Standards are objects or ideas designated as 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 “metre” length standard were attempting to do in the 19th century when they defined a metre as one ten-millionth of the distance from the equator to one of the Earth’s poles. It was later learned that the Earth’s surface is an unreliable basis for a standard, as the Earth is not spherical and it is constantly changing in shape. But the special alloy metre 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 metre’s original basis.
Historical International Prototype Metre bars
Currently, five independent units of measure are internationally recognized: temperature interval, linear distance, electrical current, frequency and mass. Any measurement can be based on one or more of these measurement units. To supplement these five, two units of angle measurement that are also independent are recognized. For example, Ohm's law 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.
Interested parties believe that eventually, standards organizations will define each of the independent units of measure in terms of the other four independent units. Length (metre) and time (second) are already connected this way. If an accurate time base is available, then a length standard can be reproduced without a metre bar artifact, using the known constant speed of light. Lesser known is the relationship between the luminance (candela) and current (ampere). The candela is defined in terms of the watt, which in turn derives from the ampere.
Non-commercial measurement details were once academic curiosities. However, engineering, manufacturing and ordinary living now routinely challenge the limits of measurement. Standards development follows the needs of technology. As a result, some units of measure have higher resolution than others. The second is reproducible to 1 part in 1014. As it became possible to measure time more precisely, solar time, believed to be a constant, proved to be very slightly irregular. This resulted in leap second adjustments to keep UTC synchronised with solar time.
The candela standard is difficult to recreate. An incandescent bulb design must be used as a secondary standard a supplementary standard, a transfer standard. These special candela standard bulbs recreate the candela when a specific amount of current is applied. 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—measurements that are not uniform over the entire temperature range. Temperature measurement is coordinated by the International Practical Temperature Scale, maintained by the BIPM.
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 particular needs for technically and economically competitive manufacturing. These standards and procedures, while drawing on the national and international standards, also address issues of what specific instrument technology to use to measure each quantity, how often to measure each quantity, and which definition of each quantity to use for process control for a particular manufacturing and product specification. An industrial metrology standard includes a dynamic control plan (DCP, also known as a dimensional control plans for their product.
In industrial metrology, several issues beyond accuracy constrain the usability of metrology methods. These include:
Speed with which measurements can be accomplished on parts or surfaces in the process of manufacturing, which must match the takt time of the production line
Completeness with which the manufactured part can be measured, such as described in high-definition metrology
Ability of the measurement mechanism to operate reliably in a manufacturing plant environment considering temperature, vibration, dust, and other hostile factors
Ability of the measurement results, as presented, to be assimilated by manufacturing operators or automation in time to effectively control manufacturing process variables
Total financial cost of measuring each part
Every country maintains its own metrology system.
In the United States, the National Institute of Standards and Technology (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 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.
A state seal on fuel pump in Nevada, United States
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 final authority in their domains, rather than the NIST. Disputes involving only metrology issues with those organizations probably would not be heard in any court.
See also: History of measurement
Metrology has existed in some form or another since antiquity. Solomon said: "A false balance is an abomination to the Lord, but an accurate weight is his delight." 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, time, and weight. These standards were established to facilitate commerce and record human activity.
Significant progress in metrology was made by various scientists, chemists, and physicists during the scientific revolution. 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, thermodynamics, 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, and was necessary for the implementation of mass production, equipment commonality, and assembly lines.
Modern metrology has its roots in the French Revolution, with the political motivation to harmonize units all over France and the concept of establishing units of measurement based on constants of nature, 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, and a unit of mass from a cube of water. The result was platinum standards for the metre and the kilogram 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. 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 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, or CGPM, its executive branch the International Committee for Weights and Measures, or CIPM, and its technical institution the International Bureau of Weights and Measures, or BIPM.
As the authorities on SI, these organizations establish and promulgate the SI, with the ambition to service all. This includes introducing new units, such as the relatively new unit, the mole, 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 (ANSI).
The development of standards also does involve individual and small group achievements. In 1893, Edward Weston (chemist) 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.
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, but are used in practice from realisations based on the Josephson effect and the quantised Hall effect.
The transfer of traceability from the primary standards to secondary and working standards. This is achieved by calibration.
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 of predefined standards for:Criterion Definition
Accuracy Degree of exactness with which the final product corresponds to the measurement standard
Reliability Consistency of accurate results over consecutive measurements over time
Traceability Ongoing validations that the measurement of the final product conforms to the original standard of measurement
These standards 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
Institute for Reference Materials and Measurements
Metrology Equipment Listing
LearningMeasure.com Metrology Training
European Association of National Metrology Institutes (EURAMET)
Training in Metrology in Chemistry (TrainMiC)
Measurement Science in ChemistryAuthority control LCCN: sh2001008470 · GND: 4169749-2