 Certified reference material

A certified reference material is a reference material supplied with authoritative documentation assigning specified property values and their associated uncertainties and traceabilities through valid procedures. The certification applies to the stated properties and conditions. It does not make every conceivable property of the material a certified value.^[ref]

A usable certificate identifies the material and relevant property, its assigned value, the uncertainty convention, conditions of use and the evidence needed for metrological traceability. Both reference material homogeneity and reference material stability matter when a portion is used later. The VIM distinguishes reference materials suitable for precision control from those with assigned values suitable for measurement calibration or measurement trueness control.^[ref]

For a physical example, the archived NIST SRM1922 mineral-oil certificate gives refractive index $n=1.46945$ at $589.3\mathrm{nm}$ and $20{}^{\circ }\mathrm{C}$, with expanded uncertainty $U=6\times 10^{-5}$ and coverage factor $k=2$.^[ref] Its corresponding standard uncertainty is $u_{\mathrm{ref}}=U/k=3\times 10^{-5}$.

For an original comparison under those same conditions, suppose a refractometer gives $1.46953$ with independent standard uncertainty $u_{\mathrm{inst}}=4\times 10^{-5}$. The discrepancy is $8\times 10^{-5}$, and its combined standard uncertainty isThus the discrepancy is $1.6u_{\Delta }$. This calculation is a diagnostic comparison, not an automatic pass/fail rule. Correlated contributions, a different temperature or an inappropriate sample invalidate the simple comparison model. The certificate supplies a documented reference and its limits; the user's complete measurement result still includes the measuring instrument and procedure.