Uncertainty is the buzzword today for persons performing calibrations, as noted in the article "Closed-Loop Control in Hardness Testing" in this issue. For example, an uncertainty statement must be submitted with most calibrations including hardness when working to ISO Guide 17025. However, customers could request an uncertainty statement with every test performed someday, as measurement capability and accuracy become more important in a global market involving increasingly sophisticated products and processes. Are you prepared to provide this information?
A measurement result is complete only when accompanied by a quantitative statement of its uncertainty according to NIST Reference on Constants, Units and Uncertainty. The uncertainty is required to decide if the result is adequate for its intended purpose and to determine if it is consistent with other similar results.
National Institute of Standards and Technology's (NIST)-the U.S. measurement authority-develops, maintains and disseminates national standards for basic measurement quantities and for many derived measurement quantities and assesses measurement uncertainties associated with the values assigned to these measurement standards. This also involves the concept of measurement traceability, which requires the establishment of an unbroken chain of comparison to stated references.
An unbroken chain of comparison is defined as a complete detailed, documented series of comparisons that successively link the value and uncertainty of a result of measurement with the values and uncertainties of each of the intermediate reference standards and the highest reference standard to which traceability for the result of measurement is claimed. Measurement accuracy needs to be judged on the basis of this documented line of accepted standards.
A stated reference is defined as a stated reference standard that is clearly described in supporting documentation, and a reference standard that (according to the International Vocabulary of Basic and General Terms in Metrology, or VIM) generally has the highest metrological quality available at a given location or in a given organization, which is used to derive measurements.
NIST ensures traceability of measurement results or values of standards that it either provides directly or through a program collaboration. Other organizations are responsible for establishing the traceability of their own results or values to those of NIST or other stated references. NIST adopted this policy statement to document its role with respect to traceability.
NIST uses and recommends others to use the definition of traceability provided in the most recent version of VIM: ¿he 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.
Stated uncertainties is defined as uncertainty of measurement that fulfills the VIM definition; the parameter associated with the result of a measurement that characterizes the dispersion of values, which reasonably could be attributed to the measurand, is evaluated and expressed according to the general rules given in the ISO Guide to the Expression of Uncertainty in Measurement and is explicitly set forth in supporting documentation.
To learn more about uncertainty of measurement results, visit NIST Reference on Constants, Units and Uncertainty web site at http://physics. nist.gov/cuu/uncertainty.