Calibration Standard Glossary – Key Definitions and Concepts

Calibration Standard

A calibration standard is a device, material, or artifact whose value for a specific physical property is precisely established and officially recognized. These standards are the benchmarks for calibrating measurement instruments and systems, providing the foundation for accurate, reliable, and traceable measurement results in science, industry, and regulation.

Calibration standards are classified by their place in the traceability hierarchy:

• Primary standards directly realize or define SI units, maintained by national metrology institutes (NMIs).
• Secondary standards are calibrated against primary standards.
• Working standards are used for daily instrument calibration and are traceable to higher-level standards.

The use of calibration standards is crucial in fields like aviation, pharmaceuticals, manufacturing, energy, and environmental monitoring, where measurement integrity has direct safety, legal, and economic consequences. Calibration standards are accompanied by certificates detailing their value, uncertainty, and traceability, establishing confidence in measurement results.

Metrology

Metrology is the science of measurement. It ensures the accuracy, consistency, and reliability of measurements in all scientific, industrial, and commercial domains. Metrology includes:

• Scientific (fundamental) metrology: Development and realization of measurement standards and SI units.
• Industrial metrology: Application of standards in manufacturing and production.
• Legal metrology: Regulation of measurements in trade, safety, and environmental protection.

Internationally, the Bureau International des Poids et Mesures (BIPM) coordinates the SI system, while national metrology institutes (e.g., NIST, PTB, NPL) realize and disseminate standards. Metrology underpins traceability, enabling global comparability and mutual recognition of measurements. In aviation, metrology ensures instruments like altimeters and pressure gauges deliver trustworthy results, critical for safety and compliance.

Calibration

Calibration is the systematic, documented comparison of a device under test (DUT) with a calibration standard to determine and correct deviations. The process includes:

• Selecting an appropriate standard,
• Conducting measurements under controlled conditions,
• Recording results and uncertainties,
• Adjusting the DUT if necessary.

Calibration is documented in a certificate stating values, uncertainties, traceability, and environmental conditions. Standards like ISO/IEC 17025 and ANSI/NCSL Z540 govern calibration laboratory competence. In aviation and other safety-critical fields, regular calibration ensures instrument reliability and regulatory compliance.

Primary Standard

A primary standard has the highest metrological quality, realizing or defining a unit without reference to other standards. Maintained by NMIs, primary standards use physical constants or reproducible phenomena (e.g., the Josephson voltage standard for the volt, Kibble balance for the kilogram, cesium atomic clock for the second).

Primary standards are crucial for global measurement equivalence and are periodically compared internationally to ensure consistency. They are the foundation for calibrating secondary and working standards, ensuring traceability and measurement integrity in all sectors.

Secondary Standard

A secondary standard is calibrated by direct comparison with a primary standard. It serves as a high-accuracy reference for calibrating working standards and measurement devices. While less accurate than primary standards, secondary standards are vital for practical dissemination of traceability.

Examples include precision gauge blocks, resistance boxes, and reference weights certified against primary standards. In regulated industries, secondary standards support quality, compliance, and the effective spread of measurement traceability.

Working Standard

A working standard is used routinely to calibrate or verify measurement instruments in laboratories, manufacturing, or field settings. Calibrated against secondary (or sometimes primary) standards, working standards are designed for robustness and frequent use.

They bridge high-level standards and everyday measurement needs. For continued reliability, working standards are regularly recalibrated and carefully documented. Their accuracy should significantly exceed that of the devices they calibrate, typically by a 4:1 ratio.

Reference Material (Certified Reference Material, CRM)

A reference material (RM) is a substance with well-characterized properties used for calibrating instruments, validating methods, or assigning values to materials. When certified and traceable to SI units, it is a certified reference material (CRM).

CRMs are extensively used in chemical, physical, and biological measurements—such as calibrating spectrometers or ensuring fuel quality in aviation. Produced by specialized organizations, CRMs come with certification documents detailing their properties, uncertainties, and traceability.

Traceability

Traceability ensures a measurement result can be related to national or international standards through an unbroken chain of calibrations, each with stated uncertainties. This property guarantees measurement results are consistent, comparable, and globally accepted.

Traceability is documented in calibration certificates, forming the basis for regulatory compliance and mutual recognition. In aviation, traceable calibration is essential for flight safety and legal acceptance.

Measurement Uncertainty

Measurement uncertainty quantifies the doubt associated with a measurement result, incorporating all sources of error. It is typically expressed as ± a value at a specified confidence level.

Uncertainty is calculated by identifying and statistically combining all error sources, following guidelines like the GUM (Guide to the Expression of Uncertainty in Measurement). In calibration, the standard’s uncertainty should be much less than that of the DUT, usually using a 4:1 or 5:1 ratio.

Device Under Test (DUT)

The device under test (DUT) is the instrument or sensor being evaluated or adjusted in calibration. Its performance is compared to a standard, with results documented in a calibration certificate. DUTs in aviation include altimeters, transponders, and fuel gauges. Accurate calibration of DUTs is vital for operational safety and regulatory compliance.

Calibration Certificate

A calibration certificate is the official record of a calibration event. It includes:

• Identification of the calibrated device,
• Measured values and uncertainties,
• Information on standards used and traceability,
• Environmental conditions,
• Calibration procedures,
• Dates and recommended recalibration intervals,
• Laboratory accreditation status.

In aviation, accredited calibration certificates are necessary for regulatory compliance and quality assurance.

Recalibration Interval

The recalibration interval is the scheduled time between calibrations to ensure ongoing measurement accuracy and compliance. Determined by usage, device criticality, manufacturer guidance, and historical data, regular recalibration minimizes drift and risk. Proper interval management supports audit readiness and measurement reliability.

Accreditation (ISO/IEC 17025, ANSI/NCSL Z540)

Accreditation confirms a laboratory’s technical competence, impartiality, and traceability. The leading standards are ISO/IEC 17025 (international) and ANSI/NCSL Z540 (U.S.). Accredited labs are audited for proficiency and must maintain rigorous quality and documentation systems. Accreditation ensures calibration results are globally recognized and trusted.

International System of Units (SI Units)

The International System of Units (SI) is the basis for all scientific and industrial measurements. It defines seven base units: meter, kilogram, second, ampere, kelvin, mole, and candela. These units are realized through fundamental constants or reproducible phenomena, ensuring worldwide consistency in calibration and measurement.

Legal Metrology

Legal metrology covers laws and regulations ensuring the accuracy of measurements in trade, safety, and public welfare. It mandates regular verification and calibration of instruments like fuel dispensers, scales, and meters, with traceability to recognized standards. Compliance is enforced through inspections and audits.

ISO/IEC 17025

ISO/IEC 17025 specifies requirements for the technical competence of testing and calibration laboratories. Accreditation to this standard ensures reliable, traceable, and globally accepted measurement results. It is a requirement for many aviation, manufacturing, and regulatory applications.

ANSI/NCSL Z540

ANSI/NCSL Z540 is a U.S. standard for calibration laboratory competence and traceability, widely used in industry and defense. It complements ISO/IEC 17025 and ensures that calibration results meet rigorous national requirements.

Learn More

For detailed guidelines, best practices, and support in calibration and metrology, reach out to our experts or explore our demo solutions. Ensuring the integrity of your measurements is our priority.

Last updated: 2024-06