Standards profile: OIML R 111 and what class E2 really promises

When a weight is sold as 'class E2', OIML R 111 is the document making the promise — and the promise is more than a tolerance. The recommendation specifies maximum permissible errors by class and nominal value, but also density limits, magnetic properties, surface condition, and marking, because a weight that meets its MPE while being magnetic or porous will still ruin precision weighings.

The class ladder (E1, E2, F1, F2, M1…M3) is designed around dissemination: each class is intended to calibrate the one below it with adequate margin, from national standards down to the M-class weights that verify shop scales under OIML R 76's non-automatic weighing instrument rules.

The library's SOP 34 shows one of the less obvious corners of practice: selecting sensitivity and tare weights whose own errors don't quietly consume the budget of the weighing they support. R 111's tables plus that procedure are most of what a working mass bench needs daily.