- This article is about the Swiss Official Chronometer Testing Institute, known by its French language acronym of COSC. For the state college in Connecticut, see Charter Oak State College.
Founded in its current structure 1973, the COSC ("Contrôle Officiel Suisse des Chronomètres") is The Swiss Official Chronometer Testing Institute. It is a not-for-profit association. The COSC was founded by five watchmaking States ("Cantons") of Switzerland: Bern, Geneva, Neuchâtel, Solothurn and Vaud, together with the FH, Federation of the Swiss Watch Industry. It encompasses the laboratories / observatories that had been created independently of each other from the late 19th century onward.
Nowadays, three laboratories do the actual testing of the movements submitted by the individual Watch Brand's manufacturers to be granted the official chronometer status, located in Biel/Bienne, Geneva & Le Locle.
The criterias firstly laid out by the General Time Railway Inspector Webb C. Ball in U.S.A. in 1893 for Railroad chronometers and applied by the Waltham Watch Company, soon followed by the Elgin Watch Company and other American & Swiss Watch Manufacturers have of course evolved and been strengthened over the time, e.g. the international norm ISO 3159 provides the definition of a wrist-chronometer with sprung balance oscillator, with the COSC own addition of a permanent display of the second.
Each Officially certified COSC Chronometer is unique, identified by a serial number engraved on its movement and a certification number given by the COSC. Each movement is individually tested for 15 days, in 5 positions, under 3 different temperatures. Based on these measurements, 7 eleminatory criterias are calculated, the minimas of which must all be met e,g, for movements of a diameter over 20 mm, indicated in seconds/day:
- Average daily rate: -4 +6
- Mean variation in rates: 2
- Greatest variation in rates: 5
- Difference between rates in H & V positions: -6 +8
- Largest variation in rates: 10
- Thermal variation: +-0.6
- Rate resumption: +-5
COSC have developed their own standard for testing Quartz Chronometers with 8 eliminatory criterias:
- Average daily rate at 23 °C: +-0.07
- Rate at 8 °C: +-0.2
- Rate at 38 °C: +-0.2
- Rate stability: 0.05
- Dynamic rate: +-0.05
- Temporary effect of mechanical shocks +-0.05
- Residual effect of mechanical shocks +-0.05, 200 shocks equivalent to 100 G (981 m/s²)
- Rate resumption +-0.05
Measurements are based on a time base established by 2 independent atomic clocks synchronised on GPS time.
Over 1'000'000 official chronometer certificates are being delivered each year, representing only 3% of the Swiss watch production, a proportion that underscores the exceptional nature of a chronometer. To earn chronometer certification, a movement must not only be made for the highest quality components, but also be the object of special care on part of the finest watchmakers and timers during assembly, standing out clearly from the standard watch production.
Sources: courtesy of "Contrôle Officiel Suisse des Chronomètres", CH-2301 La Chaux-de-Fonds, Switzerland, email: info@cosc.ch
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