COSC

COSC stands for Contrôle Officiel Suisse des Chronomètres (Official Swiss Chronometer Testing Authority), a non-profit association founded in 1973. It tests uncased movements: each mechanical one spends 15 days in five positions and at 8, 23 and 38 °C, measured against the seven criteria of ISO 3159. The best known, the average daily rate, is typically −4 to +6 seconds a day. A movement that passes is a chronometer, with an engraved number; the title belongs to that piece, not to a model. Quartz movements follow COSC’s own requirements, adapted from ISO 3159.

A chronometer is not a chronograph: one is certified precision, the other a stopwatch function, and a chronograph can be certified as a chronometer.

Some standards also test the cased watch: COSC’s Excellence Chronometer Certified, to −2 to +4 seconds a day and in a 200-gauss field, and Omega’s Master Chronometer, with eight further tests set by the Swiss Federal Institute of Metrology.

In job listings

Chronometer appears in regulation and after-sales roles, from movement regulator apprenticeships to service finishers expected to know chronometer regulation and check timing against a house’s tolerances. Boutique listings at houses that certify their watches may name the certification. See current chronometer roles.