This article will concentrate on accuracy of mechanical movements, although the presumption will be that precision is also an embedded feature. What makes a mechanical movement accurate is not a linear equation or straightforward assumption. The very word ‘accuracy’ will have different meanings for different people.
Determining watch accuracy is tricky. However, a good rule of thumb regarding accuracy is that provided by the Official Swiss Chronometer Testing Institute, or Contrôle Officiel Suisse des Chronomètres (COSC). After all, the word ‘chronometer’ refers to accuracy in timekeeping. In order to achieve a chronometer certification from the COSC, a mechanical watch must be in the ‘+6/-4’ range, meaning it should not gain more than six seconds or lose more than 4 seconds per day. The COSC also bases its certification on precision, which is derived from the average daily rate on the first 10 days of testing.
Factors That Determine Mechanical Movement Accuracy
Whilst accuracy in mechanical movements is not always an exact science, there are some determining factors, which can include:
Manufacturer grades: The specs published by movement makers such as Sellita and ETA can offer guidance as to a movement’s accuracy. These two-biggest Swiss movement makers both have ‘grades’ for their movements which are entirely based on accuracy. For example, Sellita has four grades:
- Chronometer (which must of course meet the ‘+6/-4 seconds per day’ COSC certification standard);
- Special (elaboré); and
User habits: the way a mechanical watch is worn will also determine its accuracy. This is the opinion of expert watchmaker, Alkis Kotsopoulos, of New York’s Swiss Watch Repair Co., who states he will ask a client about how they intend to wear the watch, how often, what level of physical activity they will engage in while wearing the watch, how the client will store the watch when not work, and so forth. Those factors will greatly influence accuracy, according to Kotospoulos.
The balance spring: the balance spring as we know it today dates back to 1795, and regulates the oscillations of the balance due to its elasticity. This important mechanism within the movement is also made to different grades with an emphasis on precision, which in turn determine the accuracy of a movement.
Let us turn to five of the most accurate movements ever.