The method that establishes traceability when calibrations cannot be strictly made in SI units involves the use of certified reference materials to provide a physical characterization. Certified reference materials (CRMs) are well-defined substances with known properties that have been established through rigorous testing against international standards.
When direct calibration in SI units is not possible, CRMs serve as a reliable benchmark that ensures measurements are based on recognized reference points. This allows laboratories to link their measurement results back to international standards, thus maintaining traceability. By using CRMs, calibrations can be verified and linked to the broader metrological framework, even when using non-SI units.
The other options do have roles in measurement assurance, but they do not directly ensure traceability in the same manner as certified reference materials. For example, standard weights can often refer to physical standards that may not align with the current international comparison and may not have relationships to the broader system of measurement. Interlaboratory comparisons can help establish confidence in laboratory results, but they primarily serve as a means of cross-verification rather than a direct method for establishing traceability. Multiple measurements and averaging can improve precision but does not inherently link measurements to traceable standards. Thus, the use of certified reference materials is the most effective and