BTW see
https://journal.hautehorlogerie.org/...a-chronometer/
A good discussion with COSC...
And this link explains how some watchmakers are going above and beyond:
https://www.watchtime.com/wristwatch...quality-marks/
And an excerpt from Worn and Wound:
The standard the COSC developed was stringent, a demanding regimen of tests nearly on par with the grueling Neuchatel trials.
Encompassing 16 consecutive days of testing, the COSC’s method winds and measures the movement once every 24 hours. Over the course of these 16 days, each candidate watch is tested in five different positions and at temperatures ranging from eight degrees Celsius (46.4 degrees Fahrenheit) to 38 degrees Celsius (100.4 degrees Fahrenheit).
During these tests, the watch must remain accurate to within -4/+6 seconds a day on average with a maximum allowed rate variation of seven seconds a day between test days.
Five other criteria are also judged, creating a standard so strict only three percent of annual Swiss watch production (a figure slightly over 1,000,000) qualifies for the chronometer label.
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