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Understanding metrology

Metrology, or the science of measurement. What exactly is it? To gain a better understanding of this field, along with insights into its scientific aspects and a familiarity with terms that may strike you as obscure, you're invited to consult our definitions. For some of these entries, additional notes offer a technical perspective for experts seeking more in-depth treatment of the notion being defined.

Note: The definitions appearing under this heading are intended to be as practical and concise as possible; they are explained in everyday language relative to the reference document, entitled "International Metrology Vocabulary".

 

What does it mean?

A measurement standard

A measurement standard is in fact a reference, associated with a value and an uncertainty reading, used as a comparative means to establish both the accuracy and traceability of a given set of results. The lower the level of uncertainty displayed in the standard, the higher the measurement quality. An array of uses extends from a working standard (great uncertainty) to a primary standard (very low uncertainty).

Calibration

Calibration refers to the comparison of values output by a measurement instrument with those of a benchmark or standard, in association with relevant uncertainties. Such a comparison serves to estimate the bias (accuracy) of the target instrument. The values obtained during a calibration session are recorded in a "calibration certificate". This process may be performed at several points within the measurement range of the instrument under review, thereby yielding a calibration curve.

From the expert's desk

The International Metrology Vocabulary (IMV) reference has defined the term Calibration (IMV entry 2.39) as an operation that, under a set of specified conditions, establishes in a single step the relationship between measurement values and their associated uncertainties, as given by standards and corresponding indications along with their associated uncertainties. During a second step, the calibration process relies on this information to derive a relationship that yields a measurement result based on a simple indication.


Note 1:

A calibration outcome can be expressed in the form of a statement, or else a calibration function, diagram, curve or table. In some cases, the actual calibration may consist of an additive or multiplicative correction of the given indication, along with an associated measurement uncertainty.
 

Note 2:

Attention must be paid not to mistake calibration either for adjustment in the context of a measurement system (note that this process is often erroneously called "self-calibration") or for calibration verification.

A connection?

Connecting a measurement instrument implies being given the assurance of comparing the instrument with standards that themselves have been tied to reference standards. This sequential operation (i.e. an uninterrupted chain) serves to compare measurements conducted across the entire world with considerable confidence.
The act of choosing accredited professionals to perform such connections guarantees metrological traceability.

Measurement uncertainty?

This notion refers to an estimation of the doubt associated with a given measurement result. All experiments are subjected to the influence of: the specific measurement instrumentation used (e.g. in terms of resolution), the method deployed, laboratory technicians' qualifications, the measurement environment, and the quality of the measurand.
The lower the level of uncertainty, the greater the confidence in the result outcome.

From the expert's desk

The concept of uncertainty as a quantifiable attribute is relatively new in the history of measurement, even though error and error analysis are longstanding concepts practiced in the field of metrology. It is now widely recognized that when evaluating all error components, whether known or merely suspected, and even when the appropriate corrections have been applied, some uncertainty with respect to the validity of the result being expressed still remains. This residual uncertainty relates to a lingering doubt over the extent to which the measurement result accurately reflects the value of the magnitude being measured.
 

Note:
A universal consensus on both the evaluation and expression of measurement uncertainty facilitates understanding and sharpens interpretation of a broad spectrum of measurement results in science, engineering, business, industry and regulations. Such a consensus also simplifies drawing comparisons of measurement campaigns conducted in various countries.

The international system of units?

This system refers to a selection of magnitudes, recognized by all countries and comparable at the highest level of investigation. It enables associating all measurement units using a small number of fundamental standards. The accuracy of these standards is always subject to improvement, and such is one of the missions assigned to the network of national metrology laboratories including LNE.

From the expert's desk

The first consistent system of units (the metric system initiated during the French revolution) was extended to the international scale by means of a diplomatic treaty signed during the Metric Convention held on May 20th 1875. In 1960, at the time of the Eleventh General Convention of Weights and Measures (French acronym: CGPM), the International System of Units (or SI) first appeared, featuring then and still to this day two categories of units: base units (7 in all), and derived units.

 

Note:
Changes in the definition of the meter perfectly illustrate the willingness to continuously improve the SI system. Defined relative to a quarter of the meridian, the metric unit has always been intended for universal application, yet its implementation raised myriad difficulties. Its standard was initially the meter on record in the Archives, then beginning in 1889 the international prototype of the meter was launched. On August 14th 1969, the meter was redefined as equaling 1,650,763.73 times the wavelength, in a vacuum, of orange radiation from the krypton-86 atom. In 1983, its definition was once again altered in relation to the speed of light, as equal "to the length traveled by a path of light in a vacuum for 1/299,792,458 of a second".

Fundamental metrology?

Fundamental metrology is the term used for the science of measurement; it sets forth the principles and methods for guaranteeing confidence in measurement outcomes. To achieve such a guarantee, this field has developed and continues to administer internationally-recognized national reference standards, which enable industry to tie their measurement instruments to the International System of Units (SI).

LNE performs this specific mission in the fields of mechanics, thermodynamics, optics, chemistry and electricity.

Legal metrology?

This heading refers to a set of requirements and procedures imposed by the national government in order to guarantee both the quality and reliability of certain measurement instruments or measurement operations that concern the public interest: personal safety, protection of consumers, the environment and public health, transparency of commercial transactions, and the effective application of laws and regulations.

 

From the expert's desk

Legal metrology encompasses all regulatory guidelines adopted by public authorities, at both the national and European levels. This practice guarantees the quality of instruments used to conduct commercial transactions and for certain operations that could jeopardize public health and safety. Oversight of legal metrology entails three major validation stages, during: design, manufacturing, and use.

 

Note:
LNE has been designated by the Ministry of Industry to issue inspection certificates by type of measurement instrument being marketed, as regulated by the French decree enacted on May 3rd 2001. LNE also serves as the notified body for the 2009/23/EC IPFNA Directive (regarding non-automatic weighing instruments).

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