A chronometer is a very precise timekeeper, set up to stay accurate under different conditions of use. For Swiss watches, the title is certified by COSC after a test of the movement. Only about one in twenty Swiss-made mechanical watches is chronometer-certified by COSC.
How is a chronometer tested?
COSC is a non-profit organisation that took its present form in 1973. It tests each movement on its own, before it goes into a case. The trial lasts fifteen days. The movement is checked in five positions and at three temperatures: 8, 23 and 38 degrees Celsius. To pass, it must keep an average daily rate of between minus 4 and plus 6 seconds. That limit has applied since 1973. The criteria follow ISO 3159, the international standard for wrist chronometers with a balance-spring oscillator.
How is a chronometer different from an ordinary watch?
A mechanical chronometer works like any mechanical watch. A spring stores the energy, and an escapement releases it in steady steps. The difference lies in the build: its parts are made sturdier. COSC’s definition also goes beyond precision alone. A chronometer must be adjusted so that it keeps its accuracy when the conditions of use change. Good time in a single setting is not enough.
Where it comes from
The name comes from Ancient Greek and means a device that measures time. Jeremy Thacker, an Englishman from Beverley, first used the word in 1714. John Harrison invented the marine chronometer in 1730. Before COSC, chronometers were tested by observatories. Those trials usually ran for 30 to 50 days, far longer than the fifteen-day test used today.
Not to be confused with
A chronograph is not a chronometer. A chronograph is a watch with a counter for timing intervals, used like a stopwatch. A chronometer is defined by its precision, not by any extra function. People also often call a stopwatch a chronometer. That is a mistake: such a device is really a chronograph or a chronoscope.
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