gorrie wrote:Neil72 wrote:I am assuming that it comes down to build up of tolerances within the watch itself. The chronometer certification does permit a certain degree of inaccuracy (can't remember the exact figures) which would allow it to lose or gain time over a longer period.
The COSC tolerance is actually pretty wide for an automatic/mechanical movement and most watches would pass it these days.... It's down to the fact that it costs so much to get the certification that a lot of makers (or models) are not submitted for testing.
The quartz COSC tolerance much more accurate than that for auto/mech.
Mine is a chronometer, but it doesn't mean it will keep time any better than a bog standard Tag 2000 auto which is not COSC certified... although, I must say, mine has been a lot better behaved since it came back from Chris Heale for a service/fix....
Had a quick google and found this.....range of -4/+6 secs per day for a mechanical timepiece, +- 0.07 for a quartz.
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The Swiss Official Chronometer Certification
The official Chronometer Certification duly certifies that a movement meets the highest standards of precision and reliability. To earn it, a movement has to go through 16 days of exhaustive, specific tests conducted by a neutral and independent body, representing a true guarantee of quality.
According to the usual definition within professional watchmaking circles, “a chronometer is a high precision watch capable of displaying seconds, whose movement has been tested over a number of days in different positions and at different temperatures, by a neutral, official body. The movements that satisfy the criteria of precision as set out in the ISO 3159 standard, receive an official chronometer certificate.” In Switzerland, the only “official neutral body” able to confer the title of chronometer is the famous COSC (Swiss Official Swiss Chronometer Testing Institute), founded in 1973. In its three official testing offices, (the laboratories of Bienne, Geneva and Le Locle), the COSC, with the help of the ultra-sophisticated equipment developed by its engineers, tests the precision of the movements, which are submitted by the manufacturers. Each movement is individually tested over 16 days, in 5 different positions and at 3 different temperatures (8°C, 23°C, 38°C). In order to obtain the chronometer certificate, a mechanical movement with a diameter measuring more than 20 mm must fulfil 7 necessary criteria in areas such as average daily rate, average variation of rate, the difference in rate between horizontal and vertical positions and the precision of a movement in case of temperature variations. The minimum expectations in particular are that mean daily variations in rate must fall within a range of -4/+6 seconds. Each chronometer is identified by a number engraved on the movement and by a numbered certificate presented by the COSC. Inspired by the ISO 3159 norm, COSC has also established test regulations applicable to quartz chronometers. In fact, only between 4 and 5% of watches produced in Switzerland receive this official certificate each year. In this context, one should note that the COSC tests are static and conducted in a laboratory by simulating wearing the watch. Further, the COSC tests cover only the movements of watches and do not therefore concern their water resistance and robustness, since only the movement is tested and not the fully cased-up watch. One should not confuse “chronometer” with “chronograph” - which signifies a timekeeper that measures time duration.
My jeweller has me at the top of his waiting list for the next stainless steel one that comes in. I'm assured tht I'll have one by the end of the year
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