Controlling Buoyancy underwater

To dive safely, divers need to be able to control their rate of descent and ascent in the water. Ignoring other forces such as water currents and swimming, diver's overall buoyancy determines whether a diver ascends or descends. Equipment such as the diving weighting systems, diving suits (Wet, Dry & Semi-dry suits are used depending on the water temperature) and buoyancy compensators can be used to adjust the overall buoyancy. When divers want to remain at constant depth, they try to achieve neutral buoyancy. This minimises gas consumption caused by swimming to maintain depth.

The volumes and weights of the diver and all equipment attached to the diver, contribute to the diver's overall buoyancy. Volume creates an upward force and weight creates a downward force. If the force due to volume is greater than the weight, the diver ascends. If the force due to volume is less than the weight the diver descends. Diving weighting systems can be used to reduce the diver's weight and cause an ascent in an emergency. Diving suits, mostly being made of compressible materials, reduce in volume as the diver descends and expand as the diver ascends creating unwanted buoyancy changes. The diver can inject air into some diving suits to counteract this effect and squeeze. Buoyancy compensators allow easy and fine adjustments in the diver's overall volume and therefore buoyancy. For open circuit divers, changes in the diver's lung volume can be used to adjust buoyancy.

Need to see underwater

Water has a higher refractive index than air. Light entering the eye from the water behaves differently than light entering from air. This creates a distortion that affects normal vision. Diving masks and diving helmets solve this problem by creating an air partition between the diver's eyes and the water. The distortion created by the water is effectively reversed as the light travels from water to air.

Divers who require corrective lenses to see clearly outside the water would normally require the same prescription while wearing a mask. Some masks can be ground to the diver's prescription to avoid the need for additional corrective lenses.

Occasionally commando frogmen use special contact lenses instead, to see underwater without the large glass surface of a diving mask which can reflect light and give away the frogman's position.

information: Wikipedia