The Contrôle officiel suisse des Chronomètres (COSC), the Official Swiss Chronometer Testing Institute, is the institute responsible for certifying the accuracy and precision of Swiss watches.
Background
Founded in its current form in 1973, the COSC is a Swiss non-profit organization that tests chronometers that are Swiss Made. COSC is an acronym for the organization's French language name, Contrôle Officiel Suisse des Chronomètres.
COSC testing generally applies to watches manufactured or assembled in Switzerland. Notwithstanding, the normative standards are set by international agreement and are the same whether they are nominally labeled ISO or DIN standards. Some German, Japanese, and even non-certified Swiss movements can surpass the normative requirements. The Japanese have largely abandoned the accolade, replacing it with in-house testing to a slightly more strict standard as with, for example, the Grand Seiko. On the other hand, the Germans have set up their own testing facility in Saxony at the Glashütte Observatory where the DIN 8319 standards, which mirror the ISO standards used by COSC, are employed. At one time the French provided similar large scale testing at the Observatory at Besançon, however, today only a very few watches are currently tested there and carry the accolade "Observatory Chronometer".
The organization was founded by five watchmaking cantons of Switzerland: Bern, Geneva, Neuchâtel, Solothurn and Vaud, together with the Federation of the Swiss Watch Industry (FHS). It encompasses the laboratories and observatories that had been created independently of each other from the late 19th century.
Three laboratories now test the movements submitted by individual watch manufacturers to be granted chronometer status. They are in Biel/Bienne, Saint-Imier/BE and Le Locle. The Saint-Imier and Biel laboratories are almost entirely devoted to testing Rolex movements. Although not all Rolex watches are chronometers, Breitling has claimed that since 2000 all of its production is COSC certified. Omega also has much of its production certified. Thus, based upon the movements used by Rolex, Breitling, and Omega, the movement calibers that obtain most of the COSC certificates are the Rolex 3135 (since 1988) (and variants 3155, 3175, 3185, 4130) and 2235, the ETA 2892A2 (and variants) and Valjoux 7750, each of which operates at 28,800 beats per hour. Tag Heuer and Ball watches both have select watches designated as chronometers which are sent to COSC for certification.
Standards and methods
Each officially COSC certified chronometer is identified by a serial number engraved on its movement and a certification number given by the COSC.
Testing criteria are based on ISO 3159 (Timekeeping instruments — Wrist-chronometers with spring balance oscillator) which defines a wrist chronometer with spring-balance oscillator. Only movements which meet the precision criteria established under ISO 3159 are granted an official chronometer certificate. (Compare ISO 3158.) The certificate is valid for the bare movement prior to casing, and only when freshly serviced or assembled. Pre-owned and vintage chronometers generally require a complete service in order to be restored to COSC-levels of performance.
Each uncased movement is individually tested for fifteen days, in five positions, at three different temperatures. The movements are fitted with a seconds hand and the automatic winding mechanisms are disengaged for the tests. Measurements are made daily with the aid of cameras. Based on these measurements, seven eliminatory criteria are calculated, each of which must be met (e.g., for movements of a diameter over 20 mm, the requirements, indicated in seconds/day, are noted in the table below). The testing protocol requires daily winding at precisely the same time each day. There is no ISO standard for quartz timepieces, but there is development in this field. ISO 10553:2018 specifies the procedure for evaluating the accuracy of quartz watches, individually and by lot, and the relationship between the accuracy tested and the accuracy classification given by the manufacturer. It applies to quartz watches having accompanying documents on which the accuracy classification is indicated. Nevertheless, COSC has also developed its own standard for testing quartz chronometers with eight eliminatory criteria, also noted in the table below. To acquire the COSC chronometer label, a quartz instrument must benefit from thermo-compensation and rigorous encapsulation.