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The Physics of Diving NOAA Diving Manual Fourth Edition

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Overview Physics 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.

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Pressure Pressure is force acting on a unit area Pressure = force / area P = F/A In the USA pressure is typically measured in pounds per square inch (psi). Underwater a diver is underwater is affected by 2 kinds of pressure –The pressure exerted by the atmosphere –The pressure exerted by the surrounding water A diver, at any depth, must be in pressure balance with the forces at that depth.

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Atmospheric Pressure This is the pressure exerted by the earth’s 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 individual’s lungs at sea level are at equilibrium with the surrounding pressure – 1atm

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Hydrostatic Pressure This 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.

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Hydrostatic Pressure In 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.

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Absolute Pressure The 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.

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Gauge Pressure The 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 = ata

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Partial Pressure Dalton’s Law In 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.

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Density Density can be defined as weight per unit volume Density = Weight / Volume or D = W / V Density is expressed in lbs/ft 3 or g/cm 3 Gas density is related to absolute pressure. Density is directly proportional to pressure –As depth increases, the density of the breathing gas and becomes heavier per unit volume.

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Density Seawater 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.

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Specific Gravity Specific 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 1.0 –Substances 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

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Water Freshwater (H 2 O): –is odorless, tasteless and very slightly compressible. –It freezes at 32 o F (0C), and boils at 212 o F (100C). –In its purest form, it is a poor conductor of electricity.

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

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pH The 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 neutral The 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

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Units of Measurement There 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 States The International System of Units is based on the kilogram, the meter, and the second. –Used everywhere else

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Length 1 meter = in = 3.28 ft. To convert 10 feet to meters: –10 ft / 3.28 ft/m = 3.05 m Convert 10 meters to feet: –10 m X 3.28 ft/m = 32.8 ft

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Area In 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 square meters.

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Volume Volume is expressed in units of length cubed. Length x Width x Height = cubic feet (ft 3 ) or cubic meters (m 3 ) –The English System, in addition to ft 3, uses other units of volume such as gallons. –The SI uses the liter (l). A liter = 1000 cubic centimeters (cm 3 ) or.001 cubic meters (m 3 ), which is one milliliter (ml).

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Weight The “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

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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 = lbs

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Temperature Heat 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).

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Temperature Temperature 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.

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Temperature (conversions) The Fahrenheit (°F) and Rankine (R) temperature scales are used in the English System. –To convert Fahrenheit to absolute temperature Rankine o F = R The Celsius (C) and Kelvin (K) temperature scales are used in the International System of Units. –To convert Celsius to absolute temperature Kelvin C = K

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Temperature (conversions) To convert from Fahrenheit to Celsius –C = 5/9 X ( o F – 32) To convert from Celsius to Fahrenheit – o F = (9/5 X C) + 32

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Heat An often forgotten but extremely important consideration in diving Humans can only function effectively in a very narrow range of internal temperatures. Maintaining the proper body core temperature is critical –This can be don’t by utilizing the proper exposure protection suit

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Buoyancy Archimedes’ Principle “Any 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”

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Buoyancy “Positive” Buoyancy is achieved if the weight of the displaced water (total displacement) is greater than the weight of the submerged object. – Object floats “Negative” Buoyancy is achieved if the weight of the displaced water (total displacement) is less than the weight of the submerged object. –Object sinks “Neutral” Buoyancy is achieved if the weight of the displaced water (total displacement) is equal to the weight of the water. –Object is suspended)

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Buoyancy Buoyancy 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.

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Gases Associated with Diving Atmospheric Air - 21% O % N 2 Oxygen - O 2 –The most important of all gases. –Usually used for decompression gas Nitrogen – N2 Helium – He –Used extensively in deep diving Carbon Dioxide – CO2 –A natural by-product of metabolism Carbon Monoxide – CO –A poisonous gas – produced by the incomplete combustion of fuels Argon – Ar –Not very common in diving Neon – Ne –Not very common in diving Hydrogen – H2 –Not very common in diving

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Gas Laws Boyle’s Law Charles’/Gay-Lussac’s Law Dalton’s Law Henry’s Law General Gas Law

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Gas Laws Boyle’s Law “For any gas at a constant temperature, the volume of the gas will vary inversely with the pressure”

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Gas Laws Boyle’s Law P 1 V 1 = P 2 V 2 P 1 = initial pressure surface absolute V 1 = initial volume in cubic feet P 2 =final pressure absolute V 2 =final volume in cubic feet

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Gas Laws Boyle’s Law Determine the volume (V 2 ) of a 24 ft 3 open bottom diving bell with at 66 fsw: P 1 = 1 ata V 1 = 24 ft 3 P 2 = 3 ata V 2 = (P 1 V 1 ) / P 2 V 2 = (1ata x 24 ft 3 ) / 3 ata V 2 = 8 ft 3

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Gas Laws Charles’/Gay-Lussac’s Law “For 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.”

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Gas Laws Charles’ Law Volume Change (pressure remains constant) V1 / V2 = T1 / T2 V1 = volume initial T1 = temperature initial V2 = volume final T2 = temperature final

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Gas Laws Gay-Lussac’s Law Pressure Change (volume remains constant) P1 / P2 = T1 / T2 P1 = pressure initial T1 = temperature initial P2 = pressure final T2 = temperature final

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Gas Laws Dalton’s Law “The 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.”

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Gas Laws Henry’s Law “The 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.”

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Gas Laws General Gas Law Commonly called the “Ideal Gas Law” Used to predict the behavior of a given quantity of gas when changes may be expected in any or all of the variables Combines –Charles’ Law –Boyle’s Law

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Humidity Water 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 consideration –Mask fogging

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Light Human 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 refract –with a mask on the light rays are bent twice Objects appear 25 % larger. Turbidity can also effect vision by making objects appear farther than it really is.

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Light Colors –Water absorbs light according to its wavelength Red is the first color lost Blue eventually become the dominant color at deeper depths –As depth increases the ability to discern colors decreases until visible objects are distinguishable only by differences in brightness. Contrast becomes the most important factor.

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Sound Sound is produced by pressure waves triggered by vibration The more dense the medium through which sound travels, the faster the speed of sound. Sound travels roughly 4 times faster in water than in air –This makes detecting the origin of the sound very difficult.

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