Watch Education Chronometry Explained, Volume One: The Swiss COSC Certificate

Since the revival of the mechanical watch industry during the mid-1980s, enthusiasts and brands alike have been absorbed by the concept of chronometry. Like thoroughbred horse racing, the point of mechanical chronometry isn’t absolute performance – quartz and atomic radio timepieces dominate that sphere – but the pleasure of pitting favored archaic champions against each other on their own terms drives enthusiast interest.

Standards of Mechanical Chronometry

Within the self-contained world of mechanical chronometry, two standards dictate terms.

ISO 3159

ISO 3159: The baseline for all mechanical chronometer tests. While individual corporate and national chronometry certifications differ in details, the ISO 3159 is the baseline recognized by the 163 international members of the Geneva-based International Organization for Standardization. ISO 3159:2009, the latest version, last was reviewed in 2015, and is described as follows:


“[3159] lays down the definition of the term “chronometer”, describing the categories, the test programme and the acceptable minimum requirements for wrist-chronometers… Conformity to the definition of chronometer will be certified by a neutral official authority that checks the watch, or if necessary the movement, and issues an official certificate.”


Enter the dominant “neutral authority,” the “Contrôle Officiel Suisse des Chronomètres” (COSC); it issues most commonly seen chronometer certificate with just under two million movements certified annually.sealsofapprovalcosc

This standard has been in place since 1973, and it is administered through the third-party agency of the same name (a registered Swiss nonprofit) through three regional labs. Prior to this date, independent regional bureaus and observatories issued their own chronometer certifications. The basic COSC test consists of 16 days of testing in five positions, in three temperatures, and it only applies to uncased movements. Calibers without a seconds hand are not eligible for a COSC certificate, and every caliber awarded a certificate must have a serial number; these criteria disqualify a large number of movements that otherwise would be capable of passing the test itself.

COSC Standards

Current standards for the COSC certificate entail passing the above evaluations while deviating by no more than -4/+6 seconds per 24 hours; achieving a mean variation in rates of two seconds; a maximum variation in rates not exceeding five seconds, a greatest variation (i.e. greatest individual positional deviation from the mean) in rates not exceeding 10 seconds, and thermally-induced deviation not to exceed +/-(.6) seconds. Additional conditions apply, but these are the principle guiding measures that underpin COSC certification.


Two caveats apply. First, the COSC testing protocol requires daily winding at precisely the same time each day. Second, the certificate only is valid for the bare movement prior to casing, and only when freshly serviced or assembled. Pre-owned and vintage chronometers generally require a complete service in order to be restored to COSC-levels of performance.

Testing Scorecards

While all chronometers are documented in full by the COSC during the testing process, not all wristwatch manufacturers issue full chronometer certificates to end-users. The COSC’s own website is unequivocal about the fact that detailed chronometer certificates for individual movements can be requested only by the manufacturer of the movement that requested certification. Certain watch manufacturers – Swatch’s Omega is the most prominent – offer this service as an extra-cost option for owners.


Certain brands such as Ulysse Nardin and, in rare cases, Audemars Piguet and Vacheron Constantin, continue to ship comprehensive COSC scorecards. The advantage of possessing the original COSC timing test battery results is that the owner can judge not just the pass/fail status of his caliber but the actual margins by which said caliber surpassed the criteria. For example, a watch can receive a COSC certificate despite seconds of deviation per day; exceptional calibers such as the Audemars Piguet 2908 high-beat with AP Escapement have passed the test while exhibiting deviation amounting to tenths of seconds per day.

Quartz COSC Chronometer

Is there such a thing as a “quartz chronometer?” Globally, no; the COSC begs to differ.

Quartz COSC chronometer certifications are available but rarely obtained. Unlike the mechanical chronometer standard, the COSC quartz battery is not underpinned by any ISO-determined standard of performance. As ever, the certified caliber must be Swiss. Breitling is the largest subscriber to this uncommon service and claims more COSC quartz chronometers than all other brands combined.


Testing Period for Quartz

COSC’s quartz battery encompasses 11 days of testing, three temperatures, and only one position. First, the stability of the mechanical drivetrain of the quartz caliber is subjected to 200 shocks of 100-G. The second major obstacle is the temperature battery. Unlike mechanical watches, which are subject to gravity-induced timing error of the hairspring assembly, quartz watches do not suffer by position; they deviate most dramatically under the influence of temperature fluctuation.

For this reason, quartz watches are tested against three different thermal profiles, and the COSC itself opines that only thermo-compensated quartz movements such as the ETA Thermoline family and related Breitling Superquartz calibers are able to pass the test. From 1977 to 2001, Rolex also produced and certified the caliber 5035 and 5055 thermo-compensated movements for the discontinued Oysterquartz series.


Rolex, Breitling, and Omega have dominated COSC certifications for most of two decades. Although Breitling sells the lowest volume of the three, since 2000, the Grenchen-based firm has boasted of certifying all watches – mechanical and quartz – as chronometers. Omega remains the second largest recipient of Swiss chronometer certificates, but as of 2015, the Swatch brand began the process of shifting all chronometer production to the new METAS standard that Omega itself helped to pioneer.


TAG Heuer leads the line of mid-sized brands receiving COSC certificates. That group also includes Chopard, Panerai, Ulysse Nardin, Tissot, Mido, Rado, and Corum. Zenith, Audemars Piguet, Carl. F. Bucherer, and Vacheron Constantin receive very limited numbers of COSC certifications for certain exceptional models.

Superlative Chronometer from Rolex

As of 2016, Rolex remains the single largest subscriber to the COSC certification service. With production estimated between 800,000 and one million units per year, Rolex leads all manufacturers in certificates issued. However, Rolex, which has a long history of re-testing chronometer movements once fully-cased, recently announced a new in-house “Superlative Chronometer” standard. Under this program, calibers still ship to the COSC for Swiss chronometer certificates, but the movements are regulated to at most -2/+2 seconds once returned to the factory and cased as watches.


By convention, only Swiss brands that have received COSC certification may use the word “chronometer” or “chronometre” on timepieces. However, F.P. Journe of Geneva uses the term without authorization, and with reason; Montres Journe proudly declares its in-house chronometry tests superior to COSC’s battery. F.P. Journe’s tests belong to the vast and expanding world of in-house and non-Swiss chronometry standards that will be explored in future editions of this series.

About Tim Mosso
Tim Mosso is a lifelong watch enthusiast and historian. He is an active member of the online watch community and his passion for watches is second to none. As a self-proclaimed watch nut, he loves to bring his knowledge of horology across all different platforms.

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