ASME International

Metrological Traceability of Dimensional Measurements to the SI Unit of Length
 N B89.7.5

Annotation

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.