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Basically this is how it works: numbers on the bezel are equal to depht and the bezel tells you how long you can stay below water before making mandatory decompression stages.
Example: when your start your dive, put the triangle in front of the minute hand. If you dive at 20m, then the minute hand takes around 50min to join the number 20 on the bezel.
If you dive at 40m, then you have around 15mn of immersion, etc...