The Physics of DivingNOAA Diving ManualFourth Edition

OverviewPhysics is the field of science dealing with matter and energy and their interactions.This presentation explores physical laws and principles that pertain to the diving environment and its influence on the diver.The principles of physics provide the keystone for understanding the reasons for employing various diving procedures.

PressurePressure is force acting on a unit area

Pressure = force / areaP = F/AIn the USA pressure is typically measured in pounds per square inch (psi).Underwater a diver is underwater is affected by 2 kinds of pressureThe pressure exerted by the atmosphereThe pressure exerted by the surrounding water A diver, at any depth, must be in pressure balance with the forces at that depth.

Atmospheric PressureThis is the pressure exerted by the earths atmosphere.At sea level it is equal to 14.7 psi, or one atmosphere (atm).It decreases with altitude above sea level.For example, at 18,000 ft. atmospheric pressure is 7.35 psi or half that at sea level.An pressure inside an individuals lungs at sea level are at equilibrium with the surrounding pressure 1atm

Hydrostatic PressureThis pressure is created by the weight of water - called hydrostatic pressure.This pressure is cumulative. The deeper the dive, the more water above the diver and the greater the weight of the water.Hydrostatic pressure affects the diver from all sides equally.

Hydrostatic PressureIn seawater:hydrostatic pressure increases at a rate of .445 psi per foot you descend.One ata (14.7 psi) of hydrostatic pressure is reached at a depth of 33 & increases 1 atm for every additional 33 thereafter.In freshwater: hydrostatic pressure increases at a rate of .432 psi per foot you descend.One ata (14.7 psi) of hydrostatic pressure is reached at a depth of 34& increases 1 atm for every additional34 thereafter.

Absolute PressureThe sum of atmospheric pressure plus hydrostatic pressure is called absolute pressure.It can be expressed as: psia (pounds per square inch absolute), ata (atmospheres absolute), fswa (feet of seawater absolute), ffwa (feet of freshwater absolute), or mmHga (millimeters of mercury absolute.

Gauge PressureThe difference between atmospheric pressure and the pressure being measured is gauge pressure.The zero psi reading on a scuba cylinder pressure gauge at sea level is actually equal to 14.7 psi.Gauge pressure + 14.7 = ata

Partial PressureDaltons LawIn mixture of gases, the proportion of the total pressure contributed by each gas in the mixture is called the partial pressure.For our purposes air is composed of 21% oxygen and 79% nitrogen.

DensityDensity can be defined as weight per unit volumeDensity = Weight / Volume or D = W / VDensity is expressed in lbs/ft3 or g/cm3Gas density is related to absolute pressure.Density is directly proportional to pressureAs depth increases, the density of the breathing gas and becomes heavier per unit volume.

DensitySeawater has a density of 64 pounds per cubic foot.Freshwater has a density of 62.4 pounds per cubic foot.As a result, freshwater floats on top of seawater a diver is more buoyant, given the same conditions, in seawater than in freshwater.

Specific GravitySpecific gravity is the ratio of the weight of a given volume of a substance (density) to that of an equal volume of another substance.Water is the standard for liquids and solids.Air is the standard for gases.Freshwater has a specific gravity of 1.0Substances that are more dense than freshwater have a specific gravity greater than 1.0.The specific gravity of seawater is 64/62.4 = 1.026

WaterFreshwater (H2O):is odorless, tasteless and very slightly compressible.It freezes at 32oF (0C), and boils at 212oF (100C).In its purest form, it is a poor conductor of electricity.

WaterSeawater:Contains almost every substance known.The most abundant chemical is sodium chloride (common table salt).Seawater is a good conductor of electricity.

pHThe pH of an aqueous solution expresses the level of acids or alkalis present.The pH of a liquid can range from 0 (strongly acidic) to 14 (strongly alkaline).A pH of 7 is considered neutralThe pH level in the blood is what signals the brain the need to breathe.Too much CO2 causes the blood to become acidic. One way the body reduces the acidity is to increase ventilations

Units of MeasurementThere are two systems for specifying force, length and time: English and the International System of Units (SI). also known as Metric.The English System is based on the pound, the foot, and the second.Primarily use in the United StatesThe International System of Units is based on the kilogram, the meter, and the second.Used everywhere else

Length1 meter = 39.37 in = 3.28 ft.To convert 10 feet to meters:10 ft / 3.28 ft/m = 3.05 mConvert 10 meters to feet:10 m X 3.28 ft/m = 32.8 ft

AreaIn both the English and IS system, area is expressed as a length squared.For example: A room that is 12 feet by 10 feet would have an area of 120 square feet (12 ft x 10 ft).A room that is 3.66 m by 3.05 m would have an area of 11.16 square meters.

VolumeVolume is expressed in units of length cubed.Length x Width x Height = cubic feet (ft3) or cubic meters (m3)The English System, in addition to ft3, uses other units of volume such as gallons.The SI uses the liter (l). A liter = 1000 cubic centimeters (cm3) or .001 cubic meters (m3), which is one milliliter (ml).

WeightThe pound (lb) is the standard measure of weight in the English System.The kilogram (kg) is the standard measure of weight in the International System of Units.One liter of water at 4C weighs 1 kg or 2.2 lbs.1liter (l) = 1 kg = 2.2 lbs

Weight(conversions)

Convert 180 pounds to kilograms:180 lbs / 2.2 lbs/kg = 81.8 kg

Convert 82 kilograms to pounds:82 kg X 2.2 lbs/kg = 180.4 lbs

TemperatureHeat is associated with the motion of molecules.The more rapidly the molecules move, the higher the temperature.Temperature is usually measured either with the Fahrenheit (F) scale or with the Celsius (centigrade) scale (C).

TemperatureTemperature must be converted to absolute when the gas laws are used.The absolute temperature scales, which use Rankine (R) or Kelvin (K), are based on absolute zero (the lowest temperature that can possibly be reached).Note that the degree symbol () is only used with Fahrenheit temperatures.

Temperature(conversions)The Fahrenheit (F) and Rankine (R) temperature scales are used in the English System.To convert Fahrenheit to absolute temperature RankineoF + 460 = R

The Celsius (C) and Kelvin (K) temperature scales are used in the International System of Units.To convert Celsius to absolute temperature KelvinC + 273 = K

Temperature(conversions)To convert from Fahrenheit to CelsiusC = 5/9 X (oF 32)

To convert from Celsius to FahrenheitoF = (9/5 X C) + 32

HeatAn often forgotten but extremely important consideration in divingHumans can only function effectively in a very narrow range of internal temperatures.Maintaining the proper body core temperature is criticalThis can be dont by utilizing the proper exposure protection suit

Buoyancy

Archimedes PrincipleAny object wholly or partly immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object

BuoyancyPositive Buoyancy is achieved if the weight of the displaced water (total displacement) is greater than the weight of the submerged object. Object floatsNegative Buoyancy is achieved if the weight of the displaced water (total displacement) is less than the weight of the submerged object. Object sinksNeutral Buoyancy is achieved if the weight of the displaced water (total displacement) is equal to the weight of the water.Object is suspended)

BuoyancyBuoyancy is dependent upon the density of the surrounding liquid.Remember:Seawater has a density of 64 pounds per cubic foot.Freshwater has a density of 62.4 pounds per cubic foot.

Gases Associated with DivingAtmospheric Air - 21% O2 + 79% N2Oxygen - O2The most important of all gases.Usually used for decompression gasNitrogen N2Helium HeUsed extensively in deep divingCarbon Dioxide CO2A natural by-product of metabolism Carbon Monoxide COA poisonous gas produced by the incomplete combustion of fuelsArgon ArNot very common in divingNeon NeNot very common in divingHydrogen H2Not very common in diving

Gas LawsBoyles LawCharles/Gay-Lussacs LawDaltons LawHenrys LawGeneral Gas Law

Gas LawsBoyles LawFor any gas at a constant temperature, the volume of the gas will vary inversely with the pressure

Gas LawsBoyles LawP1V1 = P2V2

P1= initial pressure surface absoluteV1= initial volume in cubic feetP2=final pressure absoluteV2=final volume in cubic feet

Gas LawsBoyles LawDetermine the volume (V2) of a 24 ft3 open bottom diving bell with at 66 fsw:

P1 = 1 ataV1 = 24 ft3P2 = 3 ata

V2= (P1V1) / P2

V2 = (1ata x 24 ft3) / 3 ata

V2 = 8 ft3

Gas LawsCharles/Gay-Lussacs LawFor any gas at a constant pressure, the volume of the gas will vary directly with the absolute temperature or for any gas at a constant volume, the pressure of the gas will vary directly with the absolute temperature.

Gas LawsCharles Law Volume Change

(pressure remains constant)V1 / V2 = T1 / T2

V1 = volume initialT1 = temperature initialV2 = volume finalT2 = temperature final

Gas LawsGay-Lussacs Law Pressure Change

(volume remains constant)P1 / P2 = T1 / T2

P1 = pressure initialT1 = temperature initialP2 = pressure finalT2 = temperature final

Gas LawsDaltons LawThe total pressure exerted by a mixture of gases is equal to the sum of the pressures of each of the different gases making up the mixture, with each gas acting as if it alone was present and occupied the total volume.

Gas LawsHenrys LawThe amount of any gas that will dissolve in a liquid at a given temperature is proportional to the partial pressure of that gas in equilibrium with the liquid and the solubility coefficient of the gas in the particular liquid.

Gas LawsGeneral Gas LawCommonly called the Ideal Gas LawUsed to predict the behavior of a given quantity of gas when changes may be expected in any or all of the variablesCombinesCharles Law Boyles Law

HumidityWater vapor (a gas) behaves in accordance with the gas laws.The water vapor condenses at temperatures we are likely to encounter while diving, hence humidity is an important considerationMask fogging

LightHuman eyes can only perceive a very narrow range of wave lengths (visible light)Water slows the speed at which light travels. This causes the light rays to bend or refractwith a mask on the light rays are bent twiceObjects appear 25 % larger.Turbidity can also effect vision by making objects appear farther than it really is.

LightColorsWater absorbs light according to its wavelengthRed is the first color lostBlue eventually become the dominant color at deeper depthsAs depth increases the ability to discern colors decreases until visible objects are distinguishable only by differences in brightness. Contrast becomes the most important factor.

SoundSound is produced by pressure waves triggered by vibrationThe more dense the medium through which sound travels, the faster the speed of sound.Sound travels roughly 4 times faster in water than in airThis makes detecting the origin of the sound very difficult.