NIST TN 1297: Appendix D6. Uncertainty and units of the SI; proper use of the SI and quantity and unit symbols

D.6.1 As pointed out in the Guide, the result of a measurement is sometimes expressed in terms of the adopted value of a measurement standard or in terms of a conventional reference value rather than in terms of the relevant unit of the SI. (This is an example of a situation in which all significant components of uncertainty are not taken into account.) In such cases the magnitude of the uncertainty ascribable to the measurement result may be significantly smaller than when that result is expressed in the relevant SI unit. This practice is not disallowed by the NIST policy, but it should always be made clear when the practice is being followed. In addition, one should always give some indication of the values of the components of uncertainty not taken into account. The following example is taken from the Guide. (See also subsection D.4.1.)

EXAMPLE - A high-quality Zener voltage standard is calibratedby comparison with a Josephson effect voltage reference based on the conventional value of the Josephson constant recommended for international use by the CIPM. The relative combined standard uncertainty u_c(V_S)/V_S of the calibrated potential difference V_S of the Zener standard is 2 × 10^-8 when V_S is reported in terms of the conventional value, but u_c(V_S)/V_S is 4 × 10^-7 when V_S is reported in terms of the SI unit of potential difference, the volt (V), because of the additional uncertainty associated with the SI value of the Josephson constant.

D.6.2 NIST Special Publication 811, 1995 Edition [D.3], gives guidance on the use of the SI and on the rules and style conventions regarding quantity and unit symbols. In particular, it elaborates upon the NIST policy regarding the SI and explains why abbreviations such as ppm and ppb and terms such as normality and molarity should not be used. NIST authors should consult NIST SP 811 if they have any questions concerning the proper way to express the values of quantities and their uncertainties.