Definition · Plain-language
Metrological traceability
Metrological traceability is the property of a measurement result whereby it can be related to a reference through a documented, unbroken chain of calibrations.
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The unbroken chain
Metrological traceability is built from a series of calibrations, each comparing one reference against a higher one. A working instrument is calibrated against a laboratory reference, which is calibrated against a national standard, which is realised against the SI definition. This sequence is the traceability chain, and it must be unbroken and documented: every link is a calibration with a stated measurement uncertainty. Because each link contributes uncertainty, the overall uncertainty of a result grows as you move down the chain away from the primary reference. A gap anywhere breaks traceability for everything below it.
Why traceability matters
Traceability is what makes measurements comparable and trustworthy across organisations and borders. A blood-glucose result, a weight in commerce or a dimensional measurement in manufacturing means the same thing everywhere only because each is traceable to a common reference. Without traceability, two laboratories measuring the same quantity could not be sure their results were on the same scale. This is why laboratory standards such as ISO/IEC 17025 and ISO 15189 require results that affect their validity to be metrologically traceable, and why traceability underpins international trade, regulation and science.
Traceability and the SI
The reference at the top of most traceability chains is the International System of Units (SI), whose base units are defined in terms of fundamental constants and realised by national metrology institutes such as NIST or NPL, coordinated internationally by the BIPM. Where an SI unit cannot be used directly, traceability may instead be to a certified reference material or an agreed reference measurement procedure. In all cases, the essential features are the same: a stated reference, an unbroken chain of documented calibrations, and an associated measurement uncertainty at each step.
Key facts
At a glance
- Definition: a result related to a reference through an unbroken chain of calibrations
- Usual reference: the SI (International System of Units)
- Requirement: each link documented, with a stated measurement uncertainty
- Effect: uncertainty accumulates down the chain from the primary reference
- Maintained by: national metrology institutes, coordinated by the BIPM
- Required by: ISO/IEC 17025 and ISO 15189 for results that affect validity
Common misconceptions
What people often get wrong
Often heard: A calibration certificate alone guarantees traceability.
Actually: Traceability requires the whole chain to be unbroken and documented, with each reference itself traceable to a higher one and a stated uncertainty at every step. A single certificate is meaningful only if the references behind it are themselves traceable.
Often heard: Traceability means a result has no uncertainty.
Actually: Traceability does not eliminate uncertainty; it makes it known and comparable. Every calibration in the chain contributes uncertainty, so a traceable result always carries a stated measurement uncertainty that accumulates down the chain.
Often heard: Traceability is the same as keeping records of where a sample came from.
Actually: Metrological traceability concerns measurement results and their link to a reference through calibrations, not sample provenance or chain-of-custody. The two are different concepts that share the word traceability.
Going deeper
