WatchTech

Ed. W
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COSC accuracy vs the Chinese (and why it's not all that important)

Over 1 million COSC certificates are issued annually, mostly to Rolex, Omega, and Breitling. While buying my first watch proper, I took chronometer certification very seriously and demanded it. Not surprisingly, with my budget, I ended up with a Breitling. While it was quite accurate, it didn't meet my somewhat haute horologerie expectations. I was then shocked to find that most of what Patek, Vacheron, AP, JLC, and other respected brands produced were in fact not certified. Later I learned that pretty much any well made movement was capable of being adjusted to COSC specifications. 

Below is a sample COSC certificate (courtesy of Dell Deaton):

It basically certified that the movement (not the entire watch) has been tested in the 5 specified positions (mostly at STP of 23 degrees C) and also with dial up at 8 and 38 degrees C. The accuracy requirements over 24 hours are -4/+6 seconds in 24 hours.

Well I'm about to demonstrate how a sub 300 dollar seagull 1963 reissue (with caliber ST-19 based on the swiss Venus cal. 173) has been adjusted by me to better than COSC rates. It might have taken a good 30 minutes to do but the results are (All results measured on a Timegrapher 1000 accurate to 1 second/day, near full wind, rate in seconds/day, amplitude in degrees, and beat error in milliseconds):

Without chronograph running

With chronograph running

Both of the rates average out to a very accurate chronometer grade watch. But we need to remember that this is by no means meant to be a chronometer. The movement is of reasonably high quality but designed to be manufactured at minimum cost rather than maximum precision.

So what's the conclusion? COSC certification is a nice thing to have, but certainly not worth the premium frequently charged. We still have to rely on the integrity of the manufacturer to use quality parts and oils and craftsmanship.

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