A chronometer watch is a particular type of watch that has been tested by, and meets the standards for, precision of the Contrôle Officiel Suisse des Chronomètres (COSC), the Official Swiss Chronometer Testing Institute. A chronometer watch must not lose more than four, nor gain more than six, seconds per day. Roughly one million watches are certified as chronometers each year, making up 3% of total Swiss watch production. Such a watch is typically made of high quality parts and materials.
The COSC standards that a timepiece must meet to be called a chronometer watch are compiled under a rubric known as ISO 3159. In the 15-day testing process, a particular watch movement is measured in five different positions at three different temperatures. Seven criteria must be met for a watch to receive certification. These include:
- consistency in the average rate over the initial 10 testing days
- mean variation in times
- the largest difference between any two days' readings in a single position
- the difference between rates in horizontal and vertical positions
- the largest difference between the mean daily rate and any individual rate during the initial 10 days of positional testing
- the temperature error of the movement measured in seconds per degree
- rate resumption
Rate resumption is calculated by subtracting the average mean daily rate from the first two days of testing, from the mean daily rate of the last day of testing. Each of the testing measurements are compared against atomic clocks for absolute accuracy. A watch is certified after all these requirements have been met.
In addition to meeting these stringent requirements, a chronometer watch typically incorporates rare components, such as ruby or sapphire jewel bearings, or exotic metals, like titanium or platinum. These timepieces also commonly include one or more features known as complications, which can range from a perpetual calendar to a display showing the phases of the moon. Complications, as can be inferred from the name, are built into the watch mechanism and contribute to its complexity.
Though not technically a complication according to most watchmakers, the most famous of these mechanical additions is the tourbillon. French for whirlwind, a tourbillon was originally designed to counter the effects of gravity when a watch movement was rotated. While today their use is largely relegated to ornamental status, tourbillons are still seen in the most expensive and complex types of chronometer watch — which are often left exposed to display their inner workings. While the development of quartz movement in the 1960s and 1970s made the chronometer watch obsolete from a technical standpoint, they continue to enjoy healthy sales due to their aesthetic qualities and precision engineering.
