What is Measurement Traceability?

Producing Valid Results:

Risk Mitigation and Measurement Assurance

What does it mean for my calibration to be traceable? Traceability means different things to different people. The world of Metrology has been evolving and, if you haven’t kept up with it, you are likely carrying with you an outdated or incorrect definition of traceability. Some people still refer to NIST numbers, which mean nothing (see NIST’s Policy on Traceability). Others believe that traceable calibration means traceability to NIST. This is also incorrect because measurements are not traceable to an institution. Correctly stated, each calibration contains measurement results that are traceable to the international system of units (SI Units) through a national measurement institute (NMI), such as NIST in the United States, but this could also be through any country’s NMI. For example, the NMI for the United Kingdom is the National Physical Laboratory (NPL); for Canada, the National Research Council (NRC); for Germany, the Physikalisch-Technische Bundesanstalt (PTB). All the NMI’s maintain reference standards which represent the SI units of measure (and derived SI units). For reference, you may find comparisons of Calibration and Measurement Capabilities (CMC’s) for the NMI’s of the world through the International Comparisons Data Base (ICDB) on the NIST website or through the official Key Comparison Data Base (KCDB) at the BIPM website.

But traceability of measurements to SI units is only one of seven components of measurement traceability (see Related Terms above). And traceability is not just the responsibility of your calibration service provider; it’s the equipment user’s responsibility too! Think of traceability in terms of a relay race. If you are the next runner in line, you must pay attention to the approaching runner, match the pace of that runner, and be ready to grab hold of the baton before you break into your stride. The BIPM, as the lead runner, starts this ’traceability’ race, passes the baton onto the second ‘runner’ in the team (the NMI’s), who passes it to the third ‘runner’ (ISO-17025 Accredited Reference Labs/Secondary Labs), and eventually the baton is passed onto the anchor, who must not drop the baton, but carry it across the finish line. The anchor in the world of measurement traceability is any organization who manufactures products or delivers services to the business and commercial consumers of the world. Problem is, in too many cases the anchor has dropped the baton of traceability, often unintentionally. The point of this entire race is to get good products to market, as designed, with minimal rework or waste, and without product recall. If everyone on the team does their part, the anchor will cross the finish line before the competition can. If you’re not carrying the measurement uncertainty all the way through, from the BIPM to the product/service, then you’ve dropped the baton and your products may be “disqualified” as a result.

What are the Components of Traceability?

Unbroken Chain of Comparison: The BIPM (Bureau International des Poids et Mesures) establishes the definition of measuring units, particularly Le Systeme International d’unites, which are the SI units of measure. The NMI’s develop and maintain standards, which are the physical realization of the defined units of measure. A comparison of these standards must be made from one laboratory to the next to transfer the defined quantity to your instrument and, eventually, to your products.

Competence: Critical thinking skills are required to apply a scientific approach in determining all the uncertainties that can impact a calibration laboratory’s ability to assess your instrument. Education and experience are an important part of building the competence to perform Metrology work thoroughly and correctly.

Recalibration: Instrument values change over time for a variety of reasons; physical wear during normal usage, aging of electronic components, environmental factors, improper handling and/or storage, etc. Regular calibration of your instruments is required to keep it aligned (traceable) to the global network of measurements.

SI Units: There are seven base units in the SI system: temperature (Kelvin; K), time (second; s), length (metre; m), mass (kilogram; kg), luminous intensity (candela; cd), amount of substance (mole; mol), and electric current (ampere; A). All other units of measure are derived from these base units. There are also non-SI units that are still commonly used.

Uncertainty: Measurement uncertainty is a key part of passing the baton of traceability. It is one part of traceability that is often misunderstood or too easily dismissed. But, without it, your measurements have lost their link to the global network that has been established to keep you from taking unnecessary risks.

Documentation: The documentation can be viewed as “the baton”, upon which is recorded the essential information required to pass traceability on to the next runner in line, and to your products/services!

Measurement Assurance: The way you use your measuring instruments is a very important part of measurement assurance, it’s not just the calibration lab’s responsibility—you must apply the measurements properly too and understand how errors can creep into your measurements and find ways to keep them at a minimum or eliminate them altogether!

Keeping your instruments aligned to the global network of measurement via traceability is the only way you can minimize the risk associated with the instruments you use daily to make the call as to whether your product or components of your product within the manufacturing process, are good or bad. If these instruments are not providing reliable values, you could be failing good product or passing bad product, either of which can have heavy costs associated with them. Understanding where these risks lie can only be determined by carrying the baton of traceability all the way through—up to and including your products.