Diving physics

Diving Physics explains the effects that divers and their equipment are subject to underwater.

Laws of physics for diving

The main laws of physics that govern the physics of the SCUBA diver and of diving equipment are:

• Archimedes' Principle of Buoyancy - when in water, the density of the materials in the diver's body and in the diver's equipment determine whether the diver floats or sinks.^[1][2]

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.

• Boyle's law - as pressure changes, the volume of gases in the diver's body and soft equipment changes too.^[1]

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.

• Gay-Lussac's second law – as temperature increases the pressure in a diving cylinder increases (originally described by Guillaume Amontons).^[3]

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.

• Dalton's law - in mixtures of breathing gases the concentration of the individual components of the gas mix is measured by partial pressure^[1]

Partial pressure is a useful measure for expressing limits for avoiding nitrogen narcosis and oxygen toxicity.

• Henry's law - as pressure increases the quantity of gas adsorbed by the tissues of the human body increases.^[4]

This helps explain nitrogen narcosis, oxygen toxicity and decompression sickness.

• Snell's law - the index of refraction of water is similar to that of the cornea of the eye—30% greater than air.^[5]

This is the reason a diver cannot see clearly underwater without a diving mask with an internal airspace.

Physical effects of water for divers

The physical effects of water or the underwater environment are:

• Pressure - the overall pressure on a diver is the sum of the atmospheric pressure and the water pressure.^[6]
• Density - of the water, the diver's body and equipment determines the diver's buoyancy and the use of buoyant equipment.^[7]

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.

• Thermal conductivity of water is higher than that of air.^[8]

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]

• Absorption of light and loss of colour underwater.^[11][12]

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.

• Under pressure, gases are compressible but liquids are not.

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]

Physical phenomena of interest to divers

The physical phenomena found in large bodies of water are:

• Effects of weather such as wind, which causes waves, and changes of temperature and atmospheric pressure on and in the water.

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.

• Haloclines, or strong, vertical salinity gradients.

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.

• Ocean currents transport water of different temperature and salinity over thousands of kilometres.

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.

• Thermoclines, or sudden changes in temperature.

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.

• Tidal currents and changes in sea level caused by gravitational forces and the earth's rotation.

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.

References

1. ^ ^{a b c} Acott, C (1999). "The diving "Law-ers": A brief resume of their lives.". South Pacific Underwater Medicine Society Journal 29. ISSN 0813-1988. OCLC 16986801. http://archive.rubicon-foundation.org/5990. Retrieved 2008-07-07. 
2. ^ Larry "Harris" Taylor, Ph.D.. "Practical Buoyancy Control". University of Michigan. http://www-personal.umich.edu/~lpt/practical.htm. Retrieved 2008-10-10. 
3. ^ "Amonton's Law". Purdue University. http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch4/gaslaws3.html#amonton. Retrieved 2008-07-08. 
4. ^ "Henry's Law". Online Medical Dictionary. http://cancerweb.ncl.ac.uk/cgi-bin/omd?Henry%27s+law. Retrieved 2008-10-10. 
5. ^ "Snell's Law". scienceworld.wolfram. http://scienceworld.wolfram.com/physics/SnellsLaw.html. Retrieved 2008-10-10. 
6. ^ "Pressure". Oracle ThinkQuest. http://library.thinkquest.org/28170/32.html. Retrieved 2008-10-10. 
7. ^ "Density and the Diver". Diving with Deep-Six. http://www.deep-six.com/page69.htm. Retrieved 2008-10-10. 
8. ^ "Thermal Conductivity of some common Materials". The Engineering Toolbax. http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html. Retrieved 2008-10-10. 
9. ^ Nuckols ML, Giblo J, Wood-Putnam JL. (September 15–18, 2008). "Thermal Characteristics of Diving Garments When Using Argon as a Suit Inflation Gas.". Proceedings of the Oceans 08 MTS/IEEE Quebec, Canada Meeting (MTS/IEEE). http://archive.rubicon-foundation.org/7962. Retrieved 2009-03-02. 
10. ^ Eric Maiken. "Why Argon". Author. http://www.decompression.org/maiken/Why_Argon.htm. Retrieved 2011-04-11. 
11. ^ ^{a b} Luria SM, Kinney JA (March 1970). "Underwater vision". Science 167 (3924): 1454–61. doi:10.1126/science.167.3924.1454. PMID 5415277. http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=5415277. Retrieved 2008-07-06. 
12. ^ Reproduced from J. Chem. Edu., 1993, 70(8), 612. "Why is Water Blue". Dartmoth College. http://www.dartmouth.edu/~etrnsfer/water.htm. Retrieved 2008-10-10. 
13. ^ "Compressibility and Ideal Gas Approximations". UNC-Chapel Hill. http://www.shodor.org/unchem/advanced/gas/compress.html. Retrieved 2008-10-10.