Monochrome Watches
An online magazine dedicated to fine watches

The Race for Accuracy – The definition of a Chronometer

| By Ilias Giannopoulos | 8 min read |

A few months ago we reviewed here in Monochrome the new Omega Globemaster introduced in Baselworld this year. The specific model continues the company’s ever-lasting quest for accuracy squeezed out from a highly tuned mechanical wristwatch platform. From the 1948 “Century” model, to the “Constellation” series and then to the “Globemaster” and from various chronometer competitions to COSC and then to METAS standards, we witness that Omega, as any other Swiss watch company, has an obsession with the accuracy of its timepieces. However, Omega is not the only one in this quest; Patek Phillipe, Rolex, Audemars Piguet, Longines, Seiko, JLC, Chopard and many other companies, including Zenith competed in the past and still by following various standards try to produce an accurate and robust calibre. Accuracy is an intangible quality but at the same time a very useful feature for the lovers of mechanical watches. In this article therefore we will try to shed a light to the chronometry competitions and how a chronometer caliber differs from a regular one (In a coming article, we will examine in detail one of the most important chronometer calibers ever created the Zenith cal.135 and also we will inspect a cold war period story).


During the late 19th and early 20th centuries, before quartz and GPS, nations, industries and armies depended on precise mechanical timepieces. Therefore, manufacturers and their master watchmakers spent significant amounts of time and resources on preparations for Observatory trials, which were held throughout Europe. These events were not intended to showcase new products. Instead, the Observatory trials focused on the science of Chronometry and the ability to make chronometers measure time precisely. Where a highly complex watch is a bold and visible display of watchmaking art, the pursuit of absolute precision is the application of vast skill that often goes unnoticed by the consumer. The chronometer is a finely crafted and highly tuned mechanical watch that exhibits the highest degree of accuracy possible. Outside of high-complications, the building of chronometers is one of the top-tiers of watchmaking that exhibits also any given company’s technical ability and know-how.

Longines chronometer

Ad – Scroll to continue with article

The need for established accuracy from a mechanical platform born out of real world needs. The term chronometer is a slippery one, a word that has shifted in meaning over the centuries. It was originally coined in 1714 by Jeremy Thacker and simply meant “measurer of time”. At that time the only “chronometer” (in terms of performance) was the marine clock, developed by John Harrison between 1730 and 1760 as a way of accurately measuring longitude at sea. (see our article – Found: The Actual Marine Chronometer That Accompanied Charles Darwin at the HMS Beagle on his Journey to the Galapagos Islands) The next big step in the search for accuracy was yet again occurred out of real world necessity.

Commercially available pocket watches/chronometers didn’t emerge until the 19th century. They were spurred on by the need for accurate timekeeping in the new railway systems to ensure safe operation. Webster Clay Ball in the U.S.A, began by modifying movements from existing manufacturers and establishing testing for accuracy that would become the basis of modern chronometric competitions – measurement of rate and deviation in five different positions, resistance to magnetism, and isochronism of the beat.


The third big step happened in the early 1900’s. The form of the timekeeping apparatus changed in favor of the wristwatch. The first chronometer wristwatch movement came in about 1910, and it came courtesy of a certain Hans Wilsdorf and his new company – Rolex. Wilsdorf submitted a modified Aegler calibre wristwatch movement to the Biel observatory for testing in 1910 and passed the standards usually reserved for much larger pocket-watch movements. The precedent had now been set.

From that point onward more companies started to submit their calibers for testing to independent authorities, the observatories. In 1766, the Royal Observatory of Greenwich organized the first timing competition on record. In 1772, the Classe d’Industrie et de Commerce de la Société des Arts commissioned the Observatoire de Genève to administer chronometer trials for Geneva-based manufacturers. The trials became so important for advertising purposes that other manufacturers later established facilities in the canton to qualify for competition. The need for more accuracy which could benefit both the consumer (in their ever day activities) and the watch manufacturers (increasing their sales and fame) and the establishment of an authority that could provide the necessary means for measuring accuracy all culminated in the 20th century with Neuchatel, Geneva, Besancon and Kew observatory competitions.

Observatory Trials were the ultimate test of chronometry. Movements of superior technical design, perfect finish and expert regulation were selected from inventory or manufactured for the purpose. The movements were then fixed to square blocks, for easy handling, and enclosed in aluminum or wooden cases for transportation. Unsigned dials and plain hands were often used, as aesthetics were not a consideration. After 45 days of continuous testing in 5 positions and 3 temperatures (4°C, 20°C and 30°C), the most precise chronometers were awarded honors for the year while manufacturers enjoyed the publicity and resulting sales. Observatory Chronometer movements were kept by their manufacturers for years, even decades, and few were released for sale as complete watches.


Up until the 1970s individual observatories performed tests according to their own regimens. In 1973 an official testing organization was created, the now-ubiquitous Contrôle Officiel Suisse des Chronomètres (C.O.S.C.). The COSC standardized the measurements for the Swiss industry and now no watch can legally be called a chronometer without a COSC certificate. The COSC is an independent company that tests movements from any Swiss manufacturer for a fee, and today has three laboratories, one in Geneva, one in Biel, and one in Le Locle. Passing the tests means the watch can be issued with a unique certificate with the details and the results of each portion of the test, as well as the serial number of the movement. Any new chronometer that is sold should include this certificate.

The last step on this evolutionary road for accuracy happened last year with the introduction by the Swatch group of new chronometer certification process called METAS, which you can read here. In addition, in 2009, the Concours International de Chronométrie timing competition was undertaken to celebrate the 50th anniversary of the Museum of Horology in Le Locle. The issuance of timing certificates by the Glashütte and Besancon Observatories and rumors of a similar British enterprise under consideration must certainly have provided additional motivation to reawaken the heritage of time trials.

We can witness an ever-present quest for accuracy, which started from an era when the clock/pocket-watch/wristwatch was purely mechanical and was used as a tool in life changing situations. Nowadays GPS and quartz are the tools of the trade for professionals however, the romantics should always be fascinated by the continuous struggle for perfection from a purely mechanical apparatus. However, at that point we must examine what is exactly a chronometer status calibre.


The difference between a chronometer and simple mechanical caliber is vast; perhaps the perfect analogy might be the difference between an F1 engine and a consumer engine that can be found on a hatchback. Both work on the same principles but only this remains identical. As such, in horology the working parameters are worlds apart while a chronometer spec calibre must be constructed from high quality parts and in parallel must be highly tuned.

To adjust a chronometer, the basic components must be made to the highest possible levels of accuracy to minimize tolerances. Gears must be perfectly meshed and polished. Pivots must be precisely shaped and have perfect tolerances. The jewels need to be of the highest quality. All the metals must be as pure as possible and compensated for heat, cold and magnetism wherever possible. The hairspring must be very carefully formed and adjusted to ensure isochronism is perfect. There is no room for error.


The biggest threat to performance outside of different positions is heat and cold. In different temperatures certain metals expand and contract at different rates. This is particularly important for the balance wheel and the hairspring. If the balance wheel expands even a tiny amount in heat, it will run slower, while cold will have the opposite effect. Solutions included compensation balances made of different metals to counteract changes like Nivarox alloy hairsprings and beryllium alloy balance wheels, that are practically immune to temperature change. Compensation techniques were originally relegated to high-end watchmaking, but over the decades many of these once exclusive technologies have trickled down into less expensive movements and into the realm of mass-production, meaning that today’s basic movements perform better than ever.

Chronometers are not as simple as they would seem. To the average consumer, a chronometer is a highly accurate mechanical watch. In reality they are highly complex items that require a lot of labor and engineering to produce. While high-complications may get all the glory in the watch industry, the chronometer is the unsung display of quality and precision that is needed to advance fine watchmaking for the masses.

Next week, we’ll have a case study with the Zenith Calibre 135. Stay tuned!

Leave a Reply