Metrology

Metrology

Overview

Metrology is the science
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...

 of measurement
Measurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 μέτρον (metron), "measure" + "λόγος" (logos
Logos
' is an important term in philosophy, psychology, rhetoric and religion. Originally a word meaning "a ground", "a plea", "an opinion", "an expectation", "word," "speech," "account," "reason," it became a technical term in philosophy, beginning with Heraclitus ' is an important term in...

), amongst others meaning "speech, oration, discourse, quote, study, calculation, reason". In Ancient Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 the term μετρολογία (metrologia) meant "theory of ratios".

Metrology is defined by the International Bureau of Weights and Measures
International Bureau of Weights and Measures
The International Bureau of Weights and Measures , is an international standards organisation, one of three such organisations established to maintain the International System of Units under the terms of the Metre Convention...

(BIPM) as "the science of measurement, embracing both experiment
Experiment
An experiment is a methodical procedure carried out with the goal of verifying, falsifying, or establishing the validity of a hypothesis. Experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results...

al and theoretical
Theory
The English word theory was derived from a technical term in Ancient Greek philosophy. The word theoria, , meant "a looking at, viewing, beholding", and referring to contemplation or speculation, as opposed to action...

 determinations at any level of uncertainty
Uncertainty
Uncertainty is a term used in subtly different ways in a number of fields, including physics, philosophy, statistics, economics, finance, insurance, psychology, sociology, engineering, and information science...

 in any field of science and technology." The ontology and international vocabulary of metrology
International vocabulary of metrology
The International vocabulary of metrology is an attempt to find a common language and terminology in metrology, e.g. the science of measurements, across different fields of science, legislature and commerce...

 (VIM) is maintained by the International Organisation for Standardisation.

Metrology is a very broad field and may be divided into three subfields:

A core concept in metrology is metrological traceability, defined 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
Traceability
Traceability refers to the completeness of the information about every step in a process chain.The formal definition: Traceability is the ability to chronologically interrelate uniquely identifiable entities in a way that is verifiable....

 is most often obtained by calibration
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....

, establishing the relation between the indication of a measuring instrument and the value of a measurement standard.
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Encyclopedia

Metrology is the science
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...

 of measurement
Measurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 μέτρον (metron), "measure" + "λόγος" (logos
Logos
' is an important term in philosophy, psychology, rhetoric and religion. Originally a word meaning "a ground", "a plea", "an opinion", "an expectation", "word," "speech," "account," "reason," it became a technical term in philosophy, beginning with Heraclitus ' is an important term in...

), amongst others meaning "speech, oration, discourse, quote, study, calculation, reason". In Ancient Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 the term μετρολογία (metrologia) meant "theory of ratios".

Introduction


Metrology is defined by the International Bureau of Weights and Measures
International Bureau of Weights and Measures
The International Bureau of Weights and Measures , is an international standards organisation, one of three such organisations established to maintain the International System of Units under the terms of the Metre Convention...

(BIPM) as "the science of measurement, embracing both experiment
Experiment
An experiment is a methodical procedure carried out with the goal of verifying, falsifying, or establishing the validity of a hypothesis. Experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results...

al and theoretical
Theory
The English word theory was derived from a technical term in Ancient Greek philosophy. The word theoria, , meant "a looking at, viewing, beholding", and referring to contemplation or speculation, as opposed to action...

 determinations at any level of uncertainty
Uncertainty
Uncertainty is a term used in subtly different ways in a number of fields, including physics, philosophy, statistics, economics, finance, insurance, psychology, sociology, engineering, and information science...

 in any field of science and technology." The ontology and international vocabulary of metrology
International vocabulary of metrology
The International vocabulary of metrology is an attempt to find a common language and terminology in metrology, e.g. the science of measurements, across different fields of science, legislature and commerce...

 (VIM) is maintained by the International Organisation for Standardisation.

Metrology is a very broad field and may be divided into three subfields:
Subfield Definition
Scientific or fundamental metrology concerns the establishment of quantity systems
Quantity calculus
Quantity calculus is the formal method for describing the mathematical relations between abstract physical quantities. Despite the name, it is more analogous to a system of algebra than calculus in the mathematical sense of the term. Measurements are expressed as products of a numeric value with a...

, unit systems, units of measurement
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...

, the development of new measurement methods, realisation of measurement standards and the transfer of traceability from these standards to users in society.
Applied or industrial metrology concerns the application of measurement science to manufacturing and other processes and their use in society, ensuring the suitability of measurement instruments, their calibration and quality control of measurements.
Legal metrology concerns regulatory requirements of measurements and measuring instruments for the protection of health, public safety, the environment, enabling taxation, protection of consumers and fair trade.

Metrological traceability


A core concept in metrology is metrological traceability, defined 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
Traceability
Traceability refers to the completeness of the information about every step in a process chain.The formal definition: Traceability is the ability to chronologically interrelate uniquely identifiable entities in a way that is verifiable....

 is most often obtained by calibration
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....

, 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 Institute of Standards and Technology
The National Institute of Standards and Technology , known between 1901 and 1988 as the National Bureau of Standards , is a measurement standards laboratory, otherwise known as a National Metrological Institute , which is a non-regulatory agency of the United States Department of Commerce...

, National Physical Laboratory, UK
National Physical Laboratory, UK
The National Physical Laboratory is the national measurement standards laboratory for the United Kingdom, based at Bushy Park in Teddington, London, England. It is the largest applied physics organisation in the UK.-Description:...

, Physikalisch-Technische Bundesanstalt
Physikalisch-Technische Bundesanstalt
The Physikalisch-Technische Bundesanstalt is based in Braunschweig and Berlin. It is the national institute for natural and engineering sciences and the highest technical authority for metrology and physical safety engineering in Germany....

, etc.

Tracebility, accuracy, precision
Accuracy and precision
In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...

, systematic bias, evaluation of measurement uncertainty
Measurement uncertainty
In metrology, measurement uncertainty is a non-negative parameter characterizing the dispersion of the values attributed to a measured quantity. The uncertainty has a probabilistic basis and reflects incomplete knowledge of the quantity. All measurements are subject to uncertainty and a measured...

 are critical parts of a quality management
Quality management
The term Quality management has a specific meaning within many business sectors. This specific definition, which does not aim to assure 'good quality' by the more general definition , can be considered to have four main components: quality planning, quality control, quality assurance and quality...

 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
    Voltmeter
    A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to...

     with its leads shorted together: the meter should read zero exactly.
  • Measurement of an accepted constant under qualifying conditions, such as the triple point
    Triple point
    In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases of that substance coexist in thermodynamic equilibrium...

     of pure water: the thermometer
    Thermometer
    Developed during the 16th and 17th centuries, a thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor Developed during the 16th and 17th centuries, a thermometer (from the...

     should read 273.16 kelvin
    Kelvin
    The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

     (0.01 degrees Celsius, 32.018 degrees Fahrenheit) when qualified equipment is used correctly.
  • Self-checking ratio metric measurements, such as a potentiometer
    Potentiometer
    A potentiometer , informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used , it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on...

    : 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
Repeatability
Repeatability or test-retest reliability is the variation in measurements if they would have been taken by a single person or instrument on the same item and under the same conditions. A less-than-perfect test-retest reliability causes test-retest variability. Such variability can be caused by, for...

 and reproducibility
Reproducibility
Reproducibility is the ability of an experiment or study to be accurately reproduced, or replicated, by someone else working independently...

 studies help quantify the precision: one common method is an ANOVA gauge R&R study.

Calibration
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....

 is the process where metrology is applied to measurement equipment and processes to ensure conformity with a known standard
Standard (metrology)
In the science of measurement, a standard is an object, system, or experiment that bears a defined relationship to a unit of measurement of a physical quantity. Standards are the fundamental reference for a system of weights and measures, against which all other measuring devices are compared...

 of measurement, usually traceable to a national standards board.

Society


Sufficiently correct measurements are essential to commerce. About nine out of every ten people working in metrology specialize in commercial measurement, most at the technician level. Correct measurements are beneficial to manufacturing, but other methods are available and sometimes are more appropriate.

Metrology has thrived at the interface between science and manufacturing. Aerospace, commercial nuclear power, medicine, medical devices and semiconductors rely on metrology to translate theoretical science into mass produced reality.

The basic concepts of metrology appear simple on the surface, and metrology is rarely taught in a systematic manner above the technician level. Within most businesses, metrology core beliefs such as recording all setups and observations for possible future reference are opposed to the general business practice of minimizing recordkeeping to limit litigation effects.

Applied metrology


Metrology laboratories are places where both metrology and calibration work are performed. Calibration laboratories generally specialize in calibration work only.

Both metrology and calibration laboratories must isolate the work performed from influences that might affect the work. Temperature, humidity, vibration, electrical power supply, radiated energy and other influences are often controlled. Generally, it is the rate of change or instability that is more detrimental than whatever value prevails.

Calibration technicians execute calibration work. In large organizations, the work is further divided into three groups:
Group Definition
Set-up people arrange the equipment needed for calibration and verify that it works correctly.
Operators execute the calibration procedures and collect data.
Tear-down people dismantle set-ups, check the components for damage and then put the components into a stored state. This is the entry-level position for people who didn’t start in the equipment warehouse or transportation functions


Alternatively, the technicians can be divided by major discipline areas: physical, dimensional, electrical, RF, microwave and so on. But the principles are the same regardless of the equipment.

Metrology technicians perform investigation work in addition to calibrations. They also apply proven principles to known situations and evaluate unexpected or contradictory results.

Specific education in metrology was formerly limited to sub-professional work. Most of the branches of the US Military train ‘enlisted-grade’ technicians to meet their specific needs.

Large industrial organizations also develop people who demonstrate aptitude in testing functions. When this is combined with an engineering degree, it qualifies the person as a metrology engineer. Over the last 15 years, Universities such as the University of North Carolina
University of North Carolina
Chartered in 1789, the University of North Carolina was one of the first public universities in the United States and the only one to graduate students in the eighteenth century...

 at Charlotte created a specific curriculum in metrology engineering. In England, metrology was part of the fifth year of some undergraduate engineering programmes.

Metrologist
Metrologist
Metrologists perform metrology work involving precision measurement and comparison of physical quantities such as mass, length, time, force, speed, voltage and current. They calibrate precision equipment which measures these physical units. They may also certify that outside standards of such...

s are people who perform metrology work at and above the technician levels, generally without the benefit or acknowledgement of a college degree.

The metrology and calibration work described above is always accompanied by documentation. The documentation can be divided into two types; one related to the task and the other related the administrative program. Task documentation includes calibration procedures and the data collected. Administrative program documentation includes equipment identification data, 'calibration certificates’, calibration time interval information and 'as-found' or 'out-of-tolerance' notifications.

Administrative programs provide standardization of the metrology and calibration work and make it possible to independently verify that the work was performed. Generally, the administrative program is specific to the organization performing the work and addresses customer requirements. General administrative program specifications created by industry groups, such as the ANS (ANSI) Z540 series may also be covered in the administrative program. Other specifications created by the US Food and Drug Administration, US Federal Aviation Administration or other agencies would supplement or replace ANS Z540 for work performed in their domains. Often administrative programs can be as complicated and detailed as the measurement work itself.

An administrative program that has insufficient actual metrology or calibration capability is derisively referred to as a "lick and stick" program.

Standards



Standards are objects or ideas that are designated as being authoritative for some accepted reason. Whatever value they possess is useful for comparison to unknowns for the purpose of establishing or confirming an assigned value based on the standard. The design of this comparison process for measurements is metrology. The execution of measurement comparisons for the purpose of establishing the relationship between a standard and some other measuring device is calibration.

The ideal standard is independently reproducible without uncertainty. This is what the creators of the “meter” length standard were attempting to do in the 19th century when they defined a meter as one ten-millionth of the distance from the equator to one of the Earth’s poles. Later, it was learned that the Earth’s surface is an unreliable basis for a standard. The Earth is not spherical and it is constantly changing in shape. But the special alloy meter bars that were created and accepted in that time period standardized international length measurement until the 1950s. Careful calibrations allowed tolerances as small as 10 parts per million to be distributed and reproduced in metrology laboratories worldwide, regardless of whether the rest of the metric system was implemented and in spite of the shortfalls of the meter’s original basis.


Modern standards


Currently, only five independent units of measure are internationally recognized: temperature interval, linear distance, electrical current, frequency and mass. All measurements of all types are based on one or more of these independent units. Two supplemental independent units are also recognized internationally, both dealing with angle measurement.

For example, Ohm's law
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...

 is a widely known concept in electrical study. Of the three units of measure involved, only current (ampere) is an independent unit. Voltage and resistance units are dependent on current units, as defined by Ohm's law.

In the United States, ASTM Standard Practice E 380,replaced by IEEE/ASTM SI10 http://www.techstreet.com/cgi-bin/detail?product_id=2968, adapts independent unit of measure theory to practical measurement activity.

It is believed that each of independent units of measure will be defined in terms of the other four independent units eventually. Length (meter) and time (second) are already connected this way. If an accurate time base is available, then a length standard can be reproduced without a meter bar artifact, using the known constant speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

. Lesser known is the relationship between the luminance (candela) and current (ampere). The candela is defined in terms of the watt, which in turn is derived from the ampere. This difficult to recreate standard is supplemented by an incandescent bulb design that is used as a secondary and transfer standard. These bulbs recreate the candela when a specific amount of current is applied.

The development of standards follows the needs of technology. As a result, some units of measure have much more resolution than others. The second is reproducible to 1 part in 1014. As it became possible to measure time more precisely, solar time
Solar time
Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...

, believed to be a constant, proved to be very slightly irregular. This resulted in leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

 adjustments to keep UTC synchronised with solar time.

Luminance (candela) can only be reproduced to 5% of reading despite having sensors that have accuracies of +/- 50 parts per million (0.005%) precision. This is due to the standard not being accurately reproducible.

Temperature (kelvin) is defined by agreed fixed points. These points are defined by the state changes of nearly pure materials, generally as they move from liquid to solid. Between these fixed points, Standard Platinum Resistance Thermometers (SPRTs), constructed in a specified manner, are used to interpolate temperature values. This mosaic of approaches produces measurement uncertainty which is not uniform over the entire range of temperature measurement. Temperature measurement is coordinated by the International Practical Temperature Scale, maintained by the BIPM.

These non-commercial measurement details used to be academic curiosities. However, engineering, manufacturing and ordinary living now routinely challenge the limits of measurement.

Industry-specific standards


In addition to standards created by national and international standards organizations, many large and small industrial companies also define metrology standards and procedures to meet their particular needs for technically and economically competitive manufacturing. These standards and procedures, while drawing in part upon the national and international standards, also address the issues of what specific instrument technology will be used to measure each quantity, how often each quantity will be measured, and which definition of each quantity will be used as the basis for accomplishing the process control
Process control
Process control is a statistics and engineering discipline that deals with architectures, mechanisms and algorithms for maintaining the output of a specific process within a desired range...

 that their manufacturing and product specifications require. Industrial metrology standards include dynamic control plans, also known as “dimensional control plans”, or “DCPs”, for their products.

In industrial metrology, several issues beyond accuracy constrain the usability of metrology methods. These include
  • The speed with which measurements can be accomplished on parts or surfaces in the process of manufacturing, which must match the TAKT Time
    Takt time
    Takt time, derived from the German word Taktzeit which translates to cycle time, sets the pace for industrial manufacturing lines. For example, in automobile manufacturing, cars are assembled on a line, and are moved on to the next station after a certain time - the takt time...

     of the production line.
  • The completeness with which the manufactured part can be measured such as described in high-definition metrology
    High-definition metrology
    High-definition metrology refers to measurement of dimensional or other attributes of a surface or an object in which measurements are made densely across the observable extent of that surface or object, so that the measured attribute of the surface or object can be portrayed with high-definition...

    ,
  • The ability of the measurement mechanism to operate reliably in a manufacturing plant environment considering temperature, vibration, dust, and a host of other potential hostile factors,
  • The ability of the measurement results, as they are presented, to be assimilated by the manufacturing operators or automation in time to effectively control the manufacturing process variables, and
  • The total financial cost of measuring each part.

National standards


Every country maintains its own metrology system. In the United States, the National Institute of Standards and Technology
National Institute of Standards and Technology
The National Institute of Standards and Technology , known between 1901 and 1988 as the National Bureau of Standards , is a measurement standards laboratory, otherwise known as a National Metrological Institute , which is a non-regulatory agency of the United States Department of Commerce...

 (NIST) plays the dual role of maintaining and furthering both commercial and scientific metrology. NIST does not enforce measurement accuracy directly.

The accuracy and traceability of commercial measurements is enforced per the laws of the individual states. Commercial measurement generally involves any material sold by any unit of measure. Some intuitive or obvious measurement is generally exempted, such as selling cloth on a cutting table that has a yardstick fastened to it. All counting-based transactions are generally exempt also. But each state has its own rules, responding to the accumulated concerns of the state residents.

Commercial metrology is also known as "weights and measures" and is essential to commerce of any kind above the pure barter level. Every state maintains its own weights and measures functionality with traceability to the national standards maintained by NIST. Large states further divide this effort by county, where a "Sealer" or other appointee is responsible for the validity of most common commercial measurements such as mass balances (scales) in grocery stores and gasoline pump measurements of volume. The sealer's staff and agents make periodic inspections to verify compliance, maintaining the integrity of commercial measurements.

Depending on the specific state, other state government agencies can be involved. For example, electricity watt-hour meters and water delivery flow meters are commonly monitored by the state's "public utilities commission" who enforces the measurement tolerances and traceability to NIST through the utility providers. Highway State Police and the State Highway Department generally run the commercial truck weight measurement programs for safety purposes and to minimize the damage to road surfaces that overloaded trucks cause. Nearly all states license weighmasters, weighmistresses, scale calibrators and other specialists involved in commercial measuring equipment maintenance.

The term "commercial metrology" is also used to describe calibration laboratories that are not owned by the companies they serve.

Scientific metrology addresses measurement phenomena not quantified in ordinary commerce, such as the test bed pictured at the beginning of the article. Calibration laboratories that serve scientific metrology are regulated as businesses only. They may choose to have their work accredited by voluntary certification organizations based on customer desires, but there is no requirement to do so. Irresolvable disputes involving scientific metrology are generally settled in the civil court systems. Some federal government entities like the Federal Communications Commission and the Environmental Protection Administration are considered to be the final authority in their domains rather than the NIST. Disputes involving only metrology issues with those organizations probably would not be heard in any courts.

Historical development


Metrology has existed in some form or another since antiquity
Ancient history
Ancient history is the study of the written past from the beginning of recorded human history to the Early Middle Ages. The span of recorded history is roughly 5,000 years, with Cuneiform script, the oldest discovered form of coherent writing, from the protoliterate period around the 30th century BC...

. The earliest forms of metrology were simply arbitrary standards set up by regional or local authorities, often based on practical measures such as the length of an arm. The earliest examples of these standardized measures are length
Length
In geometric measurements, length most commonly refers to the longest dimension of an object.In certain contexts, the term "length" is reserved for a certain dimension of an object along which the length is measured. For example it is possible to cut a length of a wire which is shorter than wire...

, time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

, and weight
Weight
In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...

. These standards were established in order to facilitate commerce
Commerce
While business refers to the value-creating activities of an organization for profit, commerce means the whole system of an economy that constitutes an environment for business. The system includes legal, economic, political, social, cultural, and technological systems that are in operation in any...

 and record human activity.

Little progress was made with regard to proto-metrology until various scientists, chemist
Chemist
A chemist is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties such as density and acidity. Chemists carefully describe the properties they study in terms of quantities, with detail on the level of molecules and their component atoms...

s, and physicist
Physicist
A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...

s started making headway during the scientific revolution
Scientific revolution
The Scientific Revolution is an era associated primarily with the 16th and 17th centuries during which new ideas and knowledge in physics, astronomy, biology, medicine and chemistry transformed medieval and ancient views of nature and laid the foundations for modern science...

. With the advances in the sciences, the comparison of experiment to theory required a rational system of units, and something more closely resembling modern metrology began to come into being. The discovery of atoms, electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

, thermodynamics
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...

, and other fundamental scientific principles could be applied to standards of measurement, and many inventions made it easier to quantitatively or qualitatively assess physical properties, using the defined units of measurement established by science.

Metrology was thus one of the precursors to the Industrial Revolution
Industrial Revolution
The Industrial Revolution was a period from the 18th to the 19th century where major changes in agriculture, manufacturing, mining, transportation, and technology had a profound effect on the social, economic and cultural conditions of the times...

, and was necessary for the implementation of mass production
Mass production
Mass production is the production of large amounts of standardized products, including and especially on assembly lines...

, equipment commonality, and assembly line
Assembly line
An assembly line is a manufacturing process in which parts are added to a product in a sequential manner using optimally planned logistics to create a finished product much faster than with handcrafting-type methods...

s.

Modern metrology has its roots in the French Revolution
French Revolution
The French Revolution , sometimes distinguished as the 'Great French Revolution' , was a period of radical social and political upheaval in France and Europe. The absolute monarchy that had ruled France for centuries collapsed in three years...

, with the political motivation to harmonize units all over France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

 and the concept of establishing units of measurement based on constants of nature
Nature
Nature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...

, and thus making measurement units available "for all people, for all time". In this case deriving a unit of length from the dimensions of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, and a unit of mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

 from a cube
Cube
In geometry, a cube is a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex. The cube can also be called a regular hexahedron and is one of the five Platonic solids. It is a special kind of square prism, of rectangular parallelepiped and...

 of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

. The result was platinum standards for the meter and the kilogram
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...

 established as the basis of the metric system on June 22, 1799. This further led to the creation of the Système International d'Unités, or the International System of Units
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

. This system has gained unprecedented worldwide acceptance as definitions and standards of modern measurement units. Though not the official system of units of all nations, the definitions and specifications of SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

 are globally accepted and recognized. The SI is maintained under the auspices of the Metre Convention and its institutions, the General Conference on Weights and Measures
General Conference on Weights and Measures
The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures . It is one of the three organizations established to maintain the International System of Units under the terms of the Convention du Mètre of 1875...

, or CGPM, its executive branch the International Committee for Weights and Measures
International Committee for Weights and Measures
The Interglobal Committee for Weights and Measures is the English name of the Comité international des poids et mesures . It consists of eighteen persons from Member States of the Metre Convention...

, or CIPM, and its technical institution the International Bureau of Weights and Measures
International Bureau of Weights and Measures
The International Bureau of Weights and Measures , is an international standards organisation, one of three such organisations established to maintain the International System of Units under the terms of the Metre Convention...

, or BIPM.

As the authorities on SI, these organizations establish and promulgate the SI, with the ambition to be able to service all. This includes introducing new units, such as the relatively new unit, the mole
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...

, to encompass metrology in chemistry. These units are then established and maintained through various agencies in each country, and establish a hierarchy of measurement standards that can be traced back to the established standard unit, a concept known as metrological traceability. The U.S. agencies holding this responsibility are the National Institute of Standards and Technology (NIST) and the American National Standards Institute
American National Standards Institute
The American National Standards Institute is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international...

 (ANSI).

The development of standards also does involve individual and small group achievements. In 1893, Edward Weston (chemist)
Edward Weston (chemist)
Edward Weston was an English-born American chemist noted for his achievements in electroplating and his development of the electrochemical cell, named the Weston cell, for the voltage standard...

 and his company perfected his Saturated Standard Cell design, which allowed the volt to be reproduced to 1 part in ten to the fourth power directly. This advance made a huge practical difference at a critical moment in the development of modern electrical devices. Groupings of saturated cells, called banks, can still be found in some metrology and calibration laboratories today. Edward Weston did not pursue patents for his cell design. By doing this, his superior design quickly replaced similar but inferior patented devices worldwide without much discussion.

Mechanisms


At the base of metrology is the definition, realisation and dissemination of units of measurement. Physical or chemical properties are quantised by assigning a property value in some multiple of a measurement unit.

The basic 'lineage' of measurement standards are:
  • The definition of a unit, based on some physical constant, such as absolute zero, the freezing point of water, etc.; or an agreed-upon arbitrary standard.
  • The realisation of the unit by experimental methods and the scaling into multiples and submultiples, by establishment of primary standards. In some cases an approximation is used, when the realisation of the units is less precise than other methods of generating a scale of the quantity in question. This is presently the situation for the electrical units in the SI, where voltage and resistance are defined in terms of the ampere
    Ampere
    The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...

    , but are used in practice from realisations based on the Josephson effect
    Josephson effect
    The Josephson effect is the phenomenon of supercurrent across two superconductors coupled by a weak link...

     and the quantised Hall effect.
  • the transfer of traceability
    Traceability
    Traceability refers to the completeness of the information about every step in a process chain.The formal definition: Traceability is the ability to chronologically interrelate uniquely identifiable entities in a way that is verifiable....

     from the primary standard
    Standard (metrology)
    In the science of measurement, a standard is an object, system, or experiment that bears a defined relationship to a unit of measurement of a physical quantity. Standards are the fundamental reference for a system of weights and measures, against which all other measuring devices are compared...

    s to secondary and working standards. This is achieved by calibration
    Calibration
    Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....

    .


Theoretically, metrology, as the science of measurement, attempts to validate the data obtained from test equipment. Though metrology is the science of measurement, in practical applications, it is the enforcement, verification and validation
Verification and Validation
In software project management, software testing, and software engineering, verification and validation is the process of checking that a software system meets specifications and that it fulfills its intended purpose...

 of predefined standards for:
Criterion Definition
Accuracy is the degree of exactness which the final product corresponds to the measurement standard.
Precision
Accuracy and precision
In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...

refers to the ability of a measurement to be consistently reproduced.
Reliability
Reliability engineering
Reliability engineering is an engineering field, that deals with the study, evaluation, and life-cycle management of reliability: the ability of a system or component to perform its required functions under stated conditions for a specified period of time. It is often measured as a probability of...

refers to the consistency of accurate results over consecutive measurements over time.
Traceability
Traceability
Traceability refers to the completeness of the information about every step in a process chain.The formal definition: Traceability is the ability to chronologically interrelate uniquely identifiable entities in a way that is verifiable....

refers to the ongoing validations that the measurement of the final product conforms to the original standard of measurement.


These standards can vary widely, but are often mandated by governments, agencies, and treaties such as the International Organization for Standardization, the Metre Convention, or the FDA. These agencies promulgate policies and regulations that standardize industries, countries, and streamline international trade, products, and measurements. Metrology is, at its core, an analysis of the uncertainty of individual measurements, and attempts to validate each measurement made with a given instrument, and the data obtained from it. The dissemination of traceability to consumers in society is often performed by a dedicated calibration laboratory with a recognized quality system in compliance with such standards. National laboratory accreditation schemes have been established to offer third-party assessment of such quality systems. A central requirement of these accreditations is documented traceability to national or international standards.

Some common standards include:
  • ISO 17025:2005—General Requirements for Calibration Laboratories
  • ISO 9000
    ISO 9000
    The ISO 9000 family of standards relates to quality management systems and is designed to help organizations ensure they meet the needs of customers and other stakeholders . The standards are published by ISO, the International Organization for Standardization, and available through National...

    —Quality Systems Management
  • ISO 14000
    ISO 14000
    The ISO 14000 environmental management standards exist to help organizations minimize how their operations negatively affect the environment The ISO 14000 environmental management standards exist to help organizations (a) minimize how their operations (processes etc.) negatively affect the...

    —Environmental Management
  • 21 CFR Part 210/211—FDA Regulations concerning GMP (Good Manufacturing Practices) Quality Systems
  • 21 CFR Part 110—FDA Regulations concerning Food Industry GMP's.

Time and frequency metrology


This area of metrology studies components and their characteristics, especially
  • frequency standard
    Frequency standard
    A frequency standard is a stable oscillator used for frequency calibration or reference. A frequency standard generates a fundamental frequency with a high degree of accuracy and precision. Harmonics of this fundamental frequency are used to provide reference points.Since time is the reciprocal of...

    s
  • synthesizer
    Synthesizer
    A synthesizer is an electronic instrument capable of producing sounds by generating electrical signals of different frequencies. These electrical signals are played through a loudspeaker or set of headphones...

    s
  • oscillators
    Oscillation
    Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...

  • digital clocks
    Clock
    A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...


Laboratories


A measurement standards laboratory is a laboratory
Laboratory
A laboratory is a facility that provides controlled conditions in which scientific research, experiments, and measurement may be performed. The title of laboratory is also used for certain other facilities where the processes or equipment used are similar to those in scientific laboratories...

 of metrology which establishes standards for a country or organisation.

Measurement standards laboratories

  • International Bureau of Weights and Measures
    International Bureau of Weights and Measures
    The International Bureau of Weights and Measures , is an international standards organisation, one of three such organisations established to maintain the International System of Units under the terms of the Metre Convention...

    , international body, headquartered in France, one of the bodies that governs SI
  • National Institute of Standards and Technology
    National Institute of Standards and Technology
    The National Institute of Standards and Technology , known between 1901 and 1988 as the National Bureau of Standards , is a measurement standards laboratory, otherwise known as a National Metrological Institute , which is a non-regulatory agency of the United States Department of Commerce...

     (NIST), USA, formerly the National Bureau of Standards
  • National Physical Laboratory of India
  • National Physical Laboratory (United Kingdom)
  • Physikalisch-Technische Bundesanstalt
    Physikalisch-Technische Bundesanstalt
    The Physikalisch-Technische Bundesanstalt is based in Braunschweig and Berlin. It is the national institute for natural and engineering sciences and the highest technical authority for metrology and physical safety engineering in Germany....

    , Germany

See also

  • Accuracy and precision
    Accuracy and precision
    In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...

  • Certified reference materials
    Certified reference materials
    Certified Reference Materials are ‘controls’ or standards used to check the quality and traceability of products. A reference standard for a unit of measurement is an artifact that embodies the quantity of interest in a way that ties its value to the reference base for calibration.At the highest...

  • Cosmic distance ladder
    Cosmic distance ladder
    The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth...

  • Data analysis
    Data analysis
    Analysis of data is a process of inspecting, cleaning, transforming, and modeling data with the goal of highlighting useful information, suggesting conclusions, and supporting decision making...

  • Dimensional metrology
    Dimensional metrology
    Dimensional Metrology is the science of calibrating and using physical measurement equipment to quantify the physical size of or distance from any given object. Inspection is a critical step in product development and quality control...

  • Forensic metrology
    Forensic Metrology
    Forensic metrology is metrology, the science of measurement, as it applies to forensic sciences. Forensic laboratories and criminalistic laboratories perform numerous measurements and tests to support both criminal and civil legal actions...

  • Length measurement
    Length measurement
    Length measurement is implemented in practice in an amazing variety of ways. This article is restricted to only a few methods, in particular, those used with SI units. The most commonly used approaches are the transit-time methods and the interferometer methods based upon the speed of light...

  • Metrication
    Metrication
    Metrication refers to the introduction and use of the SI metric system, the international standard for physical measurements. This has involved a long process of independent and systematic conversions of countries from various local systems of weights and measures. Metrication began in France in...

  • NCSL International
    NCSL International
    NCSL International is a global, non-profit organization whose membership is open to any organization with an interest in metrology and its application in research, development, education, and commerce.-History:...

  • Quality infrastructure
    Quality infrastructure
    Quality infrastructure relates to all fields of metrology, standardization and testing, of quality management and conformity assessment, including certification and accreditation...

  • Test method
    Test method
    A test method is a definitive procedure that produces a test result.A test can be considered as technical operation that consists of determination of one or more characteristics of a given product, process or service according to a specified procedure. Often a test is part of an experiment.The test...

  • World Metrology Day
    World Metrology Day
    World Metrology Day celebrates the signature by representatives of seventeen nations of The Metre Convention on 20 May 1875. The Convention set the framework for global collaboration in the science of measurement and in its industrial, commercial and societal application...


External links