In the United States, and other countries throughout the world, the National Metrology Institutes are usually not the sole supplier of the needed reference standards for that country. Normally, secondary calibration laboratories produce and calibrate the majority of test blocks for use by industry. Whereas, NIST is planning to only provide test blocks for three hardness levels of each Rockwell scale, the secondary calibration laboratories usually provide reference blocks at multiple hardness levels, allowing a better verification for a specific level of hardness.
The secondary calibration laboratories are linked to the national Rockwell scales through the use of primary reference standards to calibrate their standardizing machines and, often, through some form of accuracy assurance program in connection with the primary standardization laboratory (NMI).
In the United States, this is accomplished through direct verification of the secondary laboratories’ standardizing machines using NIST traceable instruments and artifacts, indirect verification of the secondary laboratories’ standardizing machines using NIST SRM test block standards, and voluntary participation in a calibration laboratory accreditation program. Because NIST SRMs are currently available only for the Rockwell C scale, the secondary reference test blocks of other Rockwell scales cannot be calibrated traceable to NIST Rockwell scales.