Found in diving watches, the unidirectional rotating bezel is a unique feature that helps measure diving time in a handy and effective manner. The bezel can only rotate in a counter-clockwise direction, which ensures diver’s safety even in the case of any accidental manipulation underwater.

The first watch to feature a rotating bezel was the Blancpain Fifty Fathoms, which is considered by some to be the first modern dive watch. There is an interesting story behind the creation of the Fifty Fathoms. In the early 1950’s, the commanders of the French Combat Diving School were looking for a specialized watch for their elite dive soldiers. The watch was an essential piece of equipment needed to measure time spent underwater and coordinate with fellow divers. Among all the watches available in the market at that time, they couldn’t find any timepiece that met their particular requirements. Hence they made a list of requirements for the perfect diver’s watch and asked various watchmakers if they could build such a timepiece. As the market for such a timepiece was too niche at the time, not many showed an interest in the proposition. Fortunately for the commanders, the Blancpain CEO at the time was a diver himself and agreed to embark on the quest for the ideal diver’s watch. Thus was born the Fifty Fathoms.

One of the most distinguishing features of the watch was the 41 mm case, which was quite large for the 1950’s. The reason for this huge size was the presence of the rotating bezel which had large markings that could be clearly read underwater. It was a new development in watches and has since become a standard feature that is found on almost every diving watch manufactured today.

The rotating bezel on a diver’s watch is usually marked with Arabic numeral minute markers and a triangle at the 12 o’clock position. The bezel allows one to measure time up to 60 minutes. To time a dive, one has to simply rotate the bezel and align the triangle marker with the minute hand at the beginning of a dive. Time spent underwater can thus be read from the markers on the bezel.

Alternatively, one can measure remaining time in the dive. Scuba divers have a limited amount of oxygen supply in their tanks. It could be 30 minutes, 45 minutes or maybe even an hour. To measure the remaining time, one has to subtract total dive time from 60. For example, if you have 45 minutes of oxygen supply in your tank, then you must rotate the bezel and bring it to the point where the minute hand points at 15 (60-45) on the bezel. Now, the triangle marker on the bezel is the time when your oxygen supply will get over. In this way, you can clearly know when to start making your way back to the surface.

Unidirectional bezels rotate only in the counter-clockwise direction. The reason for this is to ensure that even in the case of any accidental manipulation underwater, you will err only on the side of caution. For example, a slight movement of the bezel may make you overestimate your dive time and cause you to come out earlier than expected. This is much better than underestimating the time and staying longer than the amount of oxygen supply you have. Unidirectional bezels are usually ratcheted so that the bezel gets locked in position unless a certain amount of force is applied by hand. In this way, unidirectional bezels ensure the diver’s safety during scuba diving.

Below are some popular diver’s watches that feature unidirectional bezels: