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Watches in Depth - Chronometers

by Jason 28 July, 2011 View Comments

Rolex Chronometer Hangtag

For centuries the pursuit of accuracy and stable rate in clocks and watches has been the driving force behind innovation and quality. The pursuit of accuracy is a cause that has significant trickle-down effects to even the least expensive mechanical watches, much moreso than the pursuit of ever-more complicated pieces. Where a highly complex watch is a bold and visible display of watchmaking prowess, the pursuit of absolute precision is the application of immense skill that often goes unnoticed by the consumer. The chronometer is a finely crafted mechanical watch that exhibits the highest degree of accuracy possible, with official certification to declare it as such. Outside of high-complications, the building of chronometers is one of the top tiers of watchmaking.


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. The problem until that time was that latitude was easy to calculate by using the position of the sun and stars, but longitude was only calculable by measuring the difference in time between your start point and your current point at sea. The idea is that you have a perfectly accurate clock that shows the time in homeport; you note the local time by comparing high noon of the sun with your ship’s clock, and you have your time difference that can then be calculated into degrees of longitude. Problem was, there was no such clock before Harrison came along. Nothing could perform reliably and accurately at sea, where conditions were variable – hot, cold, humid, dry, and of course constantly moving and tilting.

Harrison Chronometer

Harrison fought hard to have his inventions adopted by the British admiralty, despite considerably opposition from conservative thinkers (notably the Royal astronomers who were attempting to develop a method for measuring longitude by the positions of the moon). In the end Harrison’s chronometers, H1 through H4, performed remarkably well. They deviated only a few seconds over a period of a weeks at sea, measuring longitude to within a few miles. This was from a carpenter who was a self-taught watchmaker. The Harrison designs would eventually set the standard for marine chronometers. At this point a chronometer was considered a design rather than a measure of performance, with a so-called chronometer escapement being the norm in marine designs.

Chronometer Escapement

Personal (as in 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. A crash between two trains in Ohio in 1891 due to a stopped pocketwatch prompted the establishment of a railroad chronometer standard, spearheaded in the United States by Webster Clay Ball. Ball 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. Later chronometer testing would add categories for the effects of temperature, but to this day the five-position test is the official standard.

Webster Clay Ball

Now what about this isochronism? Isochronism is the variation between the swings of the balance wheel. The balance wheel of the escapement rotates back and forth on a hairspring, and an ideal beat is perfectly even swings back and forth – this is perfect isochronism. Most movements have a small variation between the swings that can affect the accuracy; to have a steady rate isochronism must be adjusted as close to perfect as possible, hence its importance in chronometer testing. Adjusting for isochronism is a bit of an art that encompasses a variety of tiny changes in the shape and position of the hairspring as well as the weight of the balance wheel.

Harrison H4 Chronometer

Wristwatches didn’t come into vogue until after the First World War, where they had become popular among soldiers as an easy way to check the time without fumbling through pockets. Up to then wristwatches were designed for women exclusively as pieces of jewellery, and accuracy was not of particular importance (variations of minutes a day was considered acceptable for women’s “wristlets”). The first chronometer wristwatch movement came about in 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 pocketwatch movements. The precedent had now been set.    


In the early 20th century chronometer performance were determined and tested by observatories in Europe, notably the Kew in Britain and the Geneva and Neuchatel observatories in Switzerland. Competitions were held between brands to achieve the highest score (by having the least amount of variation across positions and temperatures). Each observatory had its own set of rules – Kew was the strictest with a 44-day test regimen, while the Swiss observatories followed a 15-day schedule. Companies began providing production movements for certification to have official chronometers to sell to the public, Rolex being one of the biggest producers of chronometers from the early days right up to the present. Each individual movement must be tested and certified to be legally called a chronometer. Thus if the company says it is a chronometer, the movement in that particular watch was tested. Some companies had their own chronometer tests, a practice that continues to this day, but legally they could not claim to be chronometers without an official certificate from an independently sanctioned testing organization.

Kew Observatory Britain

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 the details 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 (or an equivalent document). A good performance at the COSC doesn’t guarantee the watch will maintain that level of accuracy forever – the breakdown of oil, shock, and the bedding in of parts means that most chronometers are unlikely to pass the COSC tests after a few years of use, but you can expect them to perform to a higher standard than a typical mechanical movement. Where a basic ETA calibre is expected to vary between 15-20 seconds a day, a chronometer-spec calibre should perform within 5-10 seconds per day or better when new.

COSC Certificate

Making a chronometer is no easy feat, and was particularly challenging in the early days. It’s not simply a matter of adjusting the timekeeping of a standard movement and calling it done – every part of the movement must be optimized and fine-tuned to keep perfect time in a variety of conditions. The current standard of rate is –4 to +6 seconds per 24 hour period in 5 positions, timed over 15 days, with portions of the test done in high and low temperature conditions for a total of three temperatures; anything more than –4+6 in any of the tests and the movement fails.


To adjust a chronometer, the basic components must be made to the highest possible levels of accuracy to minimize poor tolerances. In the case of graded movements, like those provided by ETA, chronometer grade movements actually have different components than their lesser versions, even though they share the same basic design, and all the parts are finished to a very high level before adjustments even begin. Gears must be perfectly meshed and polished. Arbours and 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 sloppy machining, dust, rough surfaces, average parts, or “good enough” engineering.

Valjoux 7750 Grade Comparison

The biggest threat to performance outside of different positions (which shifts the friction points of the gears and wheels making the performance change) is heat and cold. In different temperatures certain metals expand and contract at different rates. This is particularly important in the balance wheel and 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, and more recently Nivarox alloy hairsprings and beryllium alloy balance wheels that are practically immune to temperature change. Compensation techniques were originally the 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.

Superlative Chronometer

Many people have probably seen the “Superlative” tag on the dials of Rolex watches (as in “Superlative Chronometer Officially Certified”). Many people assume this distinguishes Rolex performance from a normal chronometer. This isn’t the case. A chronometer test by the COSC is pass or fail. Fall within the parameters and you get a certificate, fall outside them and you don’t. The “Superlative” bit is a piece of marketing jargon conceived to make Rolex appear superior. It does not refer to any exceptional performance or accolade from the COSC. This being said, Rolex is by far the largest producer of chronometers in Switzerland, submitting 60% or more of all tested movements each year (to the tune of approximately 800 000 movements per year).  

Breitling Chronometer

The COSC has a strict policy against competition in the results. As mentioned, the tests are pass or fail. Before 1973, competitions were held and certain observatories would grade movements according to their performance, creating a hierarchy of results. The COSC was created to stop this practice and give all manufacturers equal footing in the results. In recent years chronometer competitions have slowly begun again (with some surprising results, like inexpensive brands showing up in the standings not far off from watches costing hundreds of thousands of dollars) but these are voluntary competitions that are not related to basic chronometer certification. The COSC is a government-run organization, but with involvement from brands that use their service. A lot of politics are at play to ensure the supremacy of the COSC tests and their non-competitive approach.

Chronometer Certificates

Many people claim to get “chronometer performance” out of a watch that isn’t certified as such. Just because your watch runs within 5 seconds a day for a few days does not make it a chronometer. The watch needs to run better than that rate for at least 15 days in five positions at three different temperatures – many watches can keep time within a few seconds, but never with the precision in different conditions that a chronometer will. Chronometers are built from the ground up for maximum precision and predictability, where a base movement will have less expensive finishing and parts throughout and will be highly susceptible to positional changes and temperature variance. 


Certain companies make chronometers (in terms of performance and engineering) but abstain from participating in COSC tests. They cannot legally be called chronometers, but they will perform to the standards of the COSC. The COSC is a third party company that charges for its services, a cost that must be added to the price of the finished watch. Some companies don’t submit to keep the prices down, while others see the COSC as superfluous and unnecessary for proving their quality to the public. Audemars Piguet, Jaeger Le-Coultre, Breguet, IWC, F.P. Journe, A. Lange und Sohne – none of these brands submit to independent tests, preferring to do their own testing in-house to their own standards (which can often be stricter than the COSC guidelines).


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 labour and engineering to produce, and are governed by a strict set of standards and rules to ensure that no one abuses the official appellation of “chronometer”. 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. Unbeknownst to most consumers is the power and politics at play around the COSC and the certification of chronometers – it’s a contested arena in the watchmaking world, quite different from the early days of John Harrison’s first marine chronometers.John Harrison


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