VIM concept: MPE is not uncertainty

Two numbers get conflated on certificates and datasheets constantly: maximum permissible error and measurement uncertainty. The VIM keeps them strictly apart, and mixing them up produces real acceptance mistakes.

MPE (VIM 4.26) is a SPECIFICATION: the extreme error a class of instrument is permitted to show. It belongs to the instrument's requirements. Uncertainty (VIM 2.26) is a PROPERTY OF A RESULT: the doubt attached to what was actually measured, here, today, by this process. An instrument can be well inside its MPE while your measurement of that fact carries large uncertainty — or the reverse.

The collision happens at conformity decisions: proving an indication is within ±MPE requires accounting for the uncertainty of the proof, which is exactly what ISO 14253-1's decision rules (and the guard-band calculator in our Tools section) formalize. The dictionary entries for both concepts cite their VIM clauses — a useful pair to keep bookmarked for audit season.