Metrology

A scientist stands in front of a micro arcsecond (1 millionth of 1 arcsecond or 1 millionth of 1/3600 degree) testbed.

Metrology (from Ancient Greek metron (measure) and logos (study of)) is the science of measurement. Metrology includes all theoretical and practical aspects of measurement.

Introduction

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." [1]. The ontology and international vocabulary of metrology (VIM) is maintained by the International Organisation for Standardisation.

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

Scientific or fundamental metrology concerns the establishment of quantity systems, unit systems, units of measurement, 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.

A core concept in metrology is (metrological) traceability, defined 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 is most often obtained by calibration, establishing the relation between the indication of a measuring instrument and the value of a measurement standard. These standards are usually coordinated by national laboratories: National Institute of Standards and Technology (USA), National Physical Laboratory, UK, etc.

Tracebility, accuracy, precision, systematic bias, evaluation of measurement uncertainty are critical parts of a quality management system.

Metrology basics

Mistakes can make measurements and counts incorrect. If there are no mistakes, all counts will be exactly correct. 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 with its leads shorted together: the meter should read zero exactly.

• Measurement of an accepted constant under qualifying conditions, such as the triple point of pure water: the thermometer should read 273.16 Kelvin (0.01 degrees Celsius, 32.018 degrees Fahrenheit) when qualified equipment is used correctly.

• Self-checking ratio metric measurements, such as a potentiometer: 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 and reproducibility studies help quanitfy the precision: one common method is an ANOVA Gauge R&R study.

Calibration is the process where metrology is applied to measurement equipment and processes to determine conformity with a known standard of measurement, usually tracable to a national standards board.

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:

• Set-up people, who arrange the equipment needed for calibration and verify that it works correctly.

• Operators, who execute the calibration procedures and collect data.

• Tear-down people, who 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.

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

Metrologists 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.

Metrology in 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 are deceptively simple. Metrology is seldom recognized for its significance and 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.

The nature of engineering and engineering education in general is changing. Judgment development will replace skills conclusively yielded to computers between 1960 and 2000. Hopefully, metrology will take its rightful place in judgment development in the future.

Metrology 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. Later, it was learned that the Earth’s surface is a terrible 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.

Historical development

Metrology has existed in some form or another since antiquity. 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 in order to facilitate commerce and record human activity.

Little progress was made with regard to proto-metrology until various scientists, chemists, and physicists started making headway 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 meter 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 be able 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.

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. For example, Ohm's law is the most widely understood 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, per Ohm's law. Two supplemental independent units are also recognized internationally, both dealing with angle measurement.

In the United States, ASTM Standard Practice E 380,replaced by IEEE/ASTM SI10[2], 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. 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 10 to the 14th power. As this resolution capability increased, what was believed to be a constant proved to be very slightly irregular. See leap second for an explanation and as a case study of international cooperation under challenging conditions.

Luminance (candela) can only be reproduced to 5% of reading despite having sensors that are capable of 50 parts per million (0.0005%) precision. Reproducibility of the standard is the constraint.

Temperature (kelvin) is defined by accepted 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 a very specific way are used to interpolate temperature values. This mosaic of approaches produces uncertainty that is not uniform over the entire range of temperature measurement. Temperature measurement is coordinated by the International Practical Temperature Scale, maintained by the BIPM.

Non-commercial measurement details like these used to be academic curiosities. But as the frontiers of science moved forward, it pulled applied science along. Engineering, manufacturing and ordinary living now routinely challenge the limits of measurement.

For example, most owners of 'atomic clocks,' more correctly known as radio clocks, know that there are no radioactive materials in their clocks. Fewer users know that the clocks are synchronized by internal radio receivers for broadcasted time signals from real atomic clocks. There are too many other measurement devices used by people who don't have adequate comprehension of the basic principles involved. Without this basis, the need for metrology and calibration is difficult to substantiate.

After 40+ centuries of effort, there still are many unanswered questions and a lot of work remaining to be done. There also are plenty of surname-less units of measure waiting for new champions. They would join among others such as Kelvin, Watt, Ampere, Hertz and, in 1971, Siemens in the ranks of those who received the ultimate acknowledgement of their contributions to technology and measurement.

Industry-specific metrology 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 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 of the production line.
The completeness with which the manufactured part can be measured such as described in High-definition metrology,
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.

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, 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 precision, accuracy, traceability, and reliability.

Accuracy is the degree of exactness which the final product corresponds to the measurement standard.
Precision refers to the ability of a measurement to be consistently reproduced.
Reliability refers to the consistency of accurate results over consecutive measurements over time.
Traceability refers to the ongoing validations that the measurement of the final product conforms to the original standard of measurement.

(Fundamentals of Dimensional Metrology, Ted Busch, Wilkie Bros Foundation, Delmar Publishers, ISBN 0-8273-2127-9)

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—Quality Systems Management
ISO 14000—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

National standards

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 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 catch merchant cheaters, 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.

Time and frequency metrology

This area of metrology studies components and their characteristics, especially frequency standards, synthesizers, oscillators, and digital clocks.

Notes

References

Baber, Zaheer (1996). The Science of Empire: Scientific Knowledge, Civilization, and Colonial Rule in India. State University of New York Press. ISBN 0791429199.
Organisation Internationale de Metrologie Legale. (2000), International Vocabulary of Terms in Legal Metrology, [Online] http://www.oiml.org/publications/V/V001-ef00.pdf. (Latest version draft can be downloaded at http://www.ncsli.org/vim/wg2_doc_N318_VIM_3rd_edition_2006-08-01%20(3).pdf)
Bureau International des Poids et Mesures. (2005), "What is metrology", Copyright BIPM 2004, [Online] http://www.bipm.org/en/bipm/metrology/.
International Organisation for Standardisation. (2007), ISO Guide 99: International vocabulary of metrology—Basic and general concepts and associated terms (VIM), [Online] http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=45324
Sarle, W. (1995), Measurement theory: Frequently asked questions, Copyright 1995 by Warren S. Sarle, Cary, NC, USA [Online] SAS Institute web pages: ftp://ftp.sas.com/pub/neural/measurement.faq
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Bureau International des Poids et Mesures. (2000), The Convention of the meter, [Online] BIPM web pages: http://www.bipm.org/en/convention/
Melville, D.J. (2001). Sumerian metrological numeration systems, Mesopotamian Mathematics, [Online] St. Lawrence University web pages, http://it.stlawu.edu/%7Edmelvill/mesomath/sumerian.html
National Institute of Standards and Technology. (1999), The NIST Reference of Constants, Units, and Uncertainty, [Online] NIST web pages: http://physics.nist.gov/cuu/index.html
National Institute of Standards and Technology / Sematech. (n.d.). Engineering Statistics Handbook. [Online] NIST web pages: http://www.nist.gov/itl/div898/handbook/
National Physical Laboratory, UK—National Measurement Laboratory—Metrology related resources including many free PDF downloads including Good Practice Guides: [Online] http://www.npl.co.uk/
Franck Jedrzejewski, Histoire universelle de la mesure, Paris, Ellipses, 2002, ISBN 2729811060. (in french)
Ken Alder, "The Measure of All Things", Little, Brown 2002. (An historical account on the origin of the metric system, the meridian project).
Kimothi, S. K., "The Uncertainty of Measurements: Physical and Chemical Metrology: Impact and Analysis", 2002, ISBN 0873895355

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