COSC

The COSC — Contrôle Officiel Suisse des Chronomètres — is the independent Swiss body that has certified the precision of mechanical movements since 1973. It certifies the bare movement, not the finished watch that houses it. The case, the dial, and the strap that end up on it have never been part of the test.

The protocol follows ISO 3159 and lasts fifteen days. The movement, bare, is set running and read every twenty-four hours in five positions — crown up, crown down, crown left, dial up, dial down — and at three temperatures: 8, 23, and 38 degrees. Seven values are calculated from those readings; the most quoted is the mean daily rate, which must fall between −4 and +6 seconds per day. The tolerance is asymmetric by design: a watch that gains is easier to regulate, and the standard treats it with more lenience than one that loses. If the movement passes, it receives a certificate bearing its own serial number and the manufacturer may write «Chronometer» on the dial. If it fails, it goes back. Some houses certify their entire production.

In my experience the certificate says one precise thing and says it well: how that movement behaves in a laboratory, without a case, before it is sold. It does not say how it will behave on the wrist. Body temperature, the way you move your arm, the knocks of a day, the position in which you rest the watch at night, the oil ageing in the jewels: all of this works on the movement after the test. A certified chronometer that reads +8 seconds a day on the wrist is not a defective chronometer, it is a chronometer that needs regulating. The certificate does not cover water resistance, resistance to magnetic fields, or the stated power reserve either: separate controls exist for those. METAS, the federal metrology institute, tests the complete watch to a tolerance of 0/+5 seconds and 15,000 gauss of magnetic field; Rolex adds its own control on the finished watch at ±2 seconds on top of the COSC. Omega's Master Chronometer is the commercial name for METAS certification, not a synonym for COSC.

For someone buying a strap the certificate changes little, but one thing it does change. A certified watch is, almost always, a watch that is kept: bought to last, sent for service, passed on. The strap, on the other hand, ages with the wrist, and on a chronometer worn for ten years you change three or four of them. It is worth having each one made for that watch: exact lug width in millimetres, thickness at the case attachment that does not strain the spring bar, original buckle kept when there is one. The Submariner, certified as a chronometer in every version, takes a 20 mm lug; for other certified Rolex, Omega, Breitling, Tudor, and Longines models, the measurement is found by reference in the archive, measured at the bench.

Regulation is not my trade. The strap is.