There is no hard and fast rule which says when a diver should return to the surface in regards to the divers air supply.
Historically a diver begins his ascent when his air supply displays 50bar. In warm tropical shallow waters divers often ascend when their air supply drops to 30bar. On deeper dives divers often begin their ascent when their air supply reaches 70bar.
Whatever the dive divers must always have enough air remaining to perform a safety stop at five metres as well as have enough air in case of an emergency.
Each commercial dive charter operation has its own guidelines. These guidelines will depend on the diving conditions in their local area. Divers should always follow the advice of the Dive Supervisor on board the dive vessel.
Before entering the water divers are instructed on when to begin their ascent. This is covered in the dive brief.
Be aware that most dive charter operators frown upon divers who return to the surface with their tank completely empty.
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