Horological Meandering

Marcus Hanke
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Absolutely no problem for any mechanical movement ....

... that is in a working condition. You should keep in mind that a watch that is worn on the wrist does not change its internal temperature according to the environmental temperature, but is mainly tempered by the human body temperature.

Swiss watch expert Thomas Ernst made a test in 2002: He equipped an empty watch case with three temperature sensors: one on the outside of the crystal, one in the cases's interior under the dial, and one under the caseback.

Before entering the Finnish sauna, the caseback had 36 degrees (all values in Celsius), the case interior 32 degrees, and the crystal was at 24 degrees.

After 20 minutes in the sauna at 95 degrees ambient air temperature, the caseback was at 38 degrees, the interior at 40 degrees, and the crystal at 46 degrees.

Within two minutes after the jump into the cold water, the caseback was at 32 degrees, the interior at 30 degrees, and the crystal at 16 degrees.

The experiment clearly shows that the movement itself is not subjected to such extreme temperature changes as long as the watch is worn on the wrist. A variation of 10 to 15 degrees in the watch case should not be a problem for the lubrifiants, and certainly is not for the escapement. The only real danger from massive and instantaneous changes of the ambient temperature emerges for the gaskets, since the aforementioned test demonstrated the different grades of temperature changes between the watch's back and top. Add to this the different expansion coefficients of the materials employed (sapphire, steel, precious metals, ceramics), it shows that the gaskets have a really hard time to compensate this. Consequently, a watch worn in the sauna should be painstakingly tested for water tightness every year.

Regarding the lack of service requirement: Rolex watches are well known to keep their excellent timing performance more or less independent from the movement's physical condition - until it suddenly "dies". Service watchmakers can tell many stories of movement components damaged, or even completely ruined due to absence of maintenance. Other movements will demonstrate the need for service long before that, by collapsing accuracy or power reserve values, for example.

Marcus