What does ‘NIST traceability’ mean?
Everyone knows what “NIST traceability” is, right? Someone ‘checked’ your thermometer or hydrometer or other measuring device against another device which was “NIST traceable”, so now your instrument is ‘NIST traceable’ also, right?
Maybe. Maybe not.
Firstly, traceability is a property of a measurement, not of a device (instrument). You cannot have a thermometer which is ‘traceable to NIST’. You can have a calibrated thermometer whose indications are traceable to NIST, providing certain criteria have been met.
The International Vocabulary of Basic and General Terms in Metrology defines traceability as the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties. (International Vocabulary of Basic and General Terms in Metrology (VIM), BIPM, IEC, IFCC, ISO, IUPAC, IUPAP, OIML, 2nd ed., 1993, definition 6.10) (from the NIST website)
The American Association for Laboratory Accreditation (A2LA)’s document entitled “A2LA Policy on Measurement Traceability”, dated 10/2008, expands the explanation of this concept. We quote the following, with permission from the A2LA:
1. An unbroken chain of comparisons:
Going back to stated references acceptable to the parties, usually a national or international standard
2. Measurement uncertainty:
The uncertainty of measurement for each step in the traceability chain must be calculated or estimated according to agreed methods and must be stated so that an overall uncertainty for the whole chain may be calculated or estimated
each step in the chain must be performed according to documented and generally acknowledged procedures; and the results must be recorded
The laboratories or bodies performing one or more steps in the chain must supply evidence for their technical competence (e.g. by demonstrating that they are accredited)
5. Reference to SI units:
The chain of comparisons must, where possible, end at primary standards for realization of the SI units
6. Calibration intervals:
Calibrations must be repeated at appropriate intervals; the length in of these intervals will depend on a number of variables (e.g. uncertainty required, frequency of use, way of use, stability of equipment).
Make certain that your calibration supplier can meet these criteria – or else a knowledgeable auditor might take issue with that calibration.
Trust your critical calibrations to a competent, accredited calibration laboratory.
For more information on traceability, check the following links: