Chronometer certification

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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.

Chronometer tests

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 the rate characteristics of the Swiss COSC will always be measured only at the raw movement and are therefore documented only for the period of the chronometer certification, since each watch changes its rate after time.

The history of chronometer certification

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.

Chronometer certification today

Permitted tolerances in s/24h
  Ø (calibre) > 20mm Ø (calibre) < 20mm
Average daily rate in different positions -6 to +6 -5 to +8
Average rate deviation to 2 to 3.4
Greatest rate deviation -6 to +8 -8 to +10
Difference between hanging and lying to 5 to 7
Greatest difference between the average daily rate and one of the first 10 rates to 10 to 15
Rate deviation per degree Celsius -0.5 to +0.5 -0.7 to +0.7
Resumption of the rate -5 to +5 -6 to +6

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.

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.

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.

Glashütte Sternwarte.jpg

In the Sternwarte Glashütte (above: 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.

An additional check according to new standards, surpassing the COSC tests, is performed in Fleurier in Switzerland under the name Qualité Fleurier .


Test schedule

  • 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


  • 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