Water resistance is how well a watch is sealed against water, shown by a Water Resistant mark on the caseback. The mark usually carries a figure for the static pressure used in the leak test. It matters because the rating alone does not make a watch fit for diving or long spells in a pool.
Can a water-resistant watch be called waterproof?
No. ISO 2281, the standard for water-resistant watches, bans the word waterproof. The figure on the caseback is a test pressure. It does not promise that water can never get in. The rating is also checked only once, at the factory, when the watch is new. After that, the mark on the back tells you nothing about how well the seals are holding up.
Not to be confused with
Diver’s watches. They have their own standard, ISO 6425, which covers watches built for diving to at least 100 metres. A watch that meets only ISO 2281 is not meant for underwater diving, even if it carries an extra overpressure figure. Diving watches are tested in still water at a quarter more than their rated pressure. This gives them a safety margin.
How is a watch sealed against water?
The places where water could get in are closed off. The original Rolex Oyster of 1926 did this by screwing down the caseback, the bezel and the crown. Water-resistant watches often have a screw-down crown, or a crown sealed with a gasket. Gaskets and seals wear out over time. As they age, a watch can lose the protection its rating promises, even though the mark on the back stays the same.
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