Diving physics
Encyclopedia
Diving Physics explains the effects that divers and their equipment are subject to underwater.
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 system
s and lifting bag
s.
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.
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 suit
s 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.
The red end of the spectrum of light is absorbed even in shallow water. 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.
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 regulator
s.
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.
Laws of physics for diving
The main laws of physics that govern the physics of the SCUBA diverScuba diving
Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....
and of diving equipment
Diving equipment
Diving equipment is equipment used by underwater divers for the purpose of facilitating diving activities. This may be equipment primarily intended for this purpose, or equipment intended for other puprposes which is found to be suitable for diving use....
are:
- Archimedes' Principle of BuoyancyBuoyancyIn physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
- when in water, the densityDensityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
of the materials in the diver's body and in the diver's equipment determine whether the diver floats or sinks.
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 system
Diving weighting system
Divers wear weighting systems, weight belts or weights, generally made of lead, to counteract the buoyancy of other diving equipment, such as diving suits and aluminium diving cylinders...
s and lifting bag
Lifting bag
A lifting bag is an item of diving equipment consisting of a robust and air-tight bag with straps, which is used to lift heavy objects underwater by means of the bag's buoyancy...
s.
- Boyle's lawBoyle's lawBoyle's law is one of many gas laws and a special case of the ideal gas law. Boyle's law describes the inversely proportional relationship between the absolute pressure and volume of a gas, if the temperature is kept constant within a closed system...
- as pressure changes, the volume of gases in the diver's body and soft equipment changes too.
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 cylinderDiving cylinderA diving cylinder, scuba tank or diving tank is a gas cylinder used to store and transport high pressure breathing gas as a component of a scuba set. It provides gas to the scuba diver through the demand valve of a diving regulator....
increases (originally described by Guillaume AmontonsGuillaume AmontonsGuillaume Amontons was a French scientific instrument inventor and physicist. He was one of the pioneers in tribology, apart from Leonardo da Vinci, John Theophilus Desaguliers, Leonard Euler and Charles-Augustin de Coulomb.-Life:Guillaume was born in Paris, France. His father was a lawyer from...
).
This is why a diver who enters cold water with a warm diving cylinder, for instance after a recent quick fill
Diving air compressor
A Diving Air Compressor is a gas compressor that can provide breathing air directly to a surface-supplied diver, or fill diving cylinders with high-pressure air pure enough to be used as a breathing gas.thumb|right|Filling a cylinder from the panel...
, finds the gas pressure of the cylinder drops by an unexpectedly large amount.
- Dalton's lawDalton's lawIn chemistry and physics, Dalton's law states that the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture...
- in mixtures of breathing gasBreathing gasBreathing gas is a mixture of gaseous chemical elements and compounds used for respiration.Air is the most common and only natural breathing gas...
es the concentration of the individual components of the gas mix is measured by partial pressurePartial pressureIn a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
Partial pressure is a useful measure for expressing limits for avoiding nitrogen narcosis
Nitrogen narcosis
Narcosis while diving , is a reversible alteration in consciousness that occurs while scuba diving at depth. The Greek word ναρκωσις is derived from narke, "temporary decline or loss of senses and movement, numbness", a term used by Homer and Hippocrates...
and oxygen toxicity
Oxygen toxicity
Oxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen at elevated partial pressures. It is also known as oxygen toxicity syndrome, oxygen intoxication, and oxygen poisoning...
.
- Henry's lawHenry's lawIn physics, Henry's law is one of the gas laws formulated by William Henry in 1803. It states that:An equivalent way of stating the law is that the solubility of a gas in a liquid at a particular temperature is proportional to the pressure of that gas above the liquid...
- as pressure increases the quantity of gas adsorbed by the tissuesBiological tissueTissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
of the human body increases.
This helps explain nitrogen narcosis, oxygen toxicity and decompression sickness
Decompression sickness
Decompression sickness describes a condition arising from dissolved gases coming out of solution into bubbles inside the body on depressurization...
.
- Snell's lawSnell's lawIn optics and physics, Snell's law is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water and glass...
- the index of refractionRefractionRefraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
of water is similar to that of the corneaCorneaThe cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...
of the eyeHuman eyeThe human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
—30% greater than air.
This is the reason a diver cannot see clearly underwater without a diving mask
Diving mask
A diving mask is an item of diving equipment that allows scuba divers, free-divers, and snorkelers to see clearly underwater. When the human eye is in direct contact with water as opposed to air, its normal environment, light entering the eye is refracted by a different angle and the eye is unable...
with an internal airspace.
Physical effects of water for divers
The physical effects of water or the underwaterUnderwater
Underwater is a term describing the realm below the surface of water where the water exists in a natural feature such as an ocean, sea, lake, pond, or river. Three quarters of the planet Earth is covered by water...
environment are:
- PressurePressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
- the overall pressure on a diver is the sum of the atmospheric pressureAtmospheric pressureAtmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
and the water pressure. - DensityDensityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
- of the water, the diver's body and equipment determines the diver's buoyancy and the use of buoyant equipment.
Divers use high density materials such as lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
for diving weighting systems and low density materials such as air
Density of air
The density of air, ρ , is the mass per unit volume of Earth's atmosphere, and is a useful value in aeronautics and other sciences. Air density decreases with increasing altitude, as does air pressure. It also changes with variances in temperature or humidity...
in buoyancy compensators and lifting bags.
- Thermal conductivityThermal conductivityIn physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
of water is higher than that of air.
As water conducts heat 20 times more than air, divers in cold water must insulate their bodies with diving suit
Diving suit
A diving suit is a garment or device designed to protect a diver from the underwater environment. A diving suit typically also incorporates an air-supply .-History:...
s to avoid hypothermia
Hypothermia
Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...
. Gases used in diving have very different thermal conductivity; trimix conducts heat more than air and argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
conducts less heat than air hence the reason many trimix divers inflate their drysuits with argon.
- Absorption of lightAbsorption (electromagnetic radiation)In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
and loss of colour underwater.
The red end of the spectrum of light is absorbed even in shallow water. Divers use artificial light underwater to reveal these absorbed colours. In deeper water no light from the surface penetrates.
- Under pressure, gasGasGas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
es 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.
Physical phenomena of interest to divers
The physical phenomena found in large bodies of water are:- Effects of weatherWeatherWeather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...
such as windWindWind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
, which causes waves, and changes of temperatureTemperatureTemperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
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 regulator
Diving regulator
A diving regulator is a pressure regulator used in scuba or surface supplied diving equipment that reduces pressurized breathing gas to ambient pressure and delivers it to the diver. The gas may be air or one of a variety of specially blended breathing gases...
s.
- HaloclineHaloclineIn oceanography, a halocline is a subtype of chemocline caused by a strong, vertical salinity gradient within a body of water. Because salinity affects the density of seawater, it can play a role in its vertical stratification...
s, or strong, vertical salinitySalinitySalinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
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
North Atlantic Drift
North Atlantic Drift is:* An ocean current that continues from the North Atlantic Current* An album by Ocean Colour Scene: North Atlantic Drift this doesn't make any goddamn sense....
moderates the climate of the north west coast of Europe.
- ThermoclineThermoclineA thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...
s, 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
Slack water
Slack water, which used to be known as 'The stand of the tide', is a short period in a body of tidal water either side of high water or low water essentially when the water is completely unstressed, and therefore with no rise or fall of the tide and no movement either way in the tidal stream, and...
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
Drift diving
Drift diving is a type of recreational diving where the diver is transported by the currents caused by the tide or in a river.The current gives the diver the impression of flying and allows the diver to cover long distances underwater, possibly seeing more habitats and formations than usual...
is only possible when there is a reasonable current.