Many gage calibration and repair facilities often find themselves discussing the topic of National Institute of Standards and Technology (NIST) traceability with customers going though ISO audits. Confirming or tracking NIST gage traceability are concepts with which more ISO accredited manufacturers should familiarize themselves.
Most manufacturers have seen the statements on calibration certificates that read, “All measurements are traceable to NIST,” when they purchase a micrometer, caliper or gage block set. But many of them still question what the word traceable really means.
The definition of traceability that has achieved global acceptance in the metrology community is contained in the International Vocabulary of Metrology (VIM) 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.”
Identifying TraceabilityIt is important to note that traceability is the property of the result of a measurement, not of an instrument or calibration report or laboratory. It is not achieved by following any one particular procedure or using special equipment.
Merely having an instrument calibrated, even by NIST, is not enough to make the measurement result obtained from that instrument traceable to realizations of the appropriate International System of Units (SI) or other specified references. The measurement system by which values and uncertainties are transferred must be clearly understood and under control.
The VIM definition states that metrological traceability is a property of a measurement results by which that result is related to specified reference standards, not to institutions. Accordingly, the phrase “traceable to NIST,” in its most proper sense, is shorthand for metrologically traceable to NIST’s practical realization of the definition of a measurement unit.
Referencing the NIST NumberMany imported gage certificates will state that equipment used for inspection is directly traceable to NIST and then a NIST test number will follow. Having a NIST test number is only a reference number for tracking internal documents at NIST. But how does one know its specific application?
Again, the NIST definition for test numbers helps explain.
“Test report numbers issued by NIST are intended to be used solely for administrative purposes,” NIST says. “Although they are often used to uniquely identify documents which bear evidence of traceability, test report numbers themselves do not address the issue and should not be used nor required as the sole proof of traceability.”
When reading that statement, one should contact the manufacturer of the gage he is using and ask what instruments or measurements the manufacturer is referring to.
As the operator of that instrument, one must provide proof of an unbroken chain of traceability if an ISO auditor asks for that information, and an ISO/IEC 17025: 2005 accredited laboratory is required to prove that its processes and procedures are traceable to NIST. The above procedures make that possible.