Complications  /  Water resistance

Water resistance

Also known as — Waterproofing, sealed case, dive rating

The sealing of a case against water and dust through gaskets, threaded closures and a screw-down crown, rated by static pressure test.

How it works

A watch is a set of steel pivots turning in jewelled bearings, lubricated with oil. Water corrodes the steel, washes away the oil and leaves deposits that bind the train; condensation under the crystal will rust a hairspring within days. The pocket watch was largely protected by the pocket. On the wrist the watch met rain, washing, perspiration and immersion, and the wristwatch of the 1910s was notoriously short-lived for exactly that reason.

bezel and crystalscrewed case backgasketssqueezed betweentwo rigid facesmovementtubecrownscrews downonto the tube; a clutchlets it turn free when openoutside pressure squeezes the seal tighter
Every seal is a ring of rubber squeezed between two rigid faces, and pressure from outside squeezes it harder. The crown was the last opening to be solved, and Rolex’s clutch is what made a screwed-down crown usable.

The case had three unsealed openings: the joint at the back, the joint at the crystal, and the hole the winding stem passed through, which had to stay free enough to turn. Screwed cases such as Francois Borgel’s of 1891 dealt with the first two. The stem defeated everyone until the crown itself was made into a closure: a screw-down crown, with a clutch so that it can be turned without winding when closed, screwing onto a threaded tube. Combined with a threaded bezel and case back compressing gaskets, that produced the Oyster case of 1926.

Gaskets are consumables. They take a set, harden with age and are attacked by soaps and solvents, so a depth rating is only as good as the last service, and a crown left unscrewed makes it meaningless. The figures on the dial are also commonly misread: a rating in metres is the result of a static pressure test, not a statement about depth in use. ISO 6425 sets what a watch must actually do before it may be called a diver’s watch.

Recorded here

Milestones with this mechanism

1931The first free winding rotorRolex let the winding weight turn a full circle freely, the self-winding system nearly every automatic watch still uses.Rolex1945The first automatic chronometer with dateRolex’s Datejust of 1945 was the first self-winding, waterproof, chronometer-certified wristwatch with a date in a window.Rolex1953The first modern diver’s watchBlancpain’s Fifty Fathoms put together the features that define a diver’s watch, above all a rotating bezel that cannot be knocked out of place.Blancpain1956The first day-date wristwatchThe Rolex Day-Date of 1956 was the first wristwatch to show the day spelled out in full and the date, each in its own window.Rolex1926The first waterproof wristwatchRolex closed the last opening in the watch case with a crown that screws down onto the case, and called the result the Oyster.Rolex1932The first diving watchOmega’s Marine of 1932 was the first wristwatch sold specifically for diving.Omega