C.O.S.C. - contrôle officiel suisse des chronometres

The C.O.S.C. is a non-profit association. It was created by five watchmaking cantons (Bern, Geneva, Neuchatel, Solothurn and Vaud) as well as the Federation of the Swiss Watch Industry in 1973. It encompasses the laboratories that had been established independently of each other from the late 19th century onwards.

A chronometer is a timekeeper precise enough to be used as a portable time standard, usually in order to determine longitude by means of celestial navigation. In the world of watches, the term is also attached to those tested and certified to meet certain precision standards. In Switzerland, only timepieces certified by the C.O.S.C. can use the word "Chronometer" on them.

C.O.S.C. measurement methods:

Only the movement is tested and the tests bear absolutely no relation to a simulation of watch behaviour and performance when worn.
The test is divided into 16 days, 24-hour periods. The watch is tested in 5 positions and at 3 temperatures.


Day Temperature Position
0 23°C 6 o'clock up / 12 o'clock up (pocket-watches)
1 23°C 6 o'clock up / 12 o'clock up
2 23°C 6 o'clock up / 12 o'clock up
3 23°C 3 o'clock up
4 23°C 3 o'clock up
5 23°C 9 o'clock up
6 23°C 9 o'clock up
7 23°C Dial down
8 23°C Dial down
9 23°C Dial up
10 23°C Dial up. Chronographs run 24 hours
11 8°C Dial up
12 23°C Dial up
13 38°C Dial up
14 23°C 6 o'clock up / 12 o'clock up
15 23°C 6 o'clock up / 12 o'clock up


Criteria >20mm Diameter <=20mm Diameter Pocket Watches
Mean daily
rate: during
the first
10 days. -4+6 secs/day -5+8 secs/day -2+5 secs/day

Mean rate
variation:
average of
the 5 absolute
variations in
5 positions
during the
first 10 days
of test. 2 secs/day 3.4 secs/day 1.5 secs/day

Maximum
rate variation:
in five
positions during
the first 10
days of test. 5 secs/day 7 secs/day 2.5 secs/day

Maximum
difference in
rate between
vertical and
horizontal
positions:
mean rate
of days 1
and 2 minus
mean rate of
day 9 and 10. -6+8 secs/day -8+10 secs/day ±4 secs/day

Greatest rate
difference:
between one
of the first 10
daily rates
and the
average daily
rate for
the test. 10 secs/day 15 secs/day 7 secs/day

Rate
variation
according
to temperature:
the rate at
38°C
minus the
rate at 8°C
divided by the
temperature
difference. ±0.6 secs/day°C ±0.7 secs/day°C ±0.35 secs/day°C

Secondary
error: the
difference
between the
average
rate at 38°C
and at 8°C
(day 11 and 13)
and the
average rate
with the
dial up at
23°C (day 9
and 10). not applicable not applicable ±4.5 secs/day

Rate
resumption:
the final rate
minus the
average
rate of the
first two days ±5 secs/day ±6 secs/day ±2.5 secs/day

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