The official Swiss testing office for chronometers. The C.O.S.C is the worlds largest issuer of chronometer certificates.

For a fee C.O.S.C tests the rate of movements that have been adjusted by watch makers. These are usualy mechaical movements, those that meet the specifications for being a chronometer are awarded the title of chronometer.

Each 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:

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• 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 `

An official certificate as shown here: