Chronometer and COSC Watches Explained

Did you come across a chronometer watch? You might be wondering how it’s different from a regular wristwatch.

A chronometer watch is required to meet specific accuracy requirements in various tests.

For a mechanical watch to a chronometer, it must measure a daily average accuracy within -4 to +6 seconds per day during testing. Quartz movements are held to a much higher standard, requiring a deviation of just ±0.07 seconds per day, or 1.05 seconds per month

Chronometer watches are tested by the Contrôle Officiel Suisse des Chronomètres (COSC), an independent organization based in Switzerland.

Read on to learn more about chronometer watches, their accuracy, COSC certification requirements, and more.

What is a Chronometer Watch?

A chronometer watch is any watch that has received a chronometer certification by passing a serious of rigorous accuracy tests.

The gold standard for chronometer certification is from Contrôle Officiel Suisse des Chronomètres, or COSC for short.

Chronometer vs. COSC Watches

A COSC-certified chronometer watch is simply a watch that has passed the chronometer certification test by the Swiss organization COSC.

Contrôle Officiel Suisse des Chronomètres certification, or COSC, is the holy grail of chronometer certification. It is based in Switzerland and only certifies movements, not the entire watch.

Each year, the COSC certifies more than 1.6 million Swiss movements. They only certify Swiss-made movements promoting the “Swiss-made” label.

COSC Chronometer Testing Procedure

Here is how the COSC tests each watch

  1. COSC ensures that the identification numbers of the movements match those on the list provided to it.
  2. Watches are wound up and stored at a constant temperature for 24 hours to stabilize the movements before testing.
  3. Watches are tested in 5 different positions and three different temperatures. Each batch test takes 15 days.

The 5 positions the watches are tested in are:

  • Crown facing left
  • Crown facing up
  • Crown facing down
  • Dial facing up
  • Dial facing down

These are the most natural positions a wristwatch would rest on one’s wrist.

Each movement is maintained and measured at three specific angles for consistency: 8°, 23°, and 38°

After certification, each movement’s performance is included in the COSC database. Here, a buyer can use their movement’s identification number to check how it performed during chronometer tests.

Is the COSC Biased?

COSC is independent and supports itself financially through payments made by its clients for testing. They are not government funded and have no shareholders. This ensures impartiality in chronometer testing and certification.

Are COSC Chronometers Better Than Non-COSC Chronometers?

COSC certification is a huge status symbol for many manufacturers like Rolex. But, other luxury watchmakers like Patek Philippe subject their watches to in-house testing as they believe their watches exceed COSC standards.

For example, Patek Philippe uses its own “PP” (Patek Philippe) seal to certify its watches, while Glashütte subjects its watches to 25 days of rigorous in-house testing.

In addition, the COSC only certifies Swiss-made movements. This rules out movements from other regions, such as the fine watchmakers in Japan, such as Seiko. Still, many Seikos such as the King Seiko lineup are certified as chronometers through in-house tests.

Therefore, a COSC-certified watch isn’t necessarily better, or more accurate, than other chronometers. They are simply watches that have been tested specifically by the COSC.

How Accurate Does a COSC Chronometer Need to Be?

COSC testing allows for a small deviation of accuracy. In testing, quartz watches need to be more accurate than their mechanical counterparts.

  • For COSC to certify a mechanical movement, it must have attained a daily average deviation within -4 to +6 seconds during testing. This may vary depending on whether the movement was tested in or out of its casing.
  • Quartz movements are held to a much higher standard. They are only allowed a deviation of ±0.07 seconds per day or 1.05 seconds per month.

Rolex Chronometer Tests

Rolex subjects its COSC-certified chronometers to additional in-house testing after being COSC-certified. The movements are sent to COSC for uncased testing, so Rolex’s secondary testing is done with the movement fully cased once it is returned.

This secondary testing ensures it can promise its customers ±2 seconds of deviation per day. A secondary-tested chronometer is branded as a “Superlative Chronometer” by Rolex.

What Is the Most Accurate Chronometer Watch?

The Zenith Defy Lab is considered the world’s most accurate chronometer watch, with an astounding accuracy of 0.3 seconds per day.

Zenith achieved such accuracy by employing new technology in its oscillator. They came up with the Zenith Oscillator, which contains only one element that replaced the previous thirty-element, sprung balance oscillator used in most watches.

This scaling down of elements reduced its thickness to 0.5 mm as opposed to the average 5 mm thickness. This increases efficiency exponentially as there are fewer linkages and the mechanism is less affected by gravity.

The Zenith Defy Lab is not only the most accurate chronometer but highly precise as well. The Zenith Oscillator vibrates at 108,000 vibrations per hour (vph), whereas other modern oscillators vibrate at around 21,000-29,000 vph.

It is also made of silicon, which greatly reduces friction, eliminating the need for lubrication. The Zenith Defy Lab has a 60-hour power reserve and is precise for up to 95% of its power reserve period.

What Is the Difference Between a Chronometer and a Chronograph?

Although these two words sound extremely similar, they should never be used interchangeably.

A chronometer is an accurate watch that measures time in seconds, minutes, and hours. A chronograph, however, is a complication added to a watch to measure the amount of time that has elapsed. In simpler words, it has stopwatch functionality.

Watches with chronographs will usually feature two second hands. One tells time normally, while the other can be set to measure a certain amount of elapsed time. When not in use, the second hand, initially used as a stopwatch, will be positioned below the normal second hand, out of sight.

Watches with a chronograph function may have smaller dials on the main dial that measure elapsed seconds, minutes, and hours, displaying them separately. These sub-dials are the chronographs.

History of the Chronometer

In the first half of the 18th century, sailors could only find their position during a voyage by referencing latitudes. They used a sextant to measure the exact position north or south of the equator. Unfortunately, this position could be anywhere on that latitude.

This caused shipwrecks, so a way to calculate a ship’s longitude was also needed. For this reason, the Board of Longitude was created. It promised a 20,000-pound reward for anyone who would come up with a solution to calculate a ship’s longitude with an allowance of a 30-nautical-mile-error.

The Birth of the First Chronometer Watch

Some thought that using time could solve the longitude problem. The idea was feasible, but clocks at the time were mechanical and quite inaccurate. Variations in sea conditions like temperature, gravity, and the rocking of the ship would make them even more inaccurate.

But, in 1761, John Harrison managed to develop a fairly accurate clock. It maintained accuracy even with variations in sea conditions as it used counterbalances connected to springs. He named this first marine clock H1.

He continued to improve his design. His fourth version, the H4, was a bit larger than a regular pocket watch, and its design paved the way for modern chronometer watches. It is considered the first true chronometer.

The H4 had an average error of only 39.25 seconds after 47 days at sea. Unfortunately, this was believed to be impossible, so Harrison had to produce paperwork proving this feat and disassemble it for a team of inspectors before he could collect the reward in 1773, and only after King George III forced the board to do so.


As you can see, a chronometer watch is a mechanical or quartz timepiece that can tell time to a certain internationally accepted level of accuracy.

Chronometer watches are often, but not always, certified by the Contrôle Officiel Suisse des Chronomètres certification, or COSC. A chronometer must pass rigorous testing to be COSC-certified, so one with COSC certification is assured to be highly accurate.

While all watches keep time, a chronometer is certified to have superior accuracy.

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