To obtain official COSC chronometer certification, a movement must pass tests in five positions and three different temperatures, 8°, 23° and 38° C, over a period of 15 days, after which the average daily rate variation must be from -4 to +6 seconds. The most notable deficiency of a chronometer certification is that it only tests the movement, not the finished watch. To demonstrate that the quality of their watches exceeds that of a COSC chronometer, some brands have developed their own, stricter certification standards.
Omega and METAS
In association with the Swiss Federal Institute of Metrology (METAS), Omega has developed a new certification called Master Chronometer. It is open to all watch brands, but Omega is currently the only one to offer it, and only for its Co-Axial Master Chronometer models.
To obtain this certification, a watch must pass tests over a period of 10 days under various conditions. Placed in two different positions, the movement is subjected to a magnetic force of 15,000 gauss and controlled. After being fitted, it is again subjected to 15,000 gauss and its accuracy tested after 24 hours. Timing is checked over a period of four days and subjected to two temperatures in six different positions.
The power reserve function is also checked, with timing results recorded at 100% power reserve and then at 33%, the difference being used to ensure that the power reserve is within the margin accepted throughout. The water resistance of the watch is also tested in water. In the end, METAS only allows an average variation of 0 to + 5 seconds per day for the watch.
Rolex and the Superlative Chronometer
Rolex also adds its own tougher requirements to those of the COSC for each of its mechanical watches. Its “Superlative Chronometer” seal certifies an average precision of -2 to +2 seconds per day for the cased watch, after tests in seven static positions and on a rotating rack reproducing real conditions of use, carried out over a period of 24 hours.
To test their water resistance, all standard models are immersed to a depth corresponding to their advertised water resistance plus 10%, and diving watches are tested at their nominal depth plus 25%. Rolex even has a special high-performance box capable of simulating pressure at 4,875 meters (16,000 feet) below sea level (i.e. guaranteed water resistance at 3,900 meters from the Deepsea, plus 25%).
The automatic winding and power reserve functions are also checked as part of the certification tests.
Rolex makes no specific mention of the magnetism requirements of its movements, and the testing process remains a secret. But his Milgauss, which withstands 1000 gauss, has been around since 1956, and we can be sure that his standards are met.
We must welcome the more rigorous standards put in place by the watchmaking community, especially when they cover the watch as a whole. But it should be remembered that the two examples of certification above come in addition to the COSC certification, and do not replace it.
The COSC also positions itself as a quality control agent for the consumer, as a uniform standard, especially for smaller brands, and provides a safety net for the customer who knows that an independent external body monitors accuracy.
The COSC is a centuries-old institution that has stood the test of time and will continue into the future.