COSC is a non-profit Swiss testing institute, Contrôle Officiel Suisse des Chronomètres, which tests movements for precision. The test protocol is performed on uncased movements, which have to have a seconds hand and a serial number. Movements are individually tested over the course of 16 consecutive days, in five positions, at three different temperatures and different states of winding. The movement is wound every day, at exactly the same time every day. Those that pass are provided with a certificate and are identified by a number engraved on the movement. The resulting watch can be defined as a chronometer. COSC certification has been operating since 1973: before this date, a number of independent, regional offices and observatories performed the task.
Testing criteria are based on the standard ISO 3159. The requirements for watch movements greater than 20 mm in diameter (another more lenient set of figures applies to movements smaller than 20 mm) are as follows:
Average daily rate: −4/+6 seconds deviation per day, determined over the first 10 days of testing.
Mean variation in rates: 2 seconds/day
Greatest variation in rates: 5 seconds/day. The largest difference between the readings on two different days for a single position
Difference between rates in horizontal and vertical positions: −6/+8 seconds. Calculated by subtracting the average of the rates in the vertical position on the first and second days of testing from the average of the rates in the horizontal position on the ninth and tenth days.
Largest variation in rates: 10 seconds. This is the largest difference between the average daily rate and any rate in any position during the first 10 days of testing.
Thermal variation: ±0.6 seconds/day per degree. This measures how the rate of the watch changes with temperature
Rate resumption: ±5 seconds. This figure is obtained by subtracting the average daily rate of the first two days of testing from the average daily rate of the last test day.
The certificates issued by COSC are delivered exclusively to the manufacturer. Brands have their own policies on providing customers with the complete documentation on test results. Ulysse Nardin, and, for some of their watches, Audemars Piguet and Vacheron Constantin, supply chronometer certificates to customers.
In terms of numbers, Rolex delivers the largest number of movements to COSC for testing, presumably fairly close to a million a year. Then comes Omega. Both these brands also have their own additional internal tests, the Rolex Superlative Chronometer Certification that requires -2/+2 seconds/day precision ratings, and METAS certification for Omega, which includes resistance to magnetic fields of up to 15,000 gauss, and a precision of 0/+5 seconds/day before, during and after exposure to magnetic fields. In theory, METAS is accessible to all watch brands, but in practice, the 15,000 gauss magnetic resistance excludes just about everyone except Omega. The brand, with Swatch Group, have invested heavily in attaining this characteristic, and the METAS protocol fits the anti-magnetic performance that they have achieved to perfection.
For both Rolex and Omega’s internal test procedures, the watches are tested assembled in their cases, and so the tests reflect everyday use more accurately.
Generally speaking, watches that have received COSC certification can be called and labelled chronometers. There are some exceptions: F.P. Journe performs its own chronometry tests, at ratings superior to COSC, and calls its watches chronometers. Some German brands submit their watches for testing to the testing authority operating at Glashütte Observatory, in accordance to DIN 8319, and likewise use the term chronometer.