chronometer

Definition

Also spelled chronometre, this Swiss term that defines the performance of a mechanical watch. Most commonly, the COSC chronometer certification requires accuracy of

-5 to +6 seconds per day.

There are also requirements for [positional performance], [isochronism] and [temperature stability].

It's also known as ISO 3156. See also ISO 3158 for the positions used.

Chronometer is also the highest ETA movement grade, referring to watches that have their best materials and a COSC certificate.

Note that...

• Rolex claims 'superlative chronometer'; this is meaningless marketroid-speak. It's the same spec.
• The Grand Seiko and Patek Seal, and JLC 1000 hours specs are all more demanding; the COSC spec has been fixed since the late 1960's and really should be made more demanding.

Links

• Watchbore article on COSC - start here
• COSC versus Grand Seiko thread
• Harry Bishop has a nice post on COSC's exclusion of Seiko in their competitions.

Related pages