Works or accredited calibration certificate - which?

How do works and accredited certificates differ?

Both certificates document that an instrument was checked against a standard whose value is traceable, through an unbroken chain, to a national metrology institute (in Germany the PTB). The decisive difference is accreditation: an accredited (DAkkS) certificate comes from a laboratory that the national accreditation body has assessed to ISO/IEC 17025 for exactly that measured quantity.

A works calibration certificate (also called an ISO or manufacturer certificate) is issued by the maker or a service provider on their own authority. Traceability may well be in place, but the laboratory's competence and the stated measurement uncertainty are not monitored by an independent third party.

Works vs accredited calibration side by side

The choice depends on traceability, measurement uncertainty and proof obligations. The table below sets out the key criteria against each other.

• Works calibration: cost-effective and sufficient for non-critical uses and internal quality control.
• Accredited calibration: documented, small measurement uncertainty and recognised worldwide via the ILAC/EA agreements.
• Only accredited labs may display the accreditation symbol and the D-K-xxxxx identifier.
• Both certificates are traceable - the difference is the independent proof of competence.

When is an accredited calibration required?

An accredited calibration is called for whenever measurement results must stand up to third parties: for reference standards, in regulated sectors and whenever auditors demand an independent proof of competence.

• Reference and working standards you use to calibrate other instruments yourself.
• Regulated fields such as medical devices, aerospace, automotive and pharma.
• Test equipment whose small uncertainty feeds directly into product conformity.
• Customer contracts or audits (ISO 9001, IATF 16949) that require accredited calibration.
• Disputes where a legally sound proof of traceability is needed.

ISO 9001 requires traceability but does not mandate accreditation - a works certificate with proven traceability is often enough. Set the calibration interval on a risk basis, driven by actual usage and instrument stability, rather than a fixed default value.

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