Antireflective sapphire crystal
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Each officially COSC-certified chronometer is identified by a serial number engraved on its movement and a certification number given by the COSC. Each uncased movement is individually tested for fifteen days, in five positions, at three different temperatures. The movements are fitted with a second’s hand and the automatic winding mechanisms are disengaged for the tests. Measurements are made daily with the aid of cameras. Based on these measurements, seven eliminatory criteria are calculated, each of which must be met. The testing protocol requires daily winding at precisely the same time each day. Over a million official chronometer certificates are delivered each year, representing only 3% of the Swiss watch production. To earn chronometer certification, a movement must not only be made from the highest quality components but also be the object of special care on part of the finest watchmakers and timers during assembly. Of the Swiss-made mechanical watches, approximately 5% are chronometer-certified by the COSC.