Let’s begin with the balance wheel, the movement component that regulates the entire gear train and greatly impacts its level of precision. In short, the more regular its rate of oscillation, the more precise the watch. The maximum angle by which the balance wheel swings per alternation is expressed in degrees. This is known as the amplitude. An undamaged watch typically has an amplitude ranging from 260 to 290 degrees. Ideally, the balance wheel should take the same amount of time to swing in both directions. However, in reality, one direction often takes a millisecond longer. This difference, referred to as beat error, should never exceed 0.3 milliseconds.
Similarly, in an ideal world, the mainspring would always provide a constant and even amount of energy to the oscillation and escapement systems, regardless of whether the watch was fully wound or nearing the end of its power reserve. Moreover, as little energy as possible should be lost in the process. Well-lubricated bearings and perfectly-fitted components are, therefore, essential to efficient energy transfer.
Risks to Precision
If all of the internal components mentioned above met the ideal requirements, the watch should boast a high level of precision. However, there are external risk factors that can greatly influence how a watch functions. The impact of magnetic fields, for example, is often underestimated. Nowadays, we are more or less surrounded by magnets; they are in smartphones and tablets, refrigerators, induction stoves, speakers, and even electric toothbrushes! If a watch is exposed to a magnetic field for too long, the balance spring can become magnetized, which can have a negative effect on the function of the balance wheel. The result? More deviation.
Likewise, jolts and extreme temperatures can affect how a watch works. Very high or low temperatures can cause the expansion or contraction of certain movement components as well as changes to the properties of lubricants.
Even gravity itself has varying impacts on the accuracy of watches. When you lay your timepiece on a flat surface, for example, the force of gravity has a different influence on the movement than when the crown is pointing downward. As long as you wear your watch, these differences tend to even out across the day. If your timepiece is left unworn for long periods of time, however, be sure to move it regularly and adjust its position. Alternatively, you could store your timepiece in a watch winder, which does the work for you and has the added benefit of winding automatic timepieces.
How is accuracy measured?
There are several different ways to discover your watch’s level of precision. Professionals tend to analyze watches using a so-called timing machine or “timegrapher.” This precision instrument “listens” to your watch using a microphone and measures rate, amplitude, and beat error. The watchmaker then uses the data logged in the machine to regulate your watch.
A professional-grade timegrapher doesn’t come cheap; top models can cost thousands of dollars. That being said, simplified versions with more limited functionality can be purchased for a few hundred dollars. An even cheaper alternative is an app-based system that turns your smartphone into a timegrapher. Of course, if you’re just looking for a gross estimate of your watch’s accuracy, you could always compare the time displayed with a reference time. Online atomic clocks make a trustworthy reference point.
How are watches regulated?
If you’ve determined your watch’s deviation is unacceptable, your best bet would be to visit a professional watchmaker. They will likely start by demagnetizing the balance spring if necessary and then adjusting the beat using the regulator.
Sometimes vintage watches need a bit more attention. The watchmaker typically checks that all the movement components are still in good working order and determines whether any pivots, shafts, or bearings warrant exchange. The watch is then cleaned and old, gummy lubricant is replaced.
Which watches are the most accurate?
If you’d like to call an extra precise watch your own, look for a model that is chronometer-certified. To earn this title, a watch has to pass a series of rigorous tests. Over 15 days, watches are tested at three different temperatures (46°F, 73°F, 100°F/8°C, 23°C, 38°C) and in five different positions (dial up, dial down, crown facing left, crown facing up, crown facing down). To pass, the average deviation must not exceed +6/-4 seconds per day.
The Official Swiss Chronometer Testing Institute (Contrôle Officiel Suisse des Chronomètres, COSC) has been certifying Swiss-made chronometers since 1973. In turn, German watches can be tested by a similar organization based at the Glashütte Observatory in eastern Germany. The site was opened by Wempe KG and the State Offices for Weights and Measures of Saxony and Thuringia in 2006. Watches are put through tests that are largely identical to those of the COSC. However, in Glashütte, the whole watch is tested as opposed to just the movement.
Normal chronometer standards aren’t sufficient for manufacturers like Rolex and Omega. In addition to passing the COSC-certification process, they have developed their own standards with stricter precision limits as well as levels of anti-magnetism and water resistance. Rolex’s “Superlative Chronometers” are guaranteed to deviate by no more than an average of +/-2 seconds per day. Omega’s “Master Chronometers” earn their stamp of approval from Switzerland’s Federal Institute of Metrology (METAS). They are made to withstand magnetic fields of up to 15,000 Gauss and deviate by an average of no more than 0/+5 seconds per day.
