Diving Physics explains the effects that divers and their equipment are subject to underwater.
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The main laws of physics that govern the physics of the SCUBA diver and of diving equipment are:
Buoyancy control, and being able to maintain neutral buoyancy in particular, is an important safety skill. The diver needs to understand buoyancy to be able to effectively and safely operate drysuits, buoyancy compensators, diving weighting systems and lifting bags.
The volume of air in a non-rigid container (such as a diver's lungs or buoyancy compensation device), decreases as external pressure increases while the diver descends in the water. Likewise, the volume of air in such non-rigid containers increases on the ascent. Changes in the volume of gases in the diver and the diver's equipment affect buoyancy. This creates a positive feedback loop on both ascent and descent.
The quantity of open circuit gas breathed by a diver increases with pressure and depth.
This is why a diver who enters cold water with a warm diving cylinder, for instance after a recent quick fill, finds the gas pressure of the cylinder drops by an unexpectedly large amount.
Partial pressure is a useful measure for expressing limits for avoiding nitrogen narcosis and oxygen toxicity.
This helps explain nitrogen narcosis, oxygen toxicity and decompression sickness.
This is the reason a diver cannot see clearly underwater without a diving mask with an internal airspace.
The physical effects of water or the underwater environment are:
Divers use high density materials such as lead for diving weighting systems and low density materials such as air in buoyancy compensators and lifting bags.
As water conducts heat 20 times more than air, divers in cold water must insulate their bodies with diving suits to avoid hypothermia. Gases used in diving have very different thermal conductivity; trimix conducts heat more than air and argon conducts less heat than air hence the reason many trimix divers inflate their drysuits with argon.[9][10]
The red end of the spectrum of light is absorbed even in shallow water.[11] Divers use artificial light underwater to reveal these absorbed colours. In deeper water no light from the surface penetrates.
Air spaces in the diver's body and gas held in flexible equipment shrink as the diver descends and expand as the diver ascends.[13]
The physical phenomena found in large bodies of water are:
Even moderately high winds prevent diving because of the increased risk of becoming lost at sea or injured. Low water temperatures force divers to wear diving suits and can cause problems in the use of high pressure equipment such as freezing of diving regulators.
For instance where fresh water enters a warmer sea, the fresh water floats over the denser saline water. Sometimes visual effects, such as shimmering and reflection, occur at the boundary between the layers.
Some ocean currents have a huge effect on local climate, for instance the warm water of the North Atlantic drift moderates the climate of the north west coast of Europe.
Where the air temperature is higher than the water temperature, shallow water may be warmed by the air and the sunlight but deeper water remains cold resulting in a lowering of temperature as the diver descends.
Where cold, fresh water enters a warmer sea the fresh water floats over the denser saline water, so the temperature rises as the diver descends.
In lakes exposed to geothermal activity, the temperature of the deeper water may be warmer than the surface water.
Some dive sites can only be dived safely at slack water when the tidal cycle reverses and the current slows. Strong currents can cause problems for divers. Buoyancy control can be difficult when a strong current meets a vertical surface. Divers consume more breathing gas when swimming against currents. Divers on the surface can be separated from their boat cover by currents.
On the other hand, drift diving is only possible when there is a reasonable current.