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What does 30m water resistance actually mean?
Almost nothing that the number suggests. Pick the rating printed on your watch and see what it genuinely allows.
Rating printed on your dial or caseback
Relative, not literal depth
Rating
30 m / 3 ATM
Why the number lies
A 30 m rating does not mean you can take the watch 30 metres underwater. It means the case withstood a static pressure equivalent to 30 metres of water, in a tank, at a fixed temperature, for a short period, while brand new. Every part of that sentence matters.
Real water is not static. Swinging your arm through a swimming stroke briefly generates pressure several times the static figure at the same depth. Diving into a pool does the same in an instant. That is why watches rated for depths far greater than any swimming pool still flood in one.
Temperature is the other half of the problem. Metal and rubber expand at different rates, so a hot shower or a jump from a hot deck into cold water can open a gap at a seal that stays perfectly tight at a constant temperature.
The rating is about the watch when it was new
Water resistance depends on rubber gaskets at the caseback, the crown and the crystal. Rubber hardens, shrinks and cracks with age whether the watch is worn or sitting in a drawer. Soap, shampoo, sunscreen and chlorine all accelerate it.
This means a twenty year old 200 m dive watch that has never been serviced is not a 200 m watch. It is a watch that was 200 m in 2006. If you plan to swim with anything vintage, have the resistance pressure tested first. It is a cheap test and most watchmakers offer it.
The single most common way people flood a watch: leaving the crown unscrewed. On a screw-down crown, water resistance is close to zero until the crown is fully threaded home. Check it every time before the watch gets wet.
Common questions
Can I shower with my watch?
Better not to, at any rating. Hot water and soap are hard on gaskets, and steam passes seals that stop liquid water. A dive watch survives it, but you are wearing out the seals for no reason.
Is a "water resistant" watch with no number safe?
Treat it as splash-proof only. If a manufacturer had a figure worth printing, they would print it.
What about saltwater?
The rating is unchanged, but salt is corrosive and crystallises in the gaps around the crown and bezel. Rinse the watch in fresh water after sea swimming, every time.
Do smartwatches follow the same rules?
Mostly, though many are rated to a different standard (ISO 22810 rather than a dive standard) and some list swim-proof ratings explicitly. Speakers and microphone ports are the weak point rather than the case seals.
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