 Measurement standard

A measurement standard realizes a specified quantity with an assigned value and associated measurement uncertainty, and is used as a reference. It can be a physical artifact, a measuring system or a suitable reference material. The value and its measurement uncertainty apply under stated conditions and at the relevant time; a nominal label alone is insufficient.^[ref]

For an original comparison, take a reference resistor with assigned electrical resistance $R_s=100.000\Omega$ and standard uncertainty $u(R_s)=0.001\Omega$. A comparison measures a working resistor's ratio $q=R_x/R_s=1.00020$, with independent $u(q)=0.00001$. The measurement model givesand first-order propagation of uncertainty givesThe reference's measurement uncertainty remains in the result even if the ratio could be read with arbitrarily many digits.^[ref]

A practical comparison records reference conditions, connecting arrangements and repeated readings while checking effects such as instrumental drift. A primary measurement standard has a particular basis for its assigned value; a working reference need not be primary to transfer useful metrological traceability. Distinguish this physical reference from a written technical specification, which is another use of the word standard.