The accuracy of fineness gauges may be impaired after a certain time of use. The ISO standard recommends the depth is checked.

A digital comparator with a tapering point is fitted onto a rule with a central V-shaped opening and a contact edge bevelled to an angle of 60°, with thermal insulation on its upper surfaces.

The instrument is reset to zero beforehand on a reference surface, positioned perpendicular to the groove and provides an accurate measurement of the depth of the groove at the place being checked.

Reading in microns.