Metrology is the science of measurement and its application. It encompasses the theoretical and practical foundations of determining quantities, establishing reference standards, and assessing the reliability of results. Its scope extends from fundamental research to manufacturing and regulated commercial transactions, regardless of the level of uncertainty involved. Metrology provides the framework that allows measurements made by different instruments, laboratories, and countries to be meaningfully compared. (jcgm.bipm.org)
Scope and branches
Metrology is commonly divided into three overlapping branches. Scientific or fundamental metrology develops measurement methods, realizes units, and establishes standards. Industrial or applied metrology applies these foundations to manufacturing and other processes, including instrument calibration and quality control. Legal metrology concerns measurements and measuring instruments subject to regulatory requirements, particularly where results affect commercial transactions, public safety, or environmental protection. (tsapps.nist.gov)
These branches share methods but serve different purposes. A research laboratory may develop a more accurate standard; a manufacturer may use calibrated instruments to control production; a regulatory authority may verify that a fuel dispenser meets legally specified requirements. Legal metrology supports trade through controls on such devices as weighing instruments, utility meters, and fuel dispensers. The International Organization of Legal Metrology develops international recommendations to help harmonize these requirements. (tsapps.nist.gov)
Units, definitions, and realization
A measurement unit supplies an agreed reference for expressing a quantity value. The International System of Units (SI) provides the internationally agreed system. Its modern definitions, effective from 20 May 2019, fix the numerical values of seven defining constants, including the speed of light, Planck constant, elementary charge, and Boltzmann constant. These definitions connect units to reproducible physical relationships rather than requiring every unit to depend on a unique material artifact. (bipm.org)
A definition must be distinguished from a realization: an experimental implementation of that definition. The second is defined through the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, while the kilogram is defined through the fixed value of the Planck constant. Although defining constants have exact numerical values in SI units, practical realizations still involve measurement uncertainty. Exact definitions therefore do not imply exact instrument readings. (bipm.org)
Calibration and traceability
Calibration establishes, under specified conditions, a relationship between reference quantity values and an instrument’s indications, including their associated uncertainties. That relationship is then used to obtain measurement results from indications. Calibration is not the same as adjustment: adjustment changes a measuring system’s response, whereas calibration characterizes its relationship to reference values. Verification instead concerns whether specified requirements have been fulfilled. (jcgm.bipm.org)
Metrological traceability is a property of a measurement result: the result can be related to a specified reference through a documented, unbroken chain of calibrations, each contributing uncertainty. A reference may be a unit definition, a measurement procedure, or a measurement standard. Traceability belongs to particular results rather than simply to an instrument or laboratory, and it does not by itself establish that uncertainty is sufficiently small for the intended application. (jcgm.bipm.org)
Calibration laboratories and national metrology institutes form an infrastructure for transferring reference values to users. In addition to instrument calibration, this infrastructure can include reference materials, evaluated reference data, test methods, and laboratory competence assessment. Chemical measurements, for example, may rely on materials with well-characterized properties as well as calibrated equipment. (nist.gov)
Uncertainty, accuracy, and precision
Measurement uncertainty characterizes the dispersion of values attributable to the quantity being measured. It reflects limitations in available knowledge, including instrument behavior, reference values, environmental conditions, sampling, and the measurement model. Uncertainty differs from error: error is a difference relative to a reference value, whereas uncertainty expresses how well the result is known. Correcting a recognized systematic effect does not eliminate uncertainty in that correction. (bipm.org)
Accuracy concerns closeness to a true quantity value and is not itself assigned a numerical value in the International Vocabulary of Metrology. Precision concerns agreement among repeated measurements under specified conditions. An instrument can produce closely grouped readings while retaining a systematic offset; good precision alone therefore does not establish accuracy. (jcgm.bipm.org)
The Guide to the Expression of Uncertainty in Measurement distinguishes Type A evaluation, using statistical analysis of observations, from Type B evaluation, using other information such as calibration certificates or specifications. Both express components through quantities such as standard deviations. Components are combined through a measurement model, accounting for covariance where necessary. Expanded uncertainty commonly multiplies combined standard uncertainty by a coverage factor; interpreting the resulting interval requires stating the relevant assumptions and coverage information. (bipm.org)
International organization and applications
The Metre Convention, signed on 20 May 1875, established the International Bureau of Weights and Measures (BIPM) and a permanent framework for international cooperation on measurement units. The General Conference on Weights and Measures adopts major decisions concerning the SI. International coordination allows national standards to develop within a common system rather than as isolated references. (bipm.org)
The CIPM Mutual Recognition Arrangement, established in 1999, provides a framework through which national metrology institutes demonstrate the international equivalence of their standards and calibration and measurement certificates. Its technical basis includes measurement comparisons, supported by reviewed calibration and measurement capabilities. (bipm.org)
Practical metrology covers dimensional, electrical, optical, chemical, and radiological measurements, alongside mass, temperature, pressure, flow, and time and frequency. Applications range from checking manufactured dimensions to characterizing chemical reference materials and calibrating thermometers. The required measurement capability depends on the quantity, operating conditions, and intended use: improving resolution alone does not replace calibration, traceability, or uncertainty evaluation. (nist.gov)