What is a chronometer?

What is a chronometer?

Borna Bošnjak

An objective way to become more informed about a watch before buying it is by researching movements. Within that pursuit, it’s almost certain that you’ll come across mentions of accuracy, and perhaps the word “chronometer”. What does that really mean, and are all chronometers the same? In a nutshell, a chronometer is a watch that has been certified by some regulating body to be of very high accuracy, usually fitting into a fixed set of specifications. But it’s not quite that simple, as these specifications and testing methods vary greatly – so let’s explore further.

But before we do… chronometer vs chronograph

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An Omega Speedmaster chronograph with a Master Chronometer-certified movement

This will not be a particularly notable tripping point for more seasoned watch enthusiasts, but it is a really common question many will have when just getting into watches. So, to clarify once and for all – a chronograph is a watch that features some sort of complication that allows it to measure increments of time. A chronometer is a watch that, as mentioned above, has been certified to be particularly accurate when it comes to timekeeping. A watch can be neither, just one of, or both a chronometer and a chronograph at the same time.

How we got here

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John Harrison’s H5 chronometer from 1772.

The earliest usage of the word itself goes back to the start of the 18th century, and William Derham’s Physico-Theology book discussing the possibility of producing more accurate time-measuring devices. The invention of a highly accurate marine chronometer was crucial at this time, as it was the only way a ship could reliably navigate in the open seas. The best-known maker was certainly John Harrison, who went on to produce some of the best-known early chronometers, including the H5 which was found to have a daily deviation of just 0.33 seconds per day over a period of 10 weeks. This is just the beginning of the story of marine chronometers that Buffy has explored in-depth, and though they’ve been replaced by more reliable GPS technology for navigation, the idea lives on in wristwatches.

seiko 4520a geneva observatory chronometer movement
A Seiko Astronomical Observatory Chronometer featuring the 4520A movement, currently property of the Seiko Museum Ginza.

These tests would usually be held at astronomical observatories, also known as observatory trials. As the life-saving accuracy element became less important, watch manufacturers quickly realised that favourable chronometry trial results are a great marketing tool – after all, it’s no mean feat saying that your movements are the most accurate in a direct, head-to-head competition with rival brands. Several observatories undertook these trials, including Geneva, Besançon, Kew, and perhaps most famously, Neuchâtel. These observatory trials are the ones that gave rise to actual chronometer certification, the Neuchâtel observatory issuing its Bulletin de Marches to the best-performing watches. It’s worth mentioning that the watches being tested at observatory trials were often purpose-made by the brands, and subjected to much higher performance expectations than modern chronometer certifications require.

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Rolex states that it was one of its watches that received the first-ever Swiss Certificate of Chronometric Precision from the Official Watch Rating Centre in Bienne, while Girard-Perregaux and Seiko are notable for producing serial examples of wristwatches with observatory chronometer-grade movements in the late ’60s and early ’70s. Today, even fewer watches are certified by observatories, and competitions haven’t been held since Seiko majorly disrupted the industry with the Astron, and any watches with such certifications can be considered highly collectible.

Not all chronometers are made equal

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A COSC-certified Tudor Pelagos 39

Without the observatory trials and the advent of quartz, a gap opened for other players to enter the field of chronometric precision testing. An early entrant, and one that is still going strong today, is the Contrôle Officiel Suisse des Chronomètres, better known as COSC. Founded in 1973, the Swiss Chronometer Testing Institute has been a household name for decades, issuing the most chronometer certificates for the majority of entry-level and luxury Swiss watch brands. COSC certification is based on the ISO 3159 standard, covering seven standards for mechanical movements, and two additional ones for quartz movements, all of which Buffy has broken down in their article exploring COSC in-depth. The most notable aspect of this testing is the average rate for mechanical watches, set to satisfy -4 to +6 seconds per day.

omega aqua terra shades master chronometer movement
Omega Aqua Terra Shades with a METAS-certified Omega Calibre 8800.

The next step up, to which COSC certification is a prerequisite, is Master Chronometer, or METAS certification, performed by the Swiss Federal Institute of Metrology. Unlike COSC, METAS requires assembled watches to be sent in for testing rather than uncased movements, also testing the watches to 15,000 Gauss – far beyond what COSC standards require. The watches’ daily rates are tested with full power reserves and at two thirds depleted, across six positions and two temperatures over four days. During this time, the movements must keep within an average rate of 0/+5 seconds per day. Curiously, METAS allows the brands themselves to carry out the testing, with periodic audits and sample testing carried out by METAS to ensure compliance. You see how Tudor carries this out in Russell’s article that took a look inside the brand’s newly opened manufacture.

rolex sky dweller superlative chronometer

Some brands prefer to keep this testing fully in-house, and perhaps the best-known example of this is the Rolex Superlative Chronometer set of standards. The testing does involve COSC certification, too, but Rolex tightens up these tolerances to -2/+2 seconds per day, alongside performing waterproofness, winding, and power reserve, performing all of these tests with cased movements. Unlike METAS, there is no external body that regulates Rolex’s own testing, though I hardly see this being an issue considering the brand’s excellent reliability track record despite the lack of transparency.

Glashutte Original Senator Chronometer 7
Glashütte Original Senator Chronometer with its DIN 8319-certified movement

Since COSC only allows Swiss-made watches to be supplied for testing, the Glashütte Observatory has devised its own set of regulations. The DIN 8319 standard that outlines the testing is closely related to the ISO 3159 standard underpinning COSC certification, differing in two ways. First, the watches must have a hacking function, and the Observatory only tests assembled watches, rather than uncased movements. There are also rules regulating the origins of the watches, as they must be assembled in Glashütte, with 55% of the components manufactured in the region.

When a chronometer isn’t a chronometer?

grand seiko special standard sbgh26

The master of making everything in-house, it’s no surprise that Grand Seiko sought to set up its own version of chronometer testing. Dubbed the Grand Seiko Standard, it’s been in place since the late 1990s, covering eight different tests performed across six positions and three temperatures over a period of 17 days. The most accurate pieces are issued a Grand Seiko Special Standard certificate, meaning their daily rate was within -2/+4 seconds per day, with its performance pointed out by “SPECIAL” text on the dial.

jaeger lecoultre master control sector date

Focusing on straight-up endurance and not divulging any accuracy or deviation standards, it’s Jaeger-LeCoultre’s Master Control test. Taking 1,000 hours to complete and proudly displayed on the winding rotors, the checks include winding, amplitude, and power reserve testing over a temperature range from 4° C to 40° C. Finally, they’re subjected to inconsistent, start-stop motion for approximately two weeks meant to test real-life conditions, before ending with a water resistance test.

When it’s not just about performance

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Chopard L.U.C with a Qualité Fleurier-level movement

The Fleurier Quality Foundation was started by Bovet, Chopard, and Parmigiani Fleurier alongside movement manufacturer Vaucher. It only tests watches that are 100% Swiss Made, and have already passed the stringent chronometric testing of COSC and the Chronofiable durability test. The assembled watches are then subjected to the Fleuritest – a 24-hour evaluation recreating real-life changes in movement and stress. To pass, a watch must have a daily rate of 0/+5 seconds. That’s not all, however, as no run-of-the-mill workhorse will be given the Qualité Fleurier seal of approval, as there’s an aesthetic component to this certification, too. According to FQF, “the movement must have a finish of exclusive aesthetic quality”, concerning the categories of materials, finishing, and means of finishing.

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The other undisputed heavyweight in this category is the Patek Philippe Seal. For starters, its chronometric requirements are tighter than COSC tolerances, requiring -3/+2 seconds per day for movements over 20mm in diameter, and -5/+4 for those that are smaller. Tourbillons have the strictest tolerances, requiring a range of -2/+1 seconds per day. The testing is ongoing over the production cycle of the movement, finalised by a fully assembled and cased examination. Just like FQF, the Patek Philippe seal also guarantees the highest grade of workmanship in regards to the movements, but also the elements of the case and dial.

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A Vacheron Constantin Traditionnelle Tourbillon Chronograph Excellence Platine, certified with the Geneva Seal

Unlike FQF, the requirements for a Patek Philippe movement to be given the Seal are less exactly defined, but one could assume that they closely follow those outlined by the Poinçon de Genève certification that Patek Philippe used before transitioning to its own seal in 2009. However, the Geneva Seal only concerns the aesthetics of a movement, with no expectations placed on performance.