What exactly does COSC mean?

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What exactly does COSC mean?

Post by andrema on Thu 05 Aug 2010, 12:40 pm

Have you ever wondered what exactly COSC means, how it is measured and the history of the standard? If so, check out these links! I saw these at another site and wanted to pass them along:




Details from the WUS thread:


Founded in its current structure in 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 individual watch manufacturers to be granted the official chronometer status, located in Biel/Bienne, Geneva & Le Locle.

Standards and Methods

Each officially certified COSC chronometer is unique, and identified by a serial number engraved on its movement and a certification number given by the COSC.

Testing criteria is based on ISO 3159 which provides the definition of a wrist-chronometer with spring balance oscillator. Only movements which meet the precision criteria established under ISO 3159 are granted an official chronometer certificate.

Each movement is individually tested for fifteen days, in five positions, at three different temperatures. Based on these measurements, seven eliminatory criteria are calculated, the minima 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 eight eliminatory criteria:

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 two independent atomic clocks synchronised on GPS time.

Over 1 million official chronometer certificates are 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 from the highest quality components, but also be the object of special care on part of the finest watchmakers and timers during assembly.

Sources: courtesy of "Contrôle Officiel Suisse des Chronomètres", CH-2301 La Chaux-de-Fonds, Switzerland, email: info@cosc.ch
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