Eight-digit voltage measurements: how meaningful are they?

The voltage of each of a group of 7 V Zener reference elements is to be determined with 8-digit resolution. A crosscomparison method is described which, in three steps, relates their voltage to that of a group of 10 V standards which have a mean voltage determined from periodic measurements at the National Physical Laboratory. The residuals of the comparison predict an average of +or-1.4 mu V uncertainty (95% confidence) for the measurement of three of the Zener references from a single measurement session lasting 2 h in total. The voltages for the same three Zener references, when fitted to straight lines over a period of 120 days, have RMS errors of +1.8 mu V, which is pleasingly close (1.4 mu V uncertainty on a 7 V measurement is 0.2 parts in 10⁶).