One of the beauties of mechanical wrist watches is that they are, well, mechanical. Unlike battery powered timepieces, mechanical watches capture kinetic energy and translate it into power to tell time. The advantages are clear, – no need for worrying about batteries or finding a power source. The drawbacks with mechanical watches are the effects of gravity and friction that can make time vary from day to day and watch to watch.
To combat competing watch manufacturers’ claims on the accuracy of their watches, Contrôle Officiel Suisse des Chronomètres (COSC) was formed in 1973 to certify watch movements on both their accuracy and durability.
Watches going through COSC certification are tested with the movements only without any winding balance attached over the course of 14 days in a variety of temperatures. To attain COSC Chronometer Certification, a watch movement has to fall within these parameters:
Average daily rate: -4/+6
Mean variation in rates: 2
Greatest variation in rates: 5
Difference between rates in H & V positions: -6/+8
Largest variation in rates: 10
Thermal variation: ±0.6
Rate resumption: ±5
Once passing COSC Certification, a watch movement is Officially Certified Chronometer and only then can a Officially Certified Chronometer designation can be written on the watch face. Every COSC chronometer has a serial number engraved on the movement and a certification number provided by COSC. Most Rolex wristwatches are COSC Officially Certified but not all. When considering a vintage wristwatch, look at the dial to see if it is Officially Certified. The $1000 difference in value between a Date and a DateJust might be what is telling you on the dial.
Last year (2012), 1,732,526 movements were sent to COSC be tested. Of the watches tested, Rolex submitted the most movement to be tested. In fact, Rolex increased the number of movements tested from the previous year by over 6% as shown by the following:
- Rolex 799.000 (751.000) +6,4%
- Omega 526.000 (509.000) +3,3%
- Breitling 157.000 (154.000) +1,9%
Despite the numbers of movements submitted to COSC, the passing movements COSC only represent fewer than 3% of Swiss manufactured watches. Not all watch companies, including Rolex, provide the original COSC chronometer certificate to the buyer, but rather, they issue their own certificates attesting that the movement of the watch was certified by COSC.
Rolex watches have a self-imposed stricter than COSC standard of -1 to +5 seconds per day. With 86,400 seconds in a 24 hour period (31,536,000 sec/year), a difference of +3 seconds per 24 hours is a deviation of 0.000035 (thirty five millionths) of its daily run resulting in 99.99% accuracy. The Rolex watch balance oscillates at a frequency of 28,800 times per hour, which is equivalent to a car going 87 mph and traveling a distance of 3,600 miles a year. With this amount of precision, it is definitely worth the effort for submitting to COSC to have this mechanical feat recognized.