what is a 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.