The watch world is full of jargon that’s tough to understand. Chronometer might sound like a mouthful, but it’s just a fancy word for “really, really precise watch”. The term chronometer is a specific type of mechanical timepiece tested and certified to meet certain precision standards. In Switzerland, only timepieces certified by the Contrôle Officiel Suisse des Chronomètres (COSC) may use the word “Chronometer” on them. Outside Switzerland, equivalent bodies (such as the Japan Chronometer Inspection Institute) have in the past certified timepieces to the same internationally recognized standards, although the use of the term has not always been strictly controlled. In this article, Novatime would like to warmly show you some facts about Chronometer watch and COSC certification.
What is it?
Not to be confused with a chronograph, a chronometer is a watch that has been tested and certified to be incredibly accurate by some sort of governing body. Chronometer is a designation given to a watch that has the highest standard of precision. The designation is given to automatic and mechanical movement watches, not those that run with quartz movement. A watch carrying the chronometer certification has passed vigorous tests demanded by the Swiss Official Chronometer Control (COSC), which is the most prominent example of that today and responsible for certifying the accuracy and precision of wristwatches in Switzerland. Only the COSC can deem a Swiss watch worthy of “chronometer” status, though other countries have their own chronometer testing outlets, like the Glashütte Observatory in Germany. While quat watches can also be certified as chronometers, the term most often refers to certified mechanical watches that are accurate to within just a few seconds per day (as an example, COSC-certified chronometers must accurate to within -4/+6 seconds a day). Some brands, like Rolex and Omega, do their own, more stringent, certification programs in addition to COSC testing. Accordingly, their products are unquestionable in terms of high quality .
A chronometer’s mechanical movement is close to perfection, so the time it displays is almost always accurate (unlike other self-winding or automatic watches), therefore carries a premium price over non-chronometer watches. COSC conducted a multi-day examination that places it in five different orientations and three different temperatures to gauge its precision amid varied conditions. The final result must be a daily variation no more than -4 or +6 seconds per day for a mechanical movement. There are also standards for battery-powered watches, though very few brands produce them. In fact, only 3 percent of watches produced in Switzerland make their way through COSC’s certification process, some equally precise watches are not submitted for testing .
A lot of factors might contribute to a watchmaking it through COSC’s grueling tests (or failing, for that matter), but two qualities must be present for a watchmaker’s work to qualify as a chronometer: good materials and high-quality control. Better metal alloys and such materials as silicium are less susceptible to expanding and contracting when the temperature changes. Nor are they as easily degraded by friction. When you’re relying on a steadily beating spring and some gears to keep time, this is paramount. And even if you have the best raw components, they need to be assembled properly to function at their peak. Pinions need to be well-oiled, gears precisely cut, and everything locked together nicely and tightly.
Where did it come from?
The word chronometer goes back to the early eighteenth century when an English clockmaker called Jeremy Thacker invented a vacuum-sealed clock. Without air resistance, it was extremely accurate; Thacker dubbed his creation a chronometer. The name was later used a few decades later to describe marine chronometers—clocks suspended in gyroscopic boxes that helped ships determine longitude and traverse the world’s oceans.
In 1714, Great Britain passed the Longitude Act to solve a problem: Namely, that sailors had a difficult time calculating their precise longitude when out in open water (without land in sight). Therefore, anyone who could devise a method of calculating a ship’s precise longitude would be given a monetary award. In 1730, clockmaker John Harrison had developed his first “marine chronometer” — a super accurate ship clock that could be used to calculate longitude through celestial navigation. Though his first clock wasn’t successful, after decades of work it would eventually be perfected.
By the mid-twentieth century, after marine chronometers were largely being displaced by radio navigation systems, the tradition of super accurate timepieces continued with the Observatory Trials. Here, watchmakers submitted watches that were put through weeks of rigorous accuracy testing at European observatories like Neuchatel. Most of these were one-off watches made for the sake of competition, though Giard-Perregaux and Seiko sold some (not many) of their Observatory Chronometer watches to the public. Though the trials ceased with the dawn of the quartz watch in the early ’70s, in 1973 the COSC was formed and has since become the leading force in chronometer certification of consumer mechanical watches (at least in Switzerland).
What are standards for it?
In order to be recognized as a Chronometer, the Swiss watch must be sent to COSC and undergo accurate access tests on a standard Chronometer. Passing all tests, apart from being called a chronometer, is also:
√ Non-repeatable on the apparatus (never two identical Chronometer clocks)
√ COSC certification (not a specific result) and this certificate is included with the watch
Each unit, regardless of cost or price, is checked separately for fifteen days. They will go through five positions, at three different temperatures. The measurements are made daily with the help of the camera.Based on the measurements, seven criteria will be calculated and must be met to be certified as a Chronometer. The COSC does not have an ISO standard for quartz watch and the ISO 10.553: 2003 standard is used to evaluate the accuracy of a quartz watch.Here are the COSC standards all units in seconds unless specified 
|Average daily rate: −4/+6||Average daily rate at 23 °C: ±0.07|
|Mean variation in rates: 2||Rate at 8 °C: ±0.2|
|Greatest variation in rates: 5||Rate at 38 °C: ±0.2|
|Difference between rates in H & V positions: −6/+8||Rate stability: 0.05|
|Largest variation in rates: 10||Dynamic rates: ±0.05|
|Thermal variation: ±0.6||Temporary effect of mechanical shocks: ±0.05|
|Rate resumption: ±5||Rate resumption: ±0.05|
|n/a||Residual effect of mechanical shocks: ±0.05; 200 shocks
equivalent to 100 g (981 m/s², 3,217 ft/s²)
Why does it matter?
Aside from having to readjust your watch less often, there’s not much of an argument to be made, practically speaking. Marine chronometers were cutting edge in their day, but timekeeping has progressed to the point where cheap quartz watches and atomic clock–regulated smartphone clocks are far, far more accurate. Given this, nobody really buys a COSC watch because they need it to be accurate — considering that only about six percent of Swiss watches produced are COSC certified, it’s mostly for the bragging rights.
Though you could make the argument that chronometer-grade movements are just better quality than other mechanical watches — for example, COSC-versions of ETA movements use upgraded shock-protection systems, hairsprings and mainsprings compared to their standard-grade counterparts. And given that watches that pass the COSC’s testing keep accurate time when subject to mechanical shocks and temperature variations, a case could be made for durability, too. But at the end of the day, in terms of watch nerdery, there’s certainly something impressive about having an antiquated piece of machinery keep particularly accurate time and the watchmaking skill it takes to make it possible.
Who does it best?
Longines Record Chronometer Certified $2,000+
COSC-certified watches generally come at a premium, though at around $2,000, Longines’ first COSC-certified watch offers some solid value for the money .
Rolex Oyster Perpetual $5,700
The Oyster Perpetual, like all Rolexes, undergoes its own testing regimen in addition to COSC, which means it’s accurate to +/-2 seconds per day, exceeding the COSC’s standards .
Bremont MBIII. $6,395
The British brand might not be subject to the Swiss chronometer regulations (nor do most of their watches say chronometer on the dial), but almost every watch in its lineup is COSC-certified. The MBIII is an evolution of Bremont’s signature watch, inspired by a famous fighter-jet ejection seat. It adds a second-time zone hand to the mix, while still being easy to read—and nearly indestructible .
Ball Trainmaster Standard Time. $7,699
Just as marine chronometers pushed exploration by ship, accurate pocket watches made it possible to standardize time across the U.S. railroad system. Ball pioneered these pocket watches back in the 1800s and is still making wristwatches in the original style. The Trainmaster Standard Time has an enamel dial and bright luminous markers that are actually tiny, gas-filled tubes for maximum glow .
Omega Globemaster $7,700
Similarly, in 2015, Omega revealed the Globemaster, and with it, its new “Master Chronometer” certification program in collaboration with the Federal Institute of Metrology, which is accurate to within -0/+5 per day and antimagnetic to 15,000 gausses .
Hano Nguyen @ Novatime Lab.
This article was written in Novatime Lab’s Technical Report (Watch Research). It can be updated in the future for better quality.
 URL: https://en.wikipedia.org/wiki/Chronometer_watch
 URL: https://en.wikipedia.org/wiki/COSC#Background
 URL: https://en.wikipedia.org/wiki/COSC#The_value_of_COSC_certification
 URL: https://www.longines.com/novelties/the-longines-symphonette-steel-and-gold
 URL: https://www.rolex.com/watches/oyster-perpetual/m114300-0001.html
 URL: https://www.bremont.com/watch/mbiii/17965#selector
 URL: https://www.omegawatches.com/watches/constellation/globemaster/the-collection/product/
Author: Hoan Nguyen
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