Metrology

This article is about the science of measurement. For the study of weather, see Meteorology.

A scientist stands in front of the Microarcsecond Metrology (MAM) testbed.

Metrology, as defined by the International Bureau of Weights and Measures (BIPM), is "the science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology". It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in France, when a length standard taken from a natural source was proposed. This led to the creation of the decimal-based metric system in 1795, establishing a set of standards for other types of measurements. Several other countries adopted the metric system between 1795 and 1875; to ensure conformity between the countries, the Bureau International des Poids et Mesures (BIPM) was established by the Metre Convention. This has evolved into the International System of Units (SI) as a result of a resolution at the 11th Conference Generale des Poids et Mesures (CGPM) in 1960.

Metrology is divided into three basic, overlapping activities:

• Definition of internationally accepted units of measurement
• Realisation of these units of measurement in practice
• Traceability, linking measurements made in practice to reference standards

Metrology has three basic sub-fields, which use the three basic activities in varying degrees:

• Scientific or fundamental metrology
• Applied, technical or industrial metrology
• Legal metrology

In each country, a national measurement system (NMS) exists as a network of laboratories, calibration facilities and accreditation bodies which implement and maintain its metrology infrastructure. The NMS affects how measurements are made in a country and their recognition by the international community, which has a wide-ranging impact in its society (including economics, energy, environment, health, manufacturing, industry and consumer confidence). The effects of metrology on trade and economy are some of the easiest-observed societal impacts. To facilitate fair trade, there must be an agreed-upon system of measurement.

Overview

Metrology is defined by the International Bureau of Weights and Measures (BIPM) as "the science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology". It establishes a common understanding of units, crucial to human activity. Trading manufactured goods, the ability to accurately diagnose illnesses, and ensuring consumer confidence during the purchase of goods and services all depend on confidence in the measurements made during these processes. This confidence is achieved by metrology's three basic activities: the definition of internationally accepted units of measurement, the realisation of these units of measurement in practice, and the application of chains of traceability (linking measurements to reference standards). These concepts apply to metrology's three main fields: scientific (fundamental) metrology; applied, technical or industrial metrology, and legal metrology.

Scientific (fundamental) metrology

Scientific metrology is concerned with the establishment of units of measurement, the development of new measurement methods, the realisation of measurement standards, and the transfer of traceability from these standards to users in a society. Although fundamental metrology is formally undefined, it is considered the top level of scientific metrology which strives for the highest degree of accuracy. The BIPM maintains a database of the metrological calibration and measurement capabilities of institutes around the world. These institutes, whose activities are peer-reviewed, provide the fundamental reference points for metrological traceability. In the area of measurement, the BIPM has identified nine metrology areas (including length, mass, and time).

Applied, technical or industrial metrology

Applied, technical or industrial metrology is concerned with the application of measurement to manufacturing and other processes and their use in society, ensuring the suitability of measurement instruments, their calibration and quality control. Although the emphasis in this area of metrology is on the measurements themselves, traceability of the measuring-device calibration is necessary to ensure confidence in the measurement. Industrial metrology is important to a country's economic and industrial development, and the condition of a country's industrial-metrology program can indicate its economic status.

Legal metrology

Legal metrology "concerns activities which result from statutory requirements and concern measurement, units of measurement, measuring instruments and methods of measurement and which are performed by competent bodies". Such statutory requirements may arise from the need for protection of health, public safety, the environment, enabling taxation, protection of consumers and fair trade. The International Organization for Legal Metrology (OIML) was established to assist in harmonising regulations across national boundaries to ensure that legal requirements do not inhibit trade. WELMEC was established in 1990 to promote cooperation in the field of legal metrology in the European Union and among European Free Trade Association (EFTA) member states. In the United States legal metrology is under the authority of the Office of Weights and Measures of National Institute of Standards and Technology (NIST), enforced by the individual states.

History

See also: History of measurement

The ability to measure alone is insufficient; standardisation is crucial for measurements to be meaningful. The first record of a permanent standard was in 2900 BC, when the royal Egyptian cubit was carved from black granite. The cubit was decreed to be the length of the Pharaoh's forearm plus the width of his hand, and replica standards were given to builders. The success of a standardised length for the building of the pyramids is indicated by the lengths of their bases differing by no more than 0.05 percent.

Other civilizations produced generally-accepted measurement standards, with Roman and Greek architecture based on distinct systems of measurement. The collapse of the empires and the Dark Ages which followed them lost much measurement knowledge and standardisation. Although local systems of measurement were common, comparability was difficult since many local systems were incompatible. England established the Assize of Measures to create standards for length measurements in 1196, and the 1215 Magna Carta included a section for the measurement of wine and beer.

Modern metrology has its roots in the French Revolution. With a political motivation to harmonise units throughout France, a length standard based on a natural source was proposed. In March 1791, the metre was defined. This led to the creation of the decimal-based metric system in 1795, establishing standards for other types of measurements. Several other countries adopted the metric system between 1795 and 1875; to ensure international conformity, the International Bureau of Weights and Measures (French: Bureau International des Poids et Mesures, or BIPM) was established by the Metre Convention. Although the BIPM's original mission was to create international standards for units of measurement and relate them to national standards to ensure conformity, its scope has broadened to include electrical and photometric units and ionizing radiation measurement standards. The metric system was modernised in 1960 with the creation of the International System of Units (SI) as a result of a resolution at the 11th General Conference on Weights and Measures (French: Conference Generale des Poids et Mesures, or CGPM).

Concepts

Definition of units

The International System of Units (SI) has seven internationally recognized base units: length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. Although by convention each unit is considered to be independent, in reality they are interdependent since some definitions contain other SI base units. All other SI units are derived from combinations of the seven base units.

SI base units and standards

Base quantity	Name	Symbol	Definition
Length	metre	m	The length of the path travelled by light in a vacuum during a time interval of 1/299792458 of a second
Mass	kilogram	kg	Mass of the international prototype kilogram (IPK)
Time	second	s	The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom
Electric current	ampere	A	A constant current which, if maintained in two straight parallel conductors of infinite length and negligible circular cross-section, placed 1 metre apart in a vacuum, would produce a force equal to 2 x 10−7 newtons per metre
Thermodynamic temperature	Kelvin	K	The fraction 1/273.16 of the thermodynamic temperature of the triple point of water
Amount of substance	mole	mol	The amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12
Luminous intensity	candela	cd	The luminous intensity, in a given direction, of a source emitting monochromatic radiation of a frequency of 540 x 1012 Hz with a radiant intensity in that direction of 1/683 watt per steradian

Since the base units are the reference points for all measurements taken in SI units, if the reference value changed all prior measurements would be incorrect. If a piece of the international prototype kilogram snapped off, it would still be defined as a kilogram; all previous measured values of a kilogram would be heavier. The importance of reproducible SI units has led the BIPM to begin defining base SI units in terms of physical constants. By defining base SI units with respect to physical constants, they are realisable with a higher level of precision and reproducibility.

Realisation of units

Computer-generated image realising the international prototype kilogram (IPK), made from an alloy of 90-percent platinum and 10-percent iridium by weight

The realisation of a unit of measure is its conversion into reality. Three possible methods of realisation are defined by the international vocabulary of metrology (VIM): a physical realisation of the unit from its definition, a highly-reproducible measurement as a reproduction of the definition (such as the quantum Hall effect for the ohm), and the use of a material object as the measurement standard.

Standards

A standard (or etalon) is an object, system, or experiment with a defined relationship to a unit of measurement of a physical quantity. Standards are the fundamental reference for a system of weights and measures by realising, preserving, or reproducing a unit against which measuring devices can be compared. Certified reference materials (CRM) are examples of standards. A CRM provides direct traceability to the realisation of the unit of measure, and can be used for direct comparisons of other materials or to calibrate a measuring device.

Hierarchy

There are three levels of standards in the hierarchy of metrology: primary, secondary, and working standards. Primary standards (the highest quality) do not reference any other standards. Secondary standards are calibrated with reference to a primary standard. Working standards, used to calibrate (or check) measuring instruments or other material measures, are calibrated with respect to secondary standards. The hierarchy preserves the quality of the higher standards.

Traceability and calibration

Metrology traceability pyramid

Metrological traceability is defined by the Joint Committee for Guides in Metrology as the "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". It permits the comparison of measurements, whether the result is compared to the previous result in the same laboratory, a measurement result a year ago, or to the result of a measurement performed anywhere else in the world. The chain of traceability allows any measurement to be referenced to higher levels of measurements back to the original definition of the unit.

Traceability is most often obtained by calibration, establishing the relationship between an indication on a measuring instrument (or secondary standard) and the value of the standard. Calibration is defined as an "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 (of the calibrated instrument or secondary standard) and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication" by the Joint Committee for Guides in Metrology. The four primary reasons for calibrations are to provide traceability, to ensure that the instrument (or standard) is consistent with other measurements, to determine accuracy, and to establish reliability.

Uncertainty

Measurement uncertainty is a value associated with a measurement which expresses the spread of possible values associated with the measurand—a quantitative expression of the doubt existing in the measurement. There are two components to the uncertainty of a measurement: the width of the uncertainty interval and the confidence level (how likely the true value is to fall within the interval). Uncertainty is generally expressed as follows:

$Y=y\pm U$,

where y is the measurement value and U is the uncertainty.

According to the Guide to the Expression of Uncertainty in Measurement (GUM), each uncertainty value has two components: type A and type B. Type A uncertainty is an estimate resulting from statistical analysis; type B uncertainty has other sources, such as the instrument history, manufacturer's specifications, or published information. Type-A measurements are generally calculated by taking several repeat measurements, determining the standard deviation of the measurements, and using the following equation:

$typeA={\frac {s}{\sqrt {n}}}$,

where s is the standard deviation and n is the number of measurements. Type-B uncertainty is determined by estimating the upper and lower limits of uncertainty and selecting the type of uncertainty distribution. With the confidence interval, the type-B uncertainty can be determined. Overall uncertainty is the root sum of the squares of the type-A and type-B uncertainties.

International infrastructure

Several international organizations maintain and standardise metrology.

Metre Convention

The Metre Convention created three main international organizations to facilitate standardisation of weights and measures. The first, the General Conference on Weights and Measures (CGPM), provided a forum for representatives of member states. The second, the International Committee for Weights and Measures (CIPM), was an advisory committee of metrologists of high standing. The third, the International Bureau of Weights and Measures (BIPM), provided secretarial and laboratory facilities for the CGPM and CIPM.

General Conference on Weights and Measures

The General Conference on Weights and Measures (French: Conférence générale des poids et mesures, or CGPM) is the convention's principal decision-making body, consisting of delegates from member states and non-voting observers from associate states. The conference usually meets every four to six years to receive and discuss a CIPM report and endorse new developments in the SI as advised by the CIPM. At the 2011 meeting, it agreed to meet again in 2014 (rather than 2015) to discuss the maturity of new SI proposals.

International Committee for Weights and Measures

The International Committee for Weights and Measures (French: Comité international des poids et mesures, or CIPM) is made up of eighteen (originally fourteen) individuals from a member state of high scientific standing, nominated by the CGPM to advise the CGPM on administrative and technical matters. It is responsible for ten consultative committees (CCs), each of which investigates a different aspect of metrology; one CC discusses the measurement of temperature, another the measurement of mass, and so forth. The CIPM meets annually in Sèvres to discuss reports from the CCs, to submit an annual report to the governments of member states concerning the administration and finances of the BIPM and to advise the CGPM on technical matters as needed. Each member of the CIPM is from a different member state, with France (in recognition of its role in establishing the convention) always having one seat.

International Bureau of Weights and Measures

BIPM seal

The International Bureau of Weights and Measures (French: Bureau international des poids et mesures, or BIPM) is an organisation based in Sèvres, France which has custody of the international prototype kilogram, provides metrology services for the GCPM and CIPM, houses the secretariat for the organisations and hosts their meetings. It also has custody of the international prototype metre, which was retired in 1960. Over the years, international prototype metres and kilograms have been returned to BIPM headquarters for recalibration. The BIPM director is an ex officio member of the CIPM and a member of all consultative committees.

International Organization of Legal Metrology

The International Organization of Legal Metrology (French: Organisation Internationale de Métrologie Légale, or OIML), is an intergovernmental organization created in 1955 to promote the global harmonisation of the legal metrology procedures facilitating international trade. This harmonisation of technical requirements, test procedures and test-report formats ensure confidence in measurements for trade and reduces the costs of discrepancies and measurement duplication. The OIML publishes a number of international reports in four categories:

• Recommendations: Model regulations to establish metrological characteristics and conformity of measuring instruments
• Informative documents: To harmonise legal metrology
• Guidelines for the application of legal metrology
• Basic publications: Definitions of the operating rules of the OIML structure and system

Although the OIML has no legal authority to impose its recommendations and guidelines on its member countries, it provides a standardised legal framework for those countries to assist the development of appropriate, harmonised legislation for certification and calibration.

International Laboratory Accreditation Cooperation

The International Laboratory Accreditation Cooperation (ILAC) is an international organisation for accreditators involved in the certification of conformity-assessment bodies. It standardises accreditation practices and procedures, recognising competent calibration facilities and assisting countries developing their own accreditation bodies. The ILAC has a mutual recognition agreement (MRA) for its members allowing their work to be automatically accepted by other signatories, which helps remove technical barriers to trade.

Joint Committee for Guides in Metrology

The Joint Committee for Guides in Metrology (JCGM) is a committee which created and maintains two metrology guides: Guide to the expression of uncertainty in measurement (GUM) and International vocabulary of metrology - basic and general concepts and associated terms (VIM). The JCGM is a collaboration of eight partner organisations:

• International Bureau of Weights and Measures (BIPM)
• International Electrotechnical Commission (IEC)
• International Federation of Clinical Chemistry and Laboratory Medicine (IFCC)
• International Organization for Standardization (ISO)
• International Union of Pure and Applied Chemistry (IUPAC)
• International Union of Pure and Applied Physics (IUPAP)
• International Organization of Legal Metrology (OIML)
• International Laboratory Accreditation Cooperation (ILAC)

The JCGM has two working groups: JCGM-WG1 and JCGM-WG2. JCGM-WG1 is responsible for the GUM, and JCGM-WG2 for the VIM. Each member organization appoints one representative and up to two experts to attend each meeting, and may appoint up to three experts for each working group.

National infrastructure

A national measurement system (NMS) is a network of laboratories, calibration facilities and accreditation bodies which implement and maintain a country's measurement infrastructure. The NMS sets measurement standards, ensuring the accuracy, consistency, comparability, and reliability of measurements made in the country. The measurements of member countries of the CIPM Mutual Recognition Arrangement (CIPM MRA), an agreement of national metrology institutes, are recognized by other member countries.

Metrology institutes

Overview of a national measurement system

A national metrology institute's (NMI) role in a country's measurement system is to conduct scientific metrology, realise base units, and maintain primary national standards. An NMI provides traceability to international standards for a country, anchoring its national calibration hierarchy. For a national measurement system to be recognized internationally by the CIPM Mutual Recognition Arrangement, an NMI must participate in international comparisons of its measurement capabilities. The BIPM maintains a comparison database and a list of calibration and measurement capabilities (CMCs) of the countries participating in the CIPM MRA. Not all countries have a centralised metrology institute; some have a lead NMI and several decentralised institutes specialising in specific national standards.

Calibration laboratories

Calibration laboratories are generally responsible for calibrations of industrial instrumentation. Since the calibration laboratories are accredited, they give companies a traceability link to national metrology standards.

Accreditation bodies

An organisation is accredited when an authoritative body determines, by assessing the organisation's personnel and management systems, that it is competent to provide its services. For international recognition, a country's accreditation body must comply with international requirements and is generally the product of international and regional cooperation. A laboratory is evaluated according to international standards such as ISO/IEC 17025 general requirements for the competence of testing and calibration laboratories. To ensure objective and technically-credible accreditation, the bodies are independent of other national measurement system institutions.

Impacts

Metrology has wide-ranging impacts on a number of sectors, including economics, energy, the environment, health, manufacturing, industry, and consumer confidence. The effects of metrology on trade and the economy are two of its most-apparent societal impacts. To facilitate fair and accurate trade between countries, there must be an agreed-upon system of measurement. Accurate measurement and regulation of water, fuel, food, and electricity are critical for consumer protection and promote the flow of goods and services between trading partners. A common measurement system and quality standards benefit consumer and producer; production at a common standard reduces cost and consumer risk, ensuring that the product meets consumer needs. Transaction costs are reduced through an increased economy of scale. Several studies have indicated that increased standardization in measurement has a positive impact on GDP. In the United Kingdom, an estimated 28.4 percent of GDP growth from 1921 to 2013 was the result of standardisation; in Canada between 1981 and 2004 an estimated nine percent of GDP growth was standardisation-related, and in Germany the annual economic benefit of standardization is an estimated 0.72% of GDP.

Legal metrology has reduced accidental deaths and injuries with measuring devices such as radar guns and breathalyzers. Legal regulation of measuring devices has improved their efficiency and reliability. Measuring the human body is challenging, with poor repeatability and reproducibility, and advances in metrology help develop new techniques to improve health care and reduce costs. Environmental policy is based on research data, and accurate measurements are important for assessing climate change and environmental regulation. Accurate measurement provides a technical infrastructure and tools to pursue innovation. Measurement standards provide a technical platform for new ideas to be developed, demonstrated, and shared.

See also

• Accuracy and precision
• Data analysis
• Dimensional metrology
• Forensic metrology
• Geometric dimensioning and tolerancing
• Historical metrology
• Instrumentation
• International vocabulary of metrology
• Length measurement
• Metrication
• Metrologia (academic journal)
• NCSL International
• Test method
• World Metrology Day

External links

• Measurement Uncertainties in Science and Technology, Springer 2005
• Bureau International des Poids et Mesures (BIPM)
• National Institute of Standards and Technology (NIST)
• National Physical Laboratory (NPL)
• Physikalisch-Technische Bundesanstalt (PTB)
• U.S. Naval Observatory
• National Conference of Standards Laboratories (NCSL)
• NCSL International
• International Organization for Standardization
• Presentation about Product Quality planning that includes a typical industry "Dimensional Control Plan"
• National Metrology Network of Chile
• National Metrology Laboratory of Malaysia
• UNC Charlotte Center for Precision Metrology
• National Metrology Laboratory of México
• Institute for Reference Materials and Measurements
• LearningMeasure.com Metrology Training
• European Association of National Metrology Institutes (EURAMET)
• Training in Metrology in Chemistry (TrainMiC)
• Measurement Science in Chemistry