Knowledge
Metrology Dictionary
The complete VIM vocabulary — all numbered concepts of JCGM 200:2012 as faithful paraphrases with clause citations — plus clearly labeled practice terms from the GUM, ILAC, ISO and NIST guidance.
Accreditation
Third-party attestation, by an authoritative body, of a conformity assessment body's competence to carry out specific tasks — for laboratories, against ISO/IEC 17025, by bodies operating under ISO/IEC 17011.
ISO/IEC 17011:2017 · Quality & Accreditation
Air buoyancy correction
The correction applied in precise mass comparisons for the differing volumes of the standards and the object weighed, proportional to air density and volume difference — dominant in high-accuracy mass work.
NIST SOP 2 / OIML R 111 · Mass
As-found / as-left data
Calibration results recorded before (as-found) and after (as-left) any adjustment or repair. As-found data support reverse-traceability decisions about product accepted with the instrument since its previous calibration.
Calibration practice; NCSLI RP-1 · Calibration
Calibration and measurement capability (CMC)
The smallest uncertainty of measurement a laboratory can achieve within its accredited scope when performing more or less routine calibrations of nearly ideal devices — the uncertainty published in its scope of accreditation.
ILAC-P14:09/2020 · Quality & Accreditation
Calibration interval
The period between successive calibrations of an instrument, set and adjusted from stability history, usage, environment, and risk — commonly managed with interval-analysis methods such as those in NCSLI RP-1 and ILAC-G24.
ILAC-G24:2022 · Calibration
Check standard
A stable artifact or instrument measured regularly within a measurement assurance program; its control chart reveals drift, shifts, or loss of statistical control of the process.
NIST SOP 30 / GLP 1 · Quality & Accreditation
Conventional mass
The conventional value of the result of weighing in air, based on a reference density of 8000 kg/m³ for the weight and 1.2 kg/m³ for air at 20 °C — the basis on which standard weights are specified per OIML R 111.
OIML R 111-1:2004 · Mass
Coordinate measuring machine (CMM)
A measuring system with the means to move a probing system and determine spatial coordinates on a workpiece surface; acceptance and reverification tests are specified in the ISO 10360 series.
ISO 10360 series · Dimensional
Deadweight tester
A pressure standard in which known masses acting on a piston of known effective area generate reference pressures — the primary realization of pressure in many calibration laboratories.
Pressure metrology practice · Pressure
Decision rule
A documented rule describing how measurement uncertainty is accounted for when stating conformity with a specified requirement — required by ISO/IEC 17025:2017 when a laboratory reports pass/fail.
ISO/IEC 17025:2017, 3.7 · Quality & Accreditation
Gauge R&R
A measurement systems analysis estimating the variation due to the gauge (repeatability) and the operators (reproducibility), commonly expressed as a percentage of total variation or tolerance.
AIAG MSA manual · Statistics
Gauge block
An end standard of length with flat, parallel measuring faces of known separation, wrung into stacks to realize lengths — grades and requirements per ISO 3650.
ISO 3650:1998 · Dimensional
Guard band
An interval applied between the tolerance limit and the acceptance limit to control the risk of false accept or false reject given measurement uncertainty — commonly the expanded uncertainty U, per ILAC-G8.
ILAC-G8:09/2019 · Quality & Accreditation
ITS-90
The International Temperature Scale of 1990: defined fixed points (e.g. the triple point of water, 273.16 K) and specified interpolation instruments through which practical temperature measurement is realized worldwide.
CCT / BIPM · Temperature
Interlaboratory comparison
Organization, performance, and evaluation of measurements on the same or similar items by two or more laboratories under predetermined conditions.
ISO/IEC 17043:2023 · Quality & Accreditation
Monte Carlo method (uncertainty)
Propagation of probability distributions through a measurement model by repeated random sampling, per JCGM 101:2008. Preferred when the model is strongly non-linear or distributions are non-normal.
JCGM 101:2008 · Uncertainty
Normalized error (En)
The interlaboratory comparison statistic En = (x − X) / √(U²(x) + U²(X)). |En| ≤ 1 indicates agreement within expanded uncertainties; it is the standard scoring for calibration proficiency tests.
ISO 13528:2022 · Statistics
Process measurement assurance
A program of intermediate checks using check standards and control charts to demonstrate that a measurement process remains in statistical control between calibrations, as described in NIST SOP 30.
NIST SOP 30 (2019) · Quality & Accreditation
Proficiency testing
Evaluation of participant performance against pre-established criteria by means of interlaboratory comparisons — an ISO/IEC 17025 requirement for demonstrating ongoing competence.
ISO/IEC 17043:2023 · Quality & Accreditation
Rectangular distribution
A probability distribution assigning equal likelihood everywhere between limits ±a. Used for Type B components where only limits are known; its standard uncertainty is a/√3.
GUM (JCGM 100:2008) 4.3.7 · Uncertainty
Sensitivity coefficient
The partial derivative ∂f/∂xᵢ of the measurement model with respect to an input quantity, evaluated at the estimates: it converts an input standard uncertainty into its contribution to the uncertainty of the result.
GUM (JCGM 100:2008) 5.1.3 · Uncertainty
Standard platinum resistance thermometer (SPRT)
The interpolation instrument of the ITS-90 between defined fixed points over most of its range: a strain-free platinum resistor whose resistance ratio W(T) maps to temperature.
ITS-90 · Temperature
Test accuracy ratio (TAR)
The legacy ratio of unit-under-test tolerance to the accuracy specification of the standard used. Superseded in rigorous work by the test uncertainty ratio, which accounts for the whole measurement process.
Industry practice · Quality & Accreditation
Test uncertainty ratio (TUR)
The ratio of the tolerance of the unit under test to the expanded uncertainty of the measurement process used to test it. A 4:1 TUR is the traditional benchmark; lower ratios call for guard banding or risk analysis (ANSI/NCSL Z540.3).
ANSI/NCSL Z540.3 · Quality & Accreditation
Thermal expansion (dimensional)
The length change of a body with temperature, characterized by its coefficient of linear expansion. Dimensional results are reported at the standard reference temperature of 20 °C defined by ISO 1.
ISO 1:2022 · Dimensional
Triangular distribution
A probability distribution rising linearly to a peak midway between limits ±a. Used when values near the centre are more likely; its standard uncertainty is a/√6.
GUM (JCGM 100:2008) 4.3.9 · Uncertainty
Welch–Satterthwaite formula
The formula for estimating the effective degrees of freedom of a combined standard uncertainty from the degrees of freedom of its components — used to select a coverage factor from the t-distribution.
GUM (JCGM 100:2008) G.4 · Uncertainty
Z-score
A proficiency-testing statistic z = (x − X)/σ_pt comparing a participant result with the assigned value in units of the standard deviation for proficiency assessment; |z| ≤ 2 is conventionally satisfactory.
ISO 13528:2022 · Statistics