What is a COSC-certified chronometer?
A watch whose movement has passed the Swiss official chronometer test, keeping to −4/+6 seconds a day across a range of positions and temperatures.
The test
The Contrôle Officiel Suisse des Chronomètres, COSC, tests uncased movements over 15 days in five positions and at three temperatures. To pass, a mechanical movement must average −4 to +6 seconds a day, among other criteria. Only movements that pass can be called chronometers in Switzerland.
Some brands go further. Rolex's Superlative Chronometer is tested again after casing to −2/+2 seconds a day, and Omega's Master Chronometer adds METAS testing, including resistance to strong magnetic fields.
Does it matter?
Certification proves a movement was adjusted to a high standard when new. Real-world accuracy depends on how you wear it and when it was last serviced, and many uncertified movements run just as well. It's a mark of quality, not a guarantee.
Questions
- How accurate is a COSC chronometer?
- Its movement averaged −4 to +6 seconds a day in testing. Many run within a few seconds a day in daily wear.