Cosc Certificate Examples Rolex Tag

COSC VS Chronometer

In the watch community, COSC and chronometer are often used interchangeably, sometimes incorrectly. It’s like whiskey and bourbon: all bourbon is whiskey, but not all whiskey is bourbon…

“All COSC certified watches are chronometers, but not all chronometers are COSC certified” -Caliber Corner

What is a chronometer?

The dictionary definition of chronometer:

1) A timepiece or timing device with a special mechanism for ensuring and adjusting its accuracy, for use in determining longitude at sea or for any purpose where very exact measurement of time is required. 2) Any timepiece, especially a wristwatch, designed for the highest accuracy. –source

What is COSC?

The term COSC is an acronym for the Contrôle Officiel Suisse des Chronomètres. COSC is a Swiss testing institute that certifies the accuracy of precision instruments (a fancy way of saying watches). COSC is responsible for issuing certificates for watches tested in Switzerland that meet chronometer specifications, in accordance with ISO 3159.

COSC Chronometer Rating Requirements:

Among other requirements, a COSC certified chronometer must have the following basic attributes:

  • The watch must be Swiss Made
  • The movement must keep time within an average rate of -4/+6 seconds per day

Tests are conducted for at least 15 days, in 5 positions, in 3 different temperatures.

Paperwork and Identification:

COSC certified watches will often include a chronometer certificate. Some collectors simply refer to this as the COSC. The COSC paperwork will have a number which should correspond to a number on the movement. Just because the box and papers includes COSC paperwork doesn’t mean it is the correct certificate for the watch. To ensure that you have the correct papers, you should try to match it with the number on the movement.

According to COSC:

“Compliance with the definition of a chronometer is approved by a neutral official body, which carries out the checks on the watch or movement, and issues an official certificate. In Switzerland, each chronometer is unique, identified by a number engraved on its movement and a certificate number issued by the COSC.” –source

This Wikipedia entry claims that COSC issues over 1,000,000 certifications per year.

Examples of COSC paperwork:

Some watch companies add their own branding to the COSC certificate.

Others, like Rolex, even proudly print the designation on the dials of their watches and include a special COSC hangtag.

Like Whiskey and Bourbon

As mentioned above, not all chronometers are COSC.

Keep in mind that just because a watch is not COSC certified does not mean it doesn’t keep time within Chronometer specs, it just means that it is not officially tested and certified by COSC. For a Swiss Made watch to have the word Chronometer on the dial or case, it has to be certified by COSC.

You may find watches that were not made in Switzerland using the term Chronometer – this Ginault for example.

Additional Resources:

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Community Reviews & Questions:

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For off topic or general watch questions, post in the Caliber Corner Forum.

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Keith Nelson
Keith Nelson
4 years ago

This clarifies a lot for me. I was looking at a King Seiko 4502 with Superior Chronometer Officially Certified written on it, and I was wondering how a Japanese watch maker went about getting it certified by the Swiss. So basically the Swiss regulate the use of the word Chronometer within Switzerland but have no control over how it is used outside of Switzerland. This makes me want the 4502 even more now!

Donald Perley
Donald Perley
4 years ago
Reply to  Keith Nelson

My understanding is that COSC added the “Swiss made” requirement shortly after Seiko started submitting movements for certification, and were doing a little too well in accuracy competition. It wouldn’t look good if the majority of COSC chronometers were from Japan!

HJK
HJK
4 years ago
Reply to  Keith Nelson

Or course, the Swiss Watch Industrie has no grip on the rest of the world. The introduction of a certification in Germany is an illustration of that.

Carlos Ferraro Chavez
Carlos Ferraro Chavez
3 years ago
Reply to  Keith Nelson

So true. The mack daddy of time pieces is a a slightly hybrid watch:
Grand Seiko “Snowflake”. It does use a mechanical/ automatic movement, save for the regulator (it is a quartz one powered by the spring). It is extremely accurate and has an irresistable buttery soft smooth sweep. The regulator speeds up/ slows down the gear as needed. You have to read up on it and watch the movement on Youtube via Watchfinder to witness what I am telling you.

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HJK
HJK
4 years ago

Germany has it’s own certification, in Glashütte I believe.

And yes, other watches may be precise as well. Longines has it’s own tradition, stemming from the sports events. Now of course they have a line with COSC-Chronometers.

Seiko does not really need a COSC -Qualification. If it wanted, their association could set up a certification process, even though the association probably is all Seiko.

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Barry
Barry
3 years ago

so a Ball NM2026C-S4CAJ-SL Engineer II Pioneer Men’s Automatic Watch Water resistant at 100 meters / 330 feet. Functions: date, hour, minute, second, chronometer doesn’t mean it’s certified? I got you.

MP
MP
3 years ago
Reply to  Barry

The functions you listed have nothing to do with being a certified chronometer. The Ball NM2026C-S4CAJ-SL has the caliber RR1103-C which is a Chronometer grade ETA or Sellita and would be certified by COSC. Didn’t your watch come with the certificate?

Rickrolled
Rickrolled
3 years ago
Reply to  Barry
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Terence
Terence
2 years ago

does that mean the watch should be -4/+6 per day compared to the atomic clock or it shouldn’t lose more than 4 or gain more than 6 seconds a day on average?

Clak's
Clak's
2 years ago

seiko caliber 5 variation 2″ per day

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DeepDiveIntoWatches
DeepDiveIntoWatches
1 year ago

So concise and clear. Thanks for the education!

Mike
Mike
6 months ago

Should a COSC or chronometer movement, always be able to hit the specs? 5,10, 15, 20 years down the road? I assume some parts might need to be replaced?

jmrtsus
Member
6 months ago

I just sent back a Tissot Chemin des Tourelles Powermatic 80 COSC I had for 1 week…….lost 15 seconds a day out of the box! Tissot said send it in for service. I said its going back to the seller, how does a COSC watch run this bad? I thought they were tested?

Michael Miller
Michael Miller
4 months ago

Thank You for your work in putting together this web site. I used my 1979 Rolex GMT Master with my Plath sextant for navigation from Japan through the Singapore straits, to the Persian Gulf to load (+/-) 35,000 tons of various military fuels then to various military bases in the Far East. This was well before GPS as we know it today.I still carried my sextant to work for the first 10 years at sea. The most important requirement of any Chronometer is that it has a predictable rate. On the bridge of any ship there is a log that… Read more »

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