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Measurement Traceability In Calibration

The concept of measurement traceability can be confusing when you are the purchasing calibration services. We will explain what you need to know to make an informed decision.

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The VIM- International Vocabulary Of Metrology  - defines metrological traceability as " 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". It also refers to a measurement hierarchy which we illustrated above.

Another way to visualize traceability is the practice of re gifting. You receive a gift, but instead of opening it and re wrapping it you simply wrap over the original gift and give it to someone else and write your name on the packaging. They do the same and so on.  The original gift is the same, but all the packaging changes the dimensions of the package and you know who wrapped it each time. You have an unbroken chain because each person that gifted signed their name and left the previous intact.  If someone opened the gift and removed the original or previous packaging the traceability would be gone.

The BIPM - Bureau International des Poids et Measures- also offers an explanation of traceability.

"metrological traceability

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

NOTES

For this definition, a 'reference' can be a definition of a measurement unit through its practical realization, or a measurement procedure including the measurement unit for a non-ordinal quantity, or a measurement standard.

Metrological traceability requires an established calibration hierarchy.

Specification of the reference must include the time at which this reference was used in establishing the calibration hierarchy, along with any other relevant metrological information about the reference, such as when the first calibration in the calibration hierarchy was performed.

For measurements with more than one input quantity in the measurement model, each of the input quantity values should itself be metrologically traceable and the calibration hierarchy involved may form a branched structure or a network. The effort involved in establishing metrological traceability for each input quantity value should be commensurate with its relative contribution to the measurement result.

 Metrological traceability of a measurement result does not ensure that the measurement uncertainty is adequate for a given purpose or that there is an absence of mistakes.

A comparison between two measurement standards may be viewed as a calibration if the comparison is used to check and, if necessary, correct the quantity value and measurement uncertainty attributed to one of the measurement standards.

The ILAC considers the elements for confirming metrological traceability to be an unbroken metrological traceability chain to an international measurement standard or a national measurement standard, a documented measurement uncertainty, a documented measurement procedure, accredited technical competence, metrological traceability to the SI, and calibration intervals (see ILAC P-10:2002).

The abbreviated term "traceability" is sometimes used to mean 'metrological traceability' as well as other concepts, such as 'sample traceability' or 'document traceability' or 'instrument traceability' or 'material traceability', where the history ("trace") of an item is meant. Therefore, the full term of "metrological traceability" is preferred if there is any confusion."

In the United States, NIST- National Institute of Standards and Technology- has developed a policy on Traceability.

"Metrological traceability requires the establishment of an unbroken chain of calibrations to specified references. NIST assures the traceability of measurement results that NIST itself provides, either directly or through an official NIST program or collaboration. Other organizations are responsible for establishing the traceability of their own results to those of NIST or other specified references. NIST has adopted this policy statement to document the NIST role with respect to traceability."

It is common to see a statement of compliance similar to this:

The calibrations within the report are traceable through NIST or another National Metrology Institute to the International System of Units (SI units).

This is a requirement of ISO 17025 accredited laboratories through ILAC. Specifically, ILAC Policy on the Traceability of Measurement Results.

"5.6.3 A programme for calibration of measuring systems and verification of trueness shall be designed and performed so as to ensure that results are traceable to SI units or by reference to a natural constant or other stated reference."

 

Conclusion:

As a purchaser of calibration services you need to know if the calibration performed is traceable to the SI units through an unbroken chain.

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