A Chronometer certification is the testing and official authentication of high-precision watches.

High-class watches show only small rate deviations as a function of position and temperature. If the position and temperature dependent deviations are particularly small, the watch is a chronometer, an denomination for particularly accurate watches.

Various organizations test movements, at the request of their manufacturers, through extensive rate measurements, whether they are consistent with the chronometer requirements. After having passed the examination a tested movement receives an offical certification. In most cases the watch manufacturer will print the term Chronometer on the dial as a marketing measure.

Note, however, that some tests are of the movement only, while others test the movement and the entire watch. Movement-only tests, including those performed by COSC, are expected to change once the watch is cased. This is especially true of COSC testing, since movements must be shipped from one of three COSC laboratories to the point of final assembly. COSC will also test “watch heads” for wristwatches, but most manufacturers do not select this option.

Already in the 18th century watches were tested in observatories for their accuracy. Observatories were independent institutions and had very accurate precision pendulum clocks. Regular chronometer certifications took place after 1850. The watches were tested for 44 or 45 days in 5 positions and at several temperatures. From about 1860 to the 1970 years chronometry competitions were performed. The victories in this competitions were included in the advertising of watch manufacturer. This enforced the desire in customers to purchase chronometers.

To undergo a larger amount of watches the chronometer certification, 1877 in Biel, 1887 in La Chaux-de-Fonds and 1888 in Saint-Imier the Bureaus officiels de controle de la marche des mastres (official offices to control the rate of movements) was founded. 1893 the test duration was set to 15 days in 2 positions and at 3 temperatures. From 1904 1st class tests (15 days) and 2nd class tests (10 days) were carried out.

After 1915 the 'Suisse des Associations de Fabricants d'Horlogerie' (Association of Swiss watch manufacturer) defined a chronometer as a precision watch, which was regulated in different positions and at different temperatures and “might” get a certification. Thus, the term “chronometer” was no longer protected. Only 1951 the chronometer definition was again changed by the Swiss watch industry. A watch was only allowed to be called “chronometer”, if it had received an official certification by an independent test office. 1952, the International Commission for the coordination of the work of chronometric observatories established that a chronometer was a precision watch that was regulated in different positions and at different temperatures in order to obtain a official certification.

The ISO, an association of standardization organizations from over 150 countries, defines in the international standard ISO 3159 that compliance with the definition of chronometers has to be confirmed by a neutral official authority, which performs the control of the watch / of the watch movement and issues an official certificate. The ISO 3159 test stipulates that a chronometer movement must be accurate to -4 to +6 seconds per day.

In Switzerland since 1979 the chronometer certification is performed by the COSC (Contrôle Officiel Suisse des Chronomètres). The COSC is an independent and non-profit organization, which is neither controlled by the Swiss government nor supported financially. The watch movements are examined in the test laboratories in Geneva, Le Locle and Biel.

COSC supports four different types of watch movements

1. Wristwatch
2. Pocket watch
3. Fixed time devices (clocks)
4. Quartz wristwatches

Only Type I (wristwatches) are tested to ISO 3159, since this standard only applies to this type of watch movement. The other standards are established by COSC themselves. COSC are static movement tests in five positions and at three temperatures, and typically last for several days. Movements are then tested for 15 to 19 days and re-checked daily to ensure they still perform as expected.

• 1. + 2. day in position '6 up' at 23°C
• 3. + 4. day in position '3 up' at 23°C
• 5. + 6. day in position '9 up' at 23°C
• 7. + 8. day in position 'dial down' at 23°C
• 9. + 10. day in position 'dial up' at 23°C
  On the 10th day of the testing complications, if existing, are examined for correct operation.
• 11. day in position 'dial up' at 8°C
• 12. day in position 'dial up' at 23°C
• 13. day in position 'dial up' at 38°C
• 14. + 15. day in position '6 up' at 23°C

In Germany since 2006 there again is a chronometer certification, which is performed by the LMET and SLME according to DIN 8319, namely in the Sternwarte Glashütte (Glashütte Observatory) restored by Wempe.

In the Glashütte Observatory (see historical photo) only complete watches with zero reset mechanism of the seconds hand are checked, as opposed to the COSC that only examines the mere movements outside of the housing, where they are built in again after the end of the test.

Grand Seiko watch movements are tested in-house for certification. As of 1998, “Grand Seiko Standard” requires a movement to achieve a mean daily rate of -3 to +5 seconds per day. A higher “Grand Seiko Special Standard” requires -2 to +4 seconds per day. Grand Seiko movements are tested over 17 days in six positions.

An additional check according to new standards, surpassing the COSC tests, is performed in Fleurier in Switzerland under the name Qualité Fleurier. This is not just an accuracy standard, as it requires design, manufacture, assembly, and quality control to be carried out in Switzerland along with standards for finishing. After achieving a COSC Chronometer rating, a movement is tested by the Dubois SA laboratory in La Chaux-de-Fonds for Chronofiable ageing durability. Finally, the cased watch is tested on the “Fleuritest simulator” for 24 hours.

Rolex uses the term, “Superlative Chronometer” to indicate a watch containing a COSC-certified chronometer movement that has also passed tests of power reserve, automatic winding, and water resistance. In 2015, Rolex created a more stringent requirement for Superlative Chronometer movements -2 to +2 seconds per day. The first movement to achieve this result was the Rolex 3255 movement, but this has been expanded to every Rolex watch movement since.

In 2015, Omega established a new Master Chronometer certification for cased watches exposed to magnetic fields. This 10-day test is performed by METAS in Biel near the Omega headquarters. It is a test of both the movement and the watch, and is limited to -0/+5 seconds per day.

The first test is COSC certification, in six different positions at two temperatures, after exposure to 15,000 Gauss. The next test measures timekeeping for the movement while exposed to 15,000 Gauss in two positions, while the third test is similar and tests the entire watch. The fourth test measures daily precision before and after exposure to magnetic fields. Next, the watch is submerged and pressurized to test water resistance. It is also tested for power reserve, deviation of performance at full winding and 33% winding, and deviation of rate in six different positions.

Permitted tolerances in s/24h:

In 2009 the famous luxury watch producer Patek Philippe introduced their own company-exclusive quality standard, the Patek Philippe Seal.

See more:

• Patek Philippe Seal

• Längengrad; Autoren Sobel, Dava / Andrewes, William J. H.; ISBN 3-442-76106-9
• Das große Uhrenlexikon; Author Fritz von Osterhausen; ISBN 3898804305
• Armbanduhren, Chronometer. Mechanische Präzisionsuhren und ihre Prüfung; Author Fritz von Osterhausen; ISBN 3766712292
• Bezeichnungs- and Prüfvorschriften für Chronometer RAL 670 A; ASIN B0000BGI7V
• Beschreibung der Hemmungen der höheren Uhrmacherkunst / Beschreibung der neuen freien Chronometer-Unruh mit Ruhecylinder and Schutz gegen unzeitgemäße … jeglichem Gebrauche für Uhren zu bearbeiten; Author Jess H. Martens; ISBN 3981046145

• COSC (Controle Officiel Suisse des Chronometres)