scope:

INTRODUCTION

This report describes the requirements for a particular interpretation¹ of metrological traceability² to the SI unit of length, i.e., the meter, for dimensional measurements, consistent with the definition in The International Vocabulary of Basic and General Terms in Metrology (VIM) [7]. The purpose is to provide a functional and usable interpretation that allows producers and customers of dimensional measurement results to agree on how to establish and demonstrate metrological traceability. A benefit of this report is that it clarifies and specifies many issues that are often debated when discussing traceability and allows the reader to understand the complexity of the traceability topic.

The VIM defines measurement traceability as

(Metrological) Traceability (VIM 1993 definition 6.10) : 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.

Historically, the principal driver for demonstrated traceability in the U.S. had been military specifications intended to ensure the quality of measurements associated with equipment procurement. Traceability was primarily a paper trail of calibration report numbers leading back to a National Measurement Institute (NMI), e.g., the National Institute of Standards and Technology (NIST). Today, metrological traceability is still tied to efforts to ensure measurement quality but (using the VIM definition) has a quantitative aspect involving measurement uncertainty. The Guide to the Expression of Uncertainty in Measurement (GUM) [1, 2] provides a unified method to evaluate measurement uncertainty that represents a quantitative measure of the quality of a measurement result. Indeed, a GUM-compliant uncertainty statement must have all significant sources of uncertainty evaluated; hence, the length standard (from which the unit of length enters the measurement) must also have its uncertainty quantitatively evaluated. This implicitly means that there must be some connection back to the SI unit, as otherwise, such a quantitative evaluation could not be performed.

From the perspective of measurement uncertainty, it is clear that traceability does not require the use of an identical type of artifact for comparison during a calibration. Rather, it is a chain of information connected at one end to the SI unit and the other end to the artifact or instrument under calibration or inspection. The quality of the chain is stated quantitatively by means of a GUM-compliant uncertainty statement.

Most NMIs and high-accuracy calibration laboratories satisfy their traceability needs through compliance with, or accreditation to, such standards as ISO 17025 [8] or ANSI/NCSL Z540-1 [9]. These standards exceed the scope of B89.7.5 and include many additional requirements, such as an extensive quality plan and external measurement audits. ASME B89.7.5 seeks to address a broader range of dimensional metrology practitioners, including those performing measurements on the factory floor, who occasionally need to show metrological traceability for their measurement results. This report examines some of the current issues with the metrological traceability concept and provides guidelines for demonstrating metrological traceability according to the B89.7.5 interpretation.

¹ The requirements described in this report are an interpretation of the 1993 VIM definition of traceability by the ASME B89.7.5 committee; practitioners who seek to invoke this interpretation should cite "metrological traceability per B89.7.5." Other organizations may have other interpretations of traceability.

² In this report, the qualifier "metrological" preceding the term "traceability" is used to distinguish this concept of traceability (the property of the result of a measurement) from other uses of the same word, such as being able to trace the history, application, or location of supplied products, parts, or materials.