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How precise should my mechanical watch be?

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A mechanical watch, whether it is automatic or manual wind, consists of hundreds of microscopic screws, cylinders and gears. It is often assumed that such a watch, especially if priced on the higher end, should run to the second. However, if you miss the train every morning by a minute, or repeatedly appear too early to the weekly spinning class, then you should probably check the accuracy of your watch (we’ll cover which tools you can use in another blog post). It should be noted here, that a deviation of 1 second per day equals an accuracy rate of 99.99%! In fact, a mechanical watch will never run as accurately as a battery-powered quartz watch or smartwatch. In addition to this, a mechanical watch should not exceed a certain deviation. So, which accuracy can be expected with mechanical watches, and what should be considered when buying a new one?

What influences the accuracy of a mechanical watch?

The accuracy of a watch is measured in seconds per day. Important: the two words "Accuracy" and "Precision" should not be intertwined. It is often said colloquially that a watch runs precise, or accurate. However, these are two very different meanings. Two factors are captured when measuring a watch: accuracy and speed stability. Let’s compare this with a target (see image). If you always hit the same point in the target, but not the bullseye, your watch is very precise, but not accurate. In this case, your watch will be relatively easy to regulate. However, if you hit all over the place, your watch is inaccurate - it does not behave uniformly in different positions and temperatures and needs to be adjusted (which takes more time as each position needs to be monitored and adjusted).

Accuracy and Precision on a target

Image by https://sites.google.com/a/apaches.k12.in.us/mr-evans-science-website/accuracy-vs-precision

In addition to material and craftsmanship, two factors affect the accuracy and precision of a movement:

Position:

In fact, gravity is the primary cause of rate deviations. In every position, whether dial top, crown left or right, dial bottom, a watch movement runs at a slightly different rate. In practice, this means that depending on how the watch is worn, the rates ​​are different (e.g. a construction worker will see different rates ​​with the exact same watch as an office worker, who moves his wrist differently throughout the workday). To balance second losses and gains in different positions, the watchmaker adjusts the balance wheel. If you experience steady deviations, try to place your watch in a different position while you’re sleeping and not wearing your watch. You might just find the perfect position to balance out the deviations accrued throughout the day.

Temperature:

Extreme and rapid temperature variations can have a noticeable effect on the accuracy of a mechanical timepiece, e.g. an ice bath after a sauna. For modern mechanical watches, however, the ski or beach holiday should have little effect on rate. These are designed so that temperature influences are automatically compensated or completely eliminated, which is made possible by the temperature insensitivity of different metals and alloys. Especially when it is cold, it should be noted that as long as the watch is worn directly on the skin, the body's own temperature does not allow the inside of the watch to cool too low.

Let's take an example - a movement that is used in many watches is the Citizen-Miyota 8215, Made in Japan. The movement is specified by the manufacturer with an accuracy of minus 20 to plus 40 seconds per day. In its worst case, the watch deviates almost five minutes every week. Even a Swiss Made movement like the ETA 2824-2 - the workhorse of the Swiss watch industry - in the standard grade is regulated, between +/- 12 to +/- 30 seconds a day according to the ETA. The technically better equipped Elaboré grade is already regulated in three positions and runs with +/- 7 to +/- 20 seconds a day. Much more accurate and precise, but also significantly more expensive (about CHF 30’000.-), is the Zenith Defy Lab Movement. With a maximum deviation of 0.3 seconds per day, this watch is probably the most precise mechanical watch ever made. This high accuracy is achieved by a completely new escapement concept, with a very high frequency. While this is an engineering masterpiece and probably melts many watch enthusiast’s heart, we need to ask ourselves if mechanical precision can somehow be affordable too? That brings us to the "Chronometer".

What is a Chronometer?

Again, two terms should be distinguished; "Chronograph" and "Chronometer". The chronograph is a watch with a stopwatch function. The chronometer is a high-precision timepiece with an official chronometer certificate. Important: Officially, a watch can only be called a chronometer (mostly indicated on the dial) if it has been tested by an official testing laboratory in a standardized measuring procedure.

What is COSC?

For Swiss watch movements, this official testing laboratory is the COSC (Contrôle Officiel Suisse des Chronomètres). Only around 6% of all watches exported from Switzerland go through this certification process, including more than 800,000 chronometer movements used in the Rolex models from Datejust to Submariner. COSC, located in La Chaux-de-Fonds with three branches in Biel, Le Locle and Geneva, was established in 1973. There, a movement is measured in five positions – each during one day, at 8, 23 and 38 degrees Celsius. The entire testing process takes 15 days. The average daily rate must be between -4 and +6 seconds in every position and under every condition. For a whole day, the mean variation may not exceed two seconds (the detailed test requirements can be found here). The values ​​of the individual movement are recorded by the COSC in a certificate and provided with a number, which is also engraved on the movement.

"For our Essence Chronometer models, all movements were officially certified by COSC, under the same conditions as any Rolex movement or many other high end Swiss watch brands. After casing, we double-check the rates to rule out that any deviation has happened during final assembly. We are proud to produce such a high-value and one of the most competitively priced chronometers in Switzerland! "

Raphael Granito, CEO Formex Watch SA

Other chronometer testing centers

Another seal that tests movements and watches is the „Observatoire Chronométrique+” by Timelab. In addition to the rates ​​of the movement, the waterproofness, magnetic field protection and power reserve are tested. The Timelab testing program takes 21 days. Since September 2006, the German Wempe Chronometerwerke Glashütte i / SA has once again been the site of a German chronometer test center operated by the jewelry and watch retailer Wempe. It was certified by the Physikalisch-Technische Bundesanstalt (PTB) as a calibration laboratory and field office for chronometer tests of the Deutscher Kalibrierdienst (DKD) in cooperation with two national offices for metrology and calibration and is the only institution in Germany allowed to certify a chronometer. All test centers are based on the requirements of the international standard ISO 3159. More than 150 countries worldwide have set common standards with these standardization organizations. Omega, with its „Master Chronometer“ took it one step further. The watch brand of the Swatch Group has developed its own standard, together with the Swiss Federal Institute of Metrology (METAS). Eight different tests are performed on the functionality of the movement and the watch under the influence of magnetic fields up to 15,000 Gauss. The accuracy may vary per 24 hours by a maximum of five seconds. The METAS tests - beyond the COSC certification - in six positions as well as the power reserve and the waterproofness of the watch.

How does a chronometer run in everyday life?

What does it mean to use a chronometer in everyday life? The chronometer test is only a snapshot of the performance in exactly the conditions under which the watch was placed (position and temperature). Magnetization, drying lubricants or a fall can change the rates. Therefore, a chronometer also requires regular service of the movement in order to restore the conditions that once existed during the test. In fact, however, a chronometer movement often features better components than the lower quality levels called Standard, Elaboré or Top, plus they come with an officially documented rate test. The closer to the chronometer grade, the better the movement's components.

Of course, the certification also costs money: the higher quality level and the test program are not free and must be covered by the watch manufacturer. In addition, the movement manufacturer in some cases does not guarantee that all movements, labelled with the grade “Chronometer”, will in fact pass the COSC certification. Often the movement manufacturer is only liable if less than 90% of its tested movements are certified, which means that up to 10% of the more expensive chronometer grade movements cannot be sold by the manufacturer of the watch as an officially certified chronometer or have to be re-regulated and re-certified. Therefore, chronometers are usually quite expensive watches. Only a few manufacturers manage to offer certified movements in watches under $USD 2,500. 

Editors:

Editors Formex Blog