Saturday, June 6, 2009



A chronometer watch is a watch tested and certified to meet certain precision standards. In Switzerland, only timepieces certified by the COSC may use the word 'Chronometer' on them. However, numerous prominent Swiss watch manufacturers do not submit their movements for COSC certification, although such movements would probably easily qualify as chronometers under the COSC certification rules.

The term chronometer is also used to describe a marine chronometer used for celestial navigation. The marine chronometer was invented by John Harrison in 1730. This was the first iteration of a series of chronometers which enabled accurate marine navigation. For the next 250 years, an accurate chronometer was essential to any kind of marine or air navigation until the implementation of global satellite navigation at the end of the 20th century. The marine chronometer is no longer used for navigation.

Once mechanical timepiece movements developed sufficient precision to allow for accurate marine navigation, there eventually developed what became known as "chronometer competitions" at astronomical observatories located in Europe. The Neuchatel Observatory, Geneva Observatory, Besancon Observatory, and Kew Observatory are prominent examples of observatories that certified the accuracy of mechanical timepieces. The observatory testing regime typically lasted for 30 to 50 days and contained accuracy standards that were far more stringent and difficult than modern standards such as those set by COSC. When a movement passed the Observatory, it became certified as an Observatory Chronometer and received a Bulletin de Marche from the Observatory, stipulating the performance of the movement. Because only very few movements were ever given the attention and manufacturing level necessary to pass the Observatory standards, there are very few Observatory Chronometers in existence. Most Observatory Chronometers had movements so specialized to accuracy that they could never withstand being used as wristwatches in normal usage. They were useful only for accuracy competitions, and so never were sold to the public for usage. However,in 1966 and 1967, Girard Perregaux manufactured approximately 670 wristwatches with the Calibre 32A movement, which became Observatory Chronometers certified by the Neuchatel Observatory. These Observatory Chronometers were then sold to the public for normal usage as wristwatches, and some examples of this watch may still be found today. The Observatory competitions ended with the advent of the quartz watch movement, in the late 1960's and early 1970's.

Chronometers often included other innovations to increase their efficiency and precision. Hard stones such as diamond, ruby, and sapphire were often used as jewel bearings to decrease friction and wear of the pivots and escapement. Chronometer makers also took advantage of the physical properties of rare metals such as gold, platinum, and palladium.

COSC COSC aka C.O.S.C. is Contrôle Officiel Suisse des Chronomètres, the Official Swiss Chronometer Testing Institute, which is the institute responsible for certifying the accuracy and precision of wristwatches in Switzerland.

Founded in its current form in 1973, the COSC is a Swiss non-profit organization that tests Swiss-made chronometers. COSC is an acronym for the organization's French name, Contrôle Officiel Suisse des Chronomètres.

COSC testing generally applies to watches manufactured/assembled in Switzerland.[1] Notwithstanding, the normative standards are set by international agreement and are the same whether they are nominally labeled ISO or DIN standards. Some German, Japanese, and even non-certified Swiss movements can surpass the normative requirements. The Japanese have largely abandoned the accolade, replacing it with in-house testing to a slightly more strict standard as with, for example, the Grand Seiko. On the other hand, the Germans have set up their own testing facility in Saxony at the Glashütte Observatory [2][3] where the DIN 8319 standards, which mirror the ISO standards used by COSC, are employed. At one time the French provided similar large scale testing at the Observatory at Besançon, however, today only a very few watches are currently tested there and carry the accolade "Observatory Chronometer."

The COSC was founded by five watchmaking cantons (states) of Switzerland: Bern, Geneva, Neuchâtel, Solothurn and Vaud, together with the Federation of the Swiss Watch Industry (FHS). It encompasses the laboratories/observatories that had been created independently of each other from the late 19th century.

Three laboratories now test the movements submitted by individual watch manufacturers to be granted chronometer status. They are in Biel/Bienne, Geneva and Le Locle. The Geneva and Biel laboratories are almost entirely devoted to testing Rolex movements.[4] Although not all Rolex watches are chronometers, Breitling has claimed that since 2000 all of its production is COSC certified. Since its inception in 2006 Bremont watch has had its entire line COSC certified as well. Omega also has much of its production certified. Thus, based upon the movements used by Rolex, Breitling, and Omega, the movement calibers that obtain most of the COSC certificates[5] are the Rolex 3135[6] (since 1988) (and variants 3155, 3175, 3185, 4130) and 2235, the ETA 2892A2[7] (and variants) and Valjoux 7750,[8] each of which operates at 28,800 beats per hour.


Each officially certified COSC chronometer is identified by a serial number engraved on its movement and a certification number given by the COSC.

Testing criteria are based on ISO 3159[9] which defines a wrist chronometer with spring-balance oscillator. Only movements which meet the precision criteria established under ISO 3159 are granted an official chronometer certificate. Compare ISO 3158.[10]

Each uncased movement is individually tested for fifteen days, in five positions, at three different temperatures. The movements are fitted with a seconds hand and the automatic winding mechanisms are disengaged for the tests. Measurements are made daily with the aid of cameras. Based on these measurements, seven eliminatory criteria are calculated, each of which must be met e.g. for movements of a diameter over 20 mm, the requirements, indicated in seconds/day, are noted in the table below. There is no ISO standard for quartz timepieces, but there is develpment in this field. ISO 10553:2003 specifies the procedure for evaluating the accuracy of quartz watches, individually and by lot, and the relationship between the accuracy tested and the accuracy classification given by the manufacturer. It applies to quartz watches having accompanying documents on which the accuracy classification is indicated. Nevertheless, COSC has also developed its own standard for testing quartz chronometers with eight eliminatory criteria, also noted in the table below.

COSC Standards
Mechanical Quartz
* Average daily rate: -4/+6 [11] * Average daily rate at 23 °C: ± 0.07
* Mean variation in rates: 2 [12] * Rate at 8 °C: ± 0.2
* Greatest variation in rates: 5 [13] * Rate at 38 °C: ± 0.2
* Difference between rates in H & V positions: -6/+8 [14] * Rate stability: 0.05
* Largest variation in rates: 10 [15] * Dynamic rate: ± 0.05
* Thermal variation: ± 0.6 [16] * Temporary effect of mechanical shocks: ± 0.05
* Rate resumption: ± 5 [17] * * Rate resumption: ± 0.05
* n/a * Residual effect of mechanical shocks: ± 0.05;

200 shocks equivalent to 100 G (981 m/s²)

Measurements are compared with a time base established by two independent atomic clocks synchronized on GPS time. Not all chronometers are supplied with the report issued by the COSC as the reports are optional to the brand or maker. Each manufacturer may decide whether to reveal the results gathered during the certification process of the movement. For example, Rolex and Omega do not supply their chronometer certified watches with the COSC certificates. However, Omega can provide the COSC chronometer certificate if asked.

While competitive chronometer testing took place at the observatories in Neuchâtel (1866-1975) and Geneva (1873-1967), testing of large numbers of watches intended for public sale was conducted by the independent Bureaux officiels de contrôle de la marche des montres (B.O.s) established between 1877 and 1956. Between 1961 and 1973, [18],[19] “a chronometer [was] a precision watch, which [was] regulated in several positions and at different temperatures and which had received a certificate [from the (“B.O.)]." Collective certificates, rather than individual certificates, were usually issued. The 1961-73 standard required a mean daily rate in five positions of -1/+10. In 1973, the B.O.’s came under the C.O.S.C. which specified a daily rate of -4/+6 sec.


Over a million official chronometer certificates are delivered each year, representing only 3% of the Swiss watch production, a proportion that underscores the exceptional nature of a chronometer. To earn chronometer certification, a movement must not only be made from the highest quality components, but also be the object of special care on part of the finest watchmakers and timers during assembly

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