The Swiss organization COSC

What is it?

COSC (Contrôle Officiel Suisse des Chronomètres) is a Swiss non-profit organization founded in 1973 that tests Swiss-made chronometers.
It was founded by five Swiss watchmaking cantons: Bern, Geneva, Neuchâtel, Solothurn, and Vaud, together with the Federation of the Swiss Watch Industry (FHS). It also includes laboratories and observatories that were established independently from each other starting in the late 19th century.
COSC tests generally apply to watches produced or assembled in Switzerland, but not exclusively. The Japanese have largely abandoned the recognition, replacing it with in-house testing to a slightly more rigorous standard, such as with the Grand Seiko. On the other hand, the Germans have set up their own testing center in Saxony at the Glashütte Observatory, where DIN 8319 standards are used, mirroring the ISO standards used by the COSC. The French once provided similar large-scale tests at the Besançon Observatory, but today only a handful of watches are tested there and carry the "Observatory Chronometer" designation.

Today, three laboratories test the movements submitted by individual watch manufacturers for chronometer status: the first in Biel/Bienne, another in Saint-Imier/BE, and the last in Le Locle. The first two are almost entirely dedicated to testing Rolex movements, and while not all Rolex watches are chronometers, Breitling has stated that since 2000, all of its production has been COSC-certified. Omega also has a large portion of its production certified, and therefore, based on the movements used by Rolex, Breitling, and Omega, the movement calibers that obtain the most COSC certificates are: - Rolex 3135 and all its variants (3155, 3175, 3185, up to its evolution, the 3253, etc.), including the 4130 and 4230 chronograph movements; - ETA 2892A2 (and variants) - Valjoux 7750, each of which operates at 28,800 vibrations. Tag Heuer and Ball Watches also both have select watches designated as chronometers that are sent to the COSC for certification. Each chronometer officially certified by the COSC is identified by a serial number engraved on the movement and a certification number provided by the COSC.

THE TESTS

The testing criteria are based on the ISO 3159 standard (Chronometer-measuring instruments - Spring-balance wrist chronometers), which defines a spring-balance wrist chronometer and stipulates that only movements that meet the accuracy criteria established by the standard are awarded an official chronometer certificate.
The certificate is valid exclusively for the movement before the casing and only when the watch has been recently serviced or assembled. Used and vintage chronometers are a separate issue, as they generally require a full service to bring them back to COSC performance levels.
Each movement is tested without its case and individually for fifteen days, in five positions and at three different temperatures. The movements are equipped with a seconds hand, and the self-winding mechanisms are disengaged for the tests. Measurements are taken daily using cameras, and based on these, seven elimination criteria are deciphered, each of which must be met. Finally, another test protocol criterion is daily winding at exactly the same time every day.