Primary reference measurement procedure used to obtain a measurement result without relation to a measurement standard for a quantity of the same kind.
Primary reference measurement procedures (also known as primary methods of measurement or, more simply, ‘primary methods’) allow a quantity value
to be determined with direct reference to the definition of its measurement unit or to fundamental constants. Because there are no other intermediate steps, they provide, under the stated conditions, metrologically traceable measurement results with the highest level of accuracy. The determination of amount-of-substance concentration by coulometry, gravimetry or by isotope dilution mass spectrometry are examples of measurement methods which have the potential to form the basis of primary measurement procedures.
REF: Terminology in Analytica Measurement Introduction to VIM 3
