated with respect to a reference measurement standard.

Metrological Traceability

  • Official Definition (VIM, 2.41): property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty

  • VIM NOTE 1: For this definition, a ‘reference’ can be a definition of a measurement unit through its practical realization, or a measurement procedure including the measurement unit for a non-ordinal quantity, or a measurement standard.

Measurement Uncertainty

  • Official Definition (VIM, 2.26): non-negative parameter characterizing the dispersion of the quantity values being attributed to a measurand, based on the information used

  • VIM NOTE 1: Measurement uncertainty includes components arising from systematic effects, such as components associated with corrections and the assigned quantity values of measurement standards, as well as the definitional uncertainty. Sometimes estimated systematic effects are not corrected for but, instead, associated measurement uncertainty components are incorporated.

Many more common and less common terms can be found in the VIM. This document should be known and used by everyone who deals with metrology and must express and share measurement results. It provides all the definitions needed to make sure everyone understands and works toward the established objectives.


Reference Authors, 2008, JCGM, Mainaining and promoting the Guide to the Expression of Univertainty in Measurement and the International Vocabulary of Metrology https://www.bipm.org/en/committees/jc/jcgm/publications


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