ISO 3159 gives the definition of the balance spring oscillator wristwatch chronometer. Only movements or watches that meet the accuracy requirements of this standard receive an official chronometer certificate.

The checks performed by the COSC consist of static tests performed in the laboratory. Each individual movement/watch undergoes a battery of tests specific to each of the four types, for several consecutive days, in five positions and at three different temperatures (8°, 23° and 38°C).

On the day of receipt of the movement (day 0), the COSC carries out the following:

• tests the numbers engraved on the movement (in relation to the list supplied by the applicant).
• places the movement in a 5-slot clasp.
• winds the movements (delivered unwound) according to the instructions of the manufacturer
• places the parts in an enclosure at 23°C (± 1°C), where they must remain for at least 12 hours so that their temperature stabilises before the test phase commences.

Then, for 15 days for wristwatches and pocket watches, 19 days for carriage clocks and on-board clocks, and 13 days for quartz timekeeping instruments – a long and rigorous time period – the parts undergo daily tests. Every day, including weekends, the instruments are measured and wound. On the basis of the measurements, 7 elimination criteria are calculated for Type I. If and only if the 7 criteria are met, the movement / watch is certified as a chronometer.

What are the 7 criteria?

• average daily rate
• mean variation in rates
• greatest variation in rates
• the difference between rates in horizontal and vertical positions
• largest variation in rates
• variation in rate depending on temperature
• rate resumption