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Mechanical movements
The OL at the COSC apply the strict test conditions described in the ISO standard or its internal requirements. The results obtained give an exact measurement of the rate of a movement, and of a watch over a given period and attest to its intrinsic precision. The tests do not therefore constitute a simulation of behaviour when worn.
During the 15 days of testing, the regularity of the watch is checked with two key pieces of data:
- temperature
- the position of the watch (3 o’clock, 6 o’clock, 9 o’clock, dial on top, dial on the bottom)
Placed on clasps containing five pieces each, the movements are successively positioned in the various positions, inside the thermal enclosures corresponding to the different temperatures required. Each piece remains in a given position and temperature for 24 hours. Each day, they are briefly removed from these thermal enclosures to take measurements, using vision machines equipped with five cameras.
The daily measurements carried out by the COSC on each movement therefore consist of determining the status of each instrument in relation to a reference period. The measurement by differentiation of statuses enables the behaviour of the movement to be integrated in time and the daily rate to be calculated.
Minimum Requirements (for balance spring wristwatches) - Type I
| Symbol | Criterion | Limit [s/d] | |
| Category 1 | Category 2 | ||
| Avg R | Average daily rate | -4 +6 | -5 +8 |
| Avg V | Mean variation in rate | 2 | 3.4 |
| Max V | Greatest variation in rates | 5 | 7 |
| D | Difference between rates in horizontal and vertical positions | -6 +8 | -8 +10 |
| P | Largest variation in rates | 10 | 15 |
| C | Thermal variation | +/- 0.6 | +/- 0.7 |
| R | Rate resumption | +/- 5 | +/- 6 |
| The minimum requirements are considered absolute limits and no calculation result is rounded up or down. | |||
Status: the difference, at a specific time, between that indicated by a timekeeping instrument and the reference time given by a master clock.
The status is positive or negative depending on whether the timekeeping instrument is ahead of or behind the reference time.
Rate: the expression of the difference, per unit of time, between two states of a timekeeping instrument, separated by a given time interval (duration).
If it is positive, the instrument advances, and vice versa.
Daily rate: the expression of the difference between two states separated by a time interval of 24 hours.
Each OL uses a time base made up of two atomic clocks and a synchronised DCF77 0CX0 clock, and makes use of two different time references (GPS and DCF). The data entry and calculation are fully computerised.
Avg R: the arithmetic mean of the daily rates of the first 10 days of testing.
Avg V: the arithmetic mean of the five absolute values of the rate variations obtained for the five positions of the watch during the first 10 days of testing.
Max V: the absolute value of the largest of the five rate variations relative to the five positions of the watch during the first 10 days of testing.
D: the difference between the vertical and horizontal rates of the watch, obtained by subtracting the average rate of the 9th and 10th days of testing from the average rate of the first two days of testing.
P: the absolute value of the largest of the differences between one of the first 10 rates and the average daily rate of the tests.
C: the variation of the rate as a function of temperature, obtained by subtracting from the rate at 8°C from that at 38°C, the whole divided by the temperature interval between these two rates.
R: the variation between the last rate of the tests and the average of the first two rates of the tests.
Category 1 consists of wristwatches with a fitting diameter greater than 20 mm or where the fitting surface area is greater than 314 mm2 Category 2 includes all wristwatches with a fitting diameter of 20 mm or less and a fitting surface area not exceeding 314 mm2.