Water Resistance: Testing and Maintenance

A water resistance rating is the most misunderstood number on a watch. It is not a depth you can safely dive to, it is a laboratory pressure figure, and it degrades from the moment the watch leaves the factory. Understanding what it means is mostly about understanding what it is not.

What the rating means

The rating is a static pressure test result, expressed as an equivalent depth. It is measured on a new watch with new gaskets, in still water, at a stable temperature, with no movement of the case or crown. None of those conditions apply when a watch is actually worn.

• 30 m / 3 bar — splash resistant. Hand washing and rain, not swimming.
• 50 m / 5 bar — showering and shallow swimming, cautiously.
• 100 m / 10 bar — swimming and snorkelling.
• 200 m / 20 bar and above — recreational diving, with the caveat below.

The caveat is that a dive rating assumes the watch is pressure-tested before use, that the crown is screwed home, and that the gaskets are current. A 200 m rated watch with five-year-old seals has no meaningful rating at all.

Why seals degrade

Elastomer gaskets fail by taking a compression set — they flatten permanently under load and lose the ability to spring back. They also harden with age and temperature, and they crack. The process is accelerated by heat, sunlight, chlorine, salt and cosmetics.

This is why the rule is unconditional: replace the gaskets every time the case is opened. A seal that has been disturbed once will not reseat identically, and a gasket that has been compressed for years has already done its job.

How testing works

Dry pressure testing

The case is pressurised with air and the instrument watches for a pressure drop over a fixed interval. A sealed case holds pressure; a leaking one does not. This is the safe method, because a leaking watch does not get wet during the test.

Dry vacuum testing

A partial vacuum is drawn and the instrument measures the tiny deformation of the case — usually the crystal — as pressure equalises. It is sensitive, fast, and avoids both water and high pressure. It generally cannot test a watch with an open case back or a vented design.

Wet pressure testing

The watch sits in a chamber above water, the chamber is pressurised, and the watch is then lowered into the water. Bubbles reveal a leak. It finds the leak location, which is its advantage; its disadvantage is that a leaking watch gets wet during diagnosis.

The procedure

Maintenance between services

• Rinse a watch in fresh water after salt water or chlorinated water, then dry it
• Do not operate the crown or pushers while the watch is wet
• Avoid saunas, hot tubs and steam rooms — heat accelerates seal failure
• Have the seals replaced and the case tested at every service, and more often if the watch is used in water

A pressure test tells you whether a watch seals today. It says nothing about tomorrow, and nothing about the watch after it has been opened.