Establishing Metrological Traceability (ISO 17025 Clause 6.5)
Understanding the unbroken chain of comparisons to national standards and how to ensure your results are traceable.
Introduction
Metrological traceability is the property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.
Why is it Important?
Without traceability, results are just numbers. Traceability ensures that a “kilogram” in your lab is the same as a “kilogram” in a lab on the other side of the world. It is the basis for acceptance of results across borders.
How to Establish Traceability
1. Calibration by a Competent Lab
The most common way is to have your measuring equipment calibrated by a laboratory that demonstrates competence (usually accredited to ISO/IEC 17025) and capability (scope of accreditation).
2. Certified Reference Materials (CRMs)
Use CRMs produced by a competent producer (ISO 17034). The values of the CRM must be traceable to the SI.
3. Direct Realization of SI Units
For primary standards labs, traceability is established by direct realization of the SI unit (e.g., using a Cesium clock for time).
The “Unbroken Chain”
- SI Unit: The definition (e.g., the speed of light for the meter).
- NMI: National Metrology Institute (e.g., NIST, NPL, PTB).
- Accredited Calibration Lab: Calibrates your reference standard.
- Your Reference Standard: Used to calibrate your working standards.
- Your Working Standard: Used to calibrate your process instruments.
- Your Measurement Result: The final value you report.