Calibration – Metrological Evaluation of Measuring Instruments and Measurement Systems

Calibration comprises two consecutive steps. First, under specified conditions, the relationship is established between the quantity values provided by measurement standards, including their measurement uncertainties, and the corresponding indications of a measuring instrument or measuring system, including their measurement uncertainties. This information is then used to establish a relationship for obtaining a measurement result from an indication. Calibration itself does not involve any modification or adjustment of the measuring instrument or measuring system.

The calibration result describes the condition of the measuring equipment for the measurement points and conditions examined. Whether the measuring equipment meets the requirements of its intended use must be evaluated separately against defined criteria.

Systemic Position of Calibration Within Quality Management

In the context of quality management, metrologically traceable calibration results are an essential basis for reliable inspection results in quantitative measurements, e.g. in quality assurance, and for the evaluations of statistical process control that build on them. In addition, the suitability of the measuring system used and of the complete measurement process for the specific measurement task must be demonstrated.

Objectives and Core Logic of Calibration

The central objective of calibration is to determine the relationship between the reference values of a measurement standard and the indications of a measuring instrument or measuring system and to document it, including the associated measurement uncertainties. The basis for reliable calibration results is their metrological traceability: a measurement result is metrologically traceable if it can be related to a suitable reference through a documented, unbroken chain of calibrations. Each stage of this chain contributes to the measurement uncertainty of the result.

For SI units, the calibration chain extends to the realization of the respective unit of measurement. SI units are defined by fixed values of natural constants. In Germany, the Physikalisch-Technische Bundesanstalt (PTB), the national metrology institute, realizes and maintains the legal units and ensures their dissemination through metrologically traceable calibrations.

The metrological traceability of a measurement result must be distinguished from the traceability of a product, material, or process step. In an industrial environment, the latter is also often referred to as traceability, but it describes a different matter.

The result of a calibration is documented in a calibration certificate or calibration record. Depending on the procedure applied and the underlying requirements, the documentation contains in particular the calibration points examined, the associated indications or measured values, the deviations or corrections determined, the respective measurement uncertainties, and the relevant environmental and measurement conditions. In addition, information on the metrological traceability of the calibration results is included.

Distinction Between Calibration and Adjustment

During an adjustment, the measuring instrument or measuring system is modified in order to change its indication behavior. Calibration, by contrast, determines and documents the existing condition without changing it. After an adjustment, a new calibration is usually required, as the intervention changes the metrological properties of the measuring equipment.

Distinction Between Calibration and Legal Verification

Legal verification (German: Eichung) is a legally regulated procedure in the field of legal metrology for certain measuring instruments and intended uses. In Germany, according to Section 3 of the Measurement and Verification Act (MessEG), it is an inspection, evaluation, and marking of a measuring instrument carried out by an authority or at the instigation of an authority, combined with the permission to use the instrument within the scope of the Measurement and Verification Act.

Calibrations, on the other hand, are carried out on the basis of metrological, normative, contractual, or operational requirements. Whether a measuring instrument has to be calibrated depends on its intended use and the requirements placed on the measurement results obtained with it; it cannot be stated across the board that calibration is generally voluntary.

Calibration and Conformity Assessment

A calibration does not automatically confirm that a measuring instrument is suitable for a particular use. For this, the calibration results, including their measurement uncertainty, must be compared with the specified requirements. Such a conformity assessment requires defined acceptance criteria and, where applicable, a decision rule, and must be distinguished from the calibration itself. A calibration certificate can contain a statement of conformity if this has been requested and the decision rule has been defined – but it is not a necessary part of every calibration.

Relationship Between Calibration and Measurement System Suitability

Calibration describes the behavior of a measuring instrument or measuring system in relation to reference values. It does not replace an examination of the suitability of the complete measurement process. Depending on the procedure, a measurement system analysis takes into account further influences such as repeatability, reproducibility, operator, resolution, and environmental conditions that calibration alone does not cover.

Process capability indices can only be interpreted reliably if the measuring system used is suitable for the specific measurement task. A valid calibration is often a necessary basis for this, but does not replace the evaluation of the entire measurement process – even a calibrated measuring instrument can be unsuitable for a particular task due to insufficient resolution, high repeatability variation, or unsuitable measurement conditions.

Role of Calibration in Industrial Manufacturing

In manufacturing, recurring calibrations and suitable intermediate checks make it possible to detect changes in the measuring equipment over time. Calibration intervals are determined on the basis of the intended use, previous behavior, and the risks associated with incorrect measurement results. If impermissible deviations are found during a calibration, it must also be assessed whether earlier measurement results and the decisions based on them could be affected. The effect can be seen at several levels:

• Organization: traceable, auditable documentation of calibration results and measurement uncertainties
• Management: a reliable basis for decisions based on documented measurement results
• Processes: systematic evaluation of the possible effects of identified deviations on previous measurement results
• Culture: establishing a metrologically sound approach to measurement results

Normative Classification of Calibration

ISO 9001 requires suitable resources for monitoring and measurement. Where measurement traceability is a requirement or is considered by the organization to be an essential part of providing confidence in the validity of measurement results, measuring equipment must be calibrated, verified, or both at specified intervals or prior to use. This does not result in a general obligation to calibrate every piece of inspection equipment; the type and scope of monitoring depend on the intended use and the significance of the respective measurement result.

ISO/IEC 17025 specifies competence requirements for testing and calibration laboratories, including requirements for procedures, measurement uncertainty, metrological traceability, and reporting of results. In the automotive sector, IATF 16949 adds requirements for, among other things, calibration and verification records, internal and external laboratories, and measurement systems analysis.

FAQ – Frequently Asked Questions About Calibration

What is the goal of calibration?

The goal of calibration is to determine and document the relationship between the reference values of a measurement standard and the indications of a measuring instrument or measuring system, including the associated measurement uncertainties.

What is the difference between calibration and adjustment?

Calibration determines and documents the existing condition of a measuring instrument, while adjustment actively intervenes in the instrument to change its indication behavior.

What is the difference between calibration and legal verification?

Legal verification (Eichung) is a legally regulated, official procedure for certain measuring instruments, while calibrations are carried out on the basis of metrological, normative, contractual, or operational requirements.

Does calibration automatically confirm the suitability of a measuring instrument?

No. Whether a measuring instrument is suitable for a particular use must be assessed through a subsequent conformity assessment against defined criteria.

Does calibration replace a measurement system analysis?

No. Calibration describes the behavior of a measuring instrument in relation to reference values, while a measurement system analysis additionally takes into account influences of the entire measurement process such as repeatability, reproducibility, and operator influence.

Does calibration guarantee reliability until the next calibration?

No. A calibration describes the condition of the measuring instrument at the time of calibration. Recurring calibrations and intermediate checks make it possible to detect changes over time.

Is calibration mandatory for every measuring instrument under ISO 9001?

No. ISO 9001 requires suitable resources for monitoring and measurement; whether and to what extent calibration, verification, or both are applied depends on the intended use and the significance of the measurement result.

Standards and Norms

ISO/IEC 17025 specifies the requirements for the competence of testing and calibration laboratories. These include requirements for testing and calibration procedures, the determination of measurement uncertainty, metrological traceability, and the reporting of results. In the automotive sector, IATF 16949 supplements these requirements in particular with specifications for calibration and verification records and for measurement systems analysis.

Technical guidelines are also available for the practical monitoring of inspection equipment. The guideline series VDI/VDE/DGQ/DKD 2618 describes the inspection and monitoring of measuring equipment for geometrical quantities and contains equipment-specific calibration and inspection instructions.

However, calibration alone does not demonstrate that a measuring system or measurement process is suitable for a specific inspection task. In the automotive sector, VDA Volume 5 “Capability of Measurement Processes” and the AIAG reference manual “Measurement Systems Analysis” are used in particular for this purpose. Both approaches serve to evaluate the suitability of measuring systems or measurement processes, but set different methodological priorities.

For the digital exchange of inspection equipment and calibration data, VDI/VDE 2623 defines the Calibration Data Exchange Format. It enables the structured transfer of inspection equipment master data, calibration orders, and calibration results between inspection equipment management systems, calibration laboratories, and other software systems.

Further Internal Content

• Inspection equipment management as the organizational framework for calibration
• Process capability as a downstream analysis of indices
• Inspection equipment management with integrated calibration management in the CAQ system

AI-assisted · Editing and publication: Christian Sedlag · Technically revised and reviewed by: Jonas Voss · Last updated: 28 October 2026