A unidirectional bezel is a turning ring around the watch face that can move only one way. On dive watches it turns only anticlockwise. A knock can therefore only make the elapsed time look longer, never shorter. That keeps a diver from staying underwater longer than planned.
How does a unidirectional bezel work?
The one-way action comes from a ratchet. It lets the bezel turn in one direction and blocks it from turning back. The ratchet is usually built into the bezel itself, from springs, clicks or teeth. Some dive watches go a step further than a one-way ratchet. Their bezel can be locked in place, so it is less likely to be moved by accident underwater.
How do you time a dive with it?
On entering the water, the diver lines up the zero on the bezel with the minute hand. Elapsed time is then read off the bezel. The one-way design guards against a real danger. If the bezel could turn clockwise, a stray knock might shift it so that less time seemed to have passed than really had. Out of the water, the same bezel works as a simple timer. People use it for parking, cooking, meetings or workouts.
Where it comes from
Today the feature appears on watches from Omega, Breitling and TAG Heuer.
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