Thirty metres does not mean thirty metres

The number stamped on a caseback is the result of a static pressure test in a laboratory, conducted on a new watch, with the crown pushed in, at a constant temperature. Every one of those conditions differs from a swimming pool, and the direction of difference is always unfavourable.

Why the number overstates the case

Movement through water generates dynamic pressure far above the static pressure at that depth. A hand swung through water in a swimming stroke can briefly experience several times the pressure of standing still at the same depth. Diving into water from the side of a pool produces a pressure spike far beyond anything the rating anticipated.

Temperature is the second problem. Gaskets are elastomers, they expand and contract, and the transition from a hot shower to cold water or from a sauna to a pool is exactly the cycle that lets water past a seal that was sound a minute earlier.

The honest translation

• 30 metres, 3 bar: rain, hand washing, a splash. Take it off to shower.
• 50 metres, 5 bar: a shower if the crown is properly seated and the gaskets are fresh. Not swimming, whatever the salesman said.
• 100 metres, 10 bar: swimming, snorkelling, watersports. This is the first rating that is genuinely useful in water.
• 200 metres, 20 bar: scuba diving, and usually with a screw down crown and a rotating bezel.
• 300 metres and above: professional diving, saturation work, and a great deal of specification that most owners will never test.

If you swim, buy 100 metres. If you dive, buy 200. Everything between 30 and 50 is a watch that stays on the side of the pool.

Gaskets, which are the actual limit

Water resistance depends on a handful of rubber rings: at the crown, at the caseback, at the crystal and at any pusher. They compress, dry, harden and take a permanent set, and they do it on a calendar rather than on use. A watch that has sat in a drawer for eight years has old gaskets even though it has done nothing.

Which is why water resistance is a maintenance item and not a property. Pressure testing costs very little at any watchmaker, takes minutes, and is the only way to know. Any watch that is going near water regularly wants testing annually and gaskets replaced at every service.

The crown, which is the usual culprit

A screw down crown must be fully screwed home, and the thread must be clean. Sand and salt in a crown thread will hold it a fraction open, which is enough. A push crown offers a single gasket and no mechanical lock, which is why 100 metre watches with push crowns should be treated with more caution than the number suggests.

Never operate a crown or a pusher underwater. On a chronograph without specifically rated pushers this is the fastest way to flood a case, and it is written in almost every manual and read by almost nobody.

If water does get in

Condensation under the crystal means water is already inside and the movement is oxidising as you look at it. This is an emergency measured in hours, not days. Get it to a watchmaker immediately. If that is not possible within a day, the least bad improvisation is to keep it warm and dial down so the moisture migrates away from the movement, and to accept that a full service is now required regardless.

Steel corrodes, steel dust contaminates lubricants, and a movement that has been wet and left is frequently beyond economic repair. The watch that survives is the one taken in the same afternoon.

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