Saudi Luxury / Watch basics
COSC and Master Chronometer: what the certificates promise
What COSC, METAS Master Chronometer and Rolex Superlative Chronometer test, so you can judge whether a certificate is worth paying for.
A watch certified as a chronometer has passed an independent accuracy test, usually at COSC in Switzerland. For most full-size mechanical movements, COSC allows an average of 4 seconds slow to 6 seconds fast a day. A chronometer is not a chronograph, and the certificate records how the watch performed when tested, not how it will run for life.
The standards compared
| Standard | What is tested | Average daily rate allowed |
|---|---|---|
| COSC chronometer | The movement before casing: 15 days, five positions, three temperatures | -4 to +6 seconds |
| METAS Master Chronometer | The finished watch, including a 15,000-gauss magnetic field, water resistance and power reserve | 0 to +5 seconds |
| Rolex Superlative Chronometer | The cased watch, tested by Rolex after COSC | -2 to +2 seconds |
| Grand Seiko standard | Grand Seiko’s mechanical movements, tested in-house: 17 days, six positions, three temperatures | -3 to +5 seconds |
COSC and METAS figures are for larger movements; smaller ones get slightly more leeway. The Rolex and Grand Seiko standards are set and checked by the brands themselves. Grand Seiko’s Spring Drive and quartz watches are rated differently; the guide to how Spring Drive works gives its rating.
COSC checks the movement on its own, so it cannot catch problems introduced when the dial, hands and case are fitted. Master Chronometer and Rolex’s standard test the watch as you will wear it, starting from a movement that has already passed a chronometer test. The Rolex Datejust 126300 carries both COSC and Superlative Chronometer status. The Omega Seamaster Diver 300M 210.30.42.20.01.001, with calibre 8800, is Master Chronometer certified.
What it means on your wrist
For comparison, makers’ tolerances for uncertified entry-level automatics are far wider, as much as 45 seconds a day in some cases. A few seconds a day is good real-world performance for any mechanical watch.
In daily wear, accuracy also depends on how the watch rests at night and whether it is magnetised. Magnetism is a common reason a good mechanical watch suddenly runs fast, and phone speakers or magnetic clasps on bags and covers can be enough. That makes the magnetic test the most useful part of Master Chronometer. A watchmaker can demagnetise an affected watch quickly.
A certificate also ages. After years of wear, a chronometer runs only as well as its last service. Before buying an older one, read what a service involves and when it is due.
Papers and resale
Whether a paper certificate comes in the box depends on the brand. Either way, keep the warranty card and papers with the watch, because they tie its serial number to the model. A missing certificate does not undo the certification, which belongs to the model, but a complete set of papers helps at resale.
Do you need one
If accuracy is all you want, a good quartz watch typically keeps better time than a certified mechanical one, at far lower cost: many are rated to about 15 seconds a month. A certificate tells you a mechanical watch was adjusted with extra care and, for COSC and METAS, checked independently. At lower prices certified chronometers are uncommon; the Frederique Constant Highlife COSC is one, with COSC stated in its listing title. Confirm the certificate on the listing or in the maker’s papers; the word “chronometer” alone is not proof. Watch Codex explains how COSC tests a movement over 15 days.