Watchmaking is full of all kinds of complicated jargon. From types of movements to specific features, watch collectors seem to almost have their own language. Whether you’re planning on entering these circles or just looking to buy a watch, it’s important to be able to decode the vernacular. An acronym that you might run into often is COSC. Especially when it comes to luxury timepieces. But what is COSC?
COSC is an acronym which stands for “Contrôle Officiel Suisse des Chronomètres” which translated to the Official Swiss Chronometer Testing Institute. The COSC Institute is a non-profit organisation which tests Swiss-made timepieces. The organisation is important for the upkeep of the high-set standards of the Swiss watchmaking industry.
As mentioned above COSC only tests watches made in Switzerland. The institute aims to protect the public interest, so it has chosen to focus its attention solely on Swiss-made timepieces in order to preserve the quality and prestige of the label. The fact the organisation only test a small portion of all watches doesn’t mean it’s in any way lenient. Only 6% of all watches send for testing become certified chronometers. Due to COSC strictly observing these rules, a COSC certification is seen as extremely prestigious. It has become the mark of a truly high-quality accurate watch.
What makes COSC special?
The institute designs and maintains its own testing technology in-house. This means the laboratories are upgraded often with the most cutting-edge equipment. COSC runs three laboratories. In order to ensure the accuracy of their tools and the watches being certified, each laboratory has two atomic clocks and a rubidium clock. The watches are consistently cross-referenced to guarantee ultimate reliability. Additionally, the floors are grounded with a copper grid to prevent static electricity. There is also an airlock to filter out dust and monitor the humidity, and air temperature. Their dedication to accuracy is what makes COSC the best at what they do.
What does it mean for a watch to be COSC Certified?
In order for a watch or a movement to be certified, it needs to meet all 7 of the criteria specified by the organisation. Only then will a watch be COSC certified. Even if 1 of the tests is outside parameters, the watch will be rejected. The 7 criteria for mechanical watches are as follows:
- The average daily accuracy needs to be between −4/+6 seconds.
- The allowed mean variation in rates for the entire period is 2.
- The tolerance for the greatest variation in rates for any two-day period is 5.
- The allowed difference between rates in horizontal and vertical positions is −6/+8.
- The largest difference between the mean daily rate and any individual rate during the first 10 days of positional testing, also known as the largest variation in rates, allowed is 10.
- The variation due to the temperature allowed is ±0.60.
- The rate resumption should be ±5.
The COSC institute has different criteria for quartz, mechanical, carriage clocks and on-board clocks.
Each watch is also tested over the span of 15 days for mechanical and 19 days for quartz. They are not only tested but observed daily to ensure there is no change in conditions or any deviation. Other tests include a battery of tests specific to each of the four types of movement, for several consecutive days, in five positions and at three different temperatures (8°, 23° and 38°C).
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