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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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