COSC stands for ‘Contrôle Suisse de Chronomètres’. This official Swiss test office was founded in 1973 by the Swiss cantons Bern, Genève, Neuchâtel, Solothurn, Vaud and the FH (Fédération de l’industrie horlogère Suisse).
COSC is the biggest publisher for de so-called chronometer certificates. Altough most watches, tested by COSC, are mechanical watches, there are some highly accurate quartz watches that are tested too.
During 15 days, COSC tests if a watch performs well within the following seven criteria: Average daily rate, mean variation in rates, greatest variation in rates, difference between rates in horizontal and vertical positions, largest variation in rates, thermal variation and rate resumption.
Only the watches that meet those criteria can call themselves ‘Chronometers’. Those timepieces are provided with the official chronometer certificate. Some watch brands have ‘Officially Certified Chronometer’ marked on the dial of their models.
Only 3% of all Swiss watches are COSC certified. An old ally is Breitling. The brand has collaboration with COSC since 1999. Of all watch brands Rolex brings the most watches to COSC to let them be certified as chronometers. But COSC hasn’t a monopoly. It is just the test office for watches in Switzerland. Other countries have their own test offices. For example: If the watch brand ‘Citizen’ wants their watches to be chronometer-certified they have the ‘Japan Chronometer Inspection Institute’ (JCII). And since 2006 there is also the institute LMET in Germany. The ‘Landesamtes für Mess- und Eichwesen Thüringen’. Glashütte Original makes use of this German institute. But all institutes have just one umbrella office, the International Chronometer Certification Committee (CICC).