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Traceability, Error, and Uncertainty

8.1 Traceability

Traceability is defined by the International Vocabulary of Basic and General Terms in Metrology (VIM)(20) as “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." From this definition, it is clear that traceability can only be obtained when uncertainties are determined. The traceability hierarchy in Rockwell hardness from highest level to the hardness measurement is the following:

(1) International definition of hardness: This is similar to a “fundamental property value." This definition should precisely define all aspects of the hardness test.

(2) National definition of hardness: At this time, there is no international agreement on a well-defined definition for any Rockwell hardness scale. National definitions are used instead, which are based on national and international test method standards. These definitions vary from country to country. The U.S. national definition of Rockwell hardness, as defined by NIST, is based on the use of the NIST primary reference standardizing machine with a specific indenter and following a specific testing cycle (15).

(3) Primary reference standardizing machine: A primary reference standardizing machine is usually maintained at a country’s National Metrology Institute (NIST in the United States). The design and operation of this machine is dependent on the hardness definition. The NIST primary reference standardizing machine is shown in Figure 11.

(4) Primary reference test blocks: The primary reference test blocks are calibrated using the primary reference standardizing machine in accordance with the hardness definition.

(5) Secondary standardizing machine: The design and operation of this machine is based on the hardness definition and calibrated using primary reference test blocks.

(6) Secondary standardized test blocks: The secondary standardized test blocks are calibrated using the secondary hardness machine in accordance with the hardness definition.

(7) Laboratory hardness test machine: The design and operation of this machine is based on the hardness definition and calibrated using standardized test blocks.

errors with respect to the Rockwell hardness definition. In the first scenario, a standardizing laboratory or a testing laboratory bases its measurement uncertainty on measurement comparisons using standardized reference test blocks. Traceability can then be linked to the highest reference level in the traceability hierarchy through one or more reference levels. In the second scenario, a standardizing laboratory’s measurement traceability is with respect to the Rockwell hardness definition and is based on standardizing machine errors as defined by the definition. This is the method that is used in determining the measurement uncertainty of primary reference standardizing machines.