What is the rotating ring on wrist watches?

The purpose of rotating watch bezels,

which are often associated with Scuba

diving, is to keep track of elapsed time,

or to make other measurements, such as

average speed or distance traveled.

Rotating bezels take advantage of a

watch's primary function--timekeeping--

to allow the wearer to keep track of

something other than just the hours,

minutes and seconds of the day.

Definition

A bezel is the ring that surrounds the

dial, or face, of a watch. Commonly

constructed of gold, gold plate or

stainless steel, the bezel may be adorned

with diamonds on a jewelry watch. On

sports watches, it often has calibrated

markings and the ability to rotate in

either one or two directions. Although

rotating watch bezels perform

timekeeping functions, the primary

purpose of the bezel is to hold the crystal

covering the face of the watch in place.

Unidirectional Bezel

As the name indicates, a unidirectional

bezel is one that rotates in only one

direction--counterclockwise. This type of

rotating bezel is often used for scuba

diving. Prior to submerging, a scuba

diver with 45 minutes of air in his tank

could turn the bezel so either the zero

mark or the 45 mark is showing,

depending if it is a count-up or

countdown gauge. That lets the diver

know how much air is left with a quick

glance at the watch, rather than

subtracting the starting time from the

current time to determine how many

minutes have elapsed or how many

minutes of air are left. The bezel on a

diver's watch is often ratcheted to

prevent it from accidentally being

knocked out of its original position,

which could be disastrous for the

wearer.

Bidirectional Bezel

This type of bezel moves both clockwise

and counterclockwise, enabling it to be

used either for mathematical calculations

or for measuring how much time has

elapsed or how much time is remaining.

Tachymeter

Watches with a rotating bezel that serves

as a tachymeter help the wearer

calculate their speed of movement based

on travel time. For example, the wearer

starts the chronograph, or stopwatch

function, at a mile indicator and at the

next mile indicator the point on the scale

where the second hand lies shows his or

her miles per hour speed. It can be used

with any unit of distance, as its purpose

is simply to measure units traveled by

hour.

Slide Rule

A slide rule bezel features logarithmic or

other scales on the watch face's outer

edge, enabling the wearers to perform

mathematical calculations. The rotating

bezel is marked with one scale and is slid

around the stationary scale to make

calculations of various types, which

depend on the watch. Some models are

designed for calculating how much fuel

an airplane has used or the fuel weight.