metrology Metrology is the scientific study of measurement. 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 Fran ...

(the science of

measurement Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. In other words, measurement is a process of determining how large or small a physical quantity is as compared ...

), a standard (or etalon) is an

object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ...

, system, or experiment that bears a defined relationship to a

unit of measurement A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other quantity of that kind can be expressed as a multi ...

of a

physical quantity A physical quantity is a physical property of a material or system that can be quantified by measurement. A physical quantity can be expressed as a ''value'', which is the algebraic multiplication of a ' Numerical value ' and a ' Unit '. For exam ...

. Standards are the fundamental reference for a

system of weights and measures A system of measurement is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce. Systems of measurement i ...

, against which all other measuring devices are compared. Historical standards for

length Length is a measure of distance. In the International System of Quantities, length is a quantity with dimension distance. In most systems of measurement a base unit for length is chosen, from which all other units are derived. In the Inte ...

volume Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). ...

, and

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different ele ...

were defined by many different authorities, which resulted in confusion and inaccuracy of measurements. Modern measurements are defined in relationship to internationally standardized reference objects, which are used under carefully controlled laboratory conditions to define the units of length, mass,

electrical potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...

, and other physical quantities.

Hierarchy of standards

There is a three-level hierarchy of physical measurement standards. At the top of the tree are the master standards – these are known as ''primary standards''. Primary standards are made to the highest metrological quality and are the definitive definition or realization of their unit of measure. Historically, units of measure were generally defined with reference to unique artifacts which were the legal basis of units of measure. A continuing trend in metrology is to eliminate as many as possible of the artifact standards and instead define practical units of measure in terms of fundamental physical constants, as demonstrated by standardized technique. One advantage of elimination of artifact standards is that inter-comparison of artifacts is no longer required. Another advantage would be that the loss or damage of the artifact standards would not disrupt the system of measures. The next quality standard in the hierarchy is known as a ''secondary standard''. Secondary standards are calibrated with reference to a primary standard. The third level of standard, a standard which is periodically calibrated against a secondary standard, is known as a ''working standard''. Working standards are used for the calibration of commercial and industrial measurement equipment.

Primary standards

An example of a primary standard was the

international prototype of the kilogram The International Prototype of the Kilogram (referred to by metrology, metrologists as the IPK or Le Grand K; sometimes called the ''wiktionary:ur-#Prefix, ur-kilogram,'' or ''urkilogram,'' particularly by German-language authors writing in Engli ...

(IPK) which was the master

kilogram The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially. ...

and the primary mass standard for the

International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. ...

(SI). The IPK is a one kilogram mass of a platinum-iridium alloy maintained by the

International Bureau of Weights and Measures The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental organisation, through which its 59 member-states act together on measurement standards in four areas: chemistry ...

(BIPM) in Sèvres, France. Another example is the unit of electrical potential, the

volt The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827). Defin ...

. Formerly it was defined in terms of

standard cell In semiconductor design, standard cell methodology is a method of designing application-specific integrated circuits (ASICs) with mostly digital-logic features. Standard cell methodology is an example of design abstraction, whereby a low-level v ...

electrochemical batteries, which limited the stability and precision of the definition. Currently the volt is defined in terms of the output of a Josephson junction, which bears a direct relationship to fundamental physical constants. In contrast, the reference standard for the

metre The metre ( British spelling) or meter ( American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its pre ...

is no longer defined by a physical object (as the former international prototype of the metre (IPM) or originally the '' mètre des Archives''). In 1983, the standard metre was redefined as the distance light travels in a vacuum during 1/ of a second.

Secondary reference standards

Secondary reference standards are very close approximations of primary reference standards. For example, major national measuring laboratories such as the US's National Institute of Standards and Technology (NIST) will hold several "national standard" kilograms, which are periodically calibrated against the IPK and each other.

Working standards

A machine shop will have physical working standards (

gauge block Gauge blocks (also known as gage blocks, Johansson gauges, slip gauges, or Jo blocks) are a system for producing precision lengths. The individual gauge block is a metal or ceramic block that has been precision ground and lapped to a specific ...

s for example) that are used for checking its measuring instruments. Working standards and

certified reference materials Certified reference materials (CRMs) are 'controls' or standards used to check the quality and metrological traceability of products, to validate analytical measurement methods, or for the calibration of instruments. A certified reference materi ...

used in commerce and industry have a traceable relationship to the secondary and primary standards. Working standards are expected to deteriorate, and are no longer considered traceable to a national standard after a time period or use count expires.

Laboratory standards

National organizations provide calibration and private industrial laboratories with items, processes and/or certification so they can provide certified traceability to national standards. (In the United States, NIST operates the NVLAP program.) These laboratory standards are kept in controlled conditions to maintain their precision, and used as a reference for calibration and creating working standards. Sometimes they are (incorrectly) called "secondary standards" because of their high quality and reference suitability.

References