The Physics of Diving

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The Physics of Diving

• NOAA Diving Manual
• Fourth Edition

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The Physics of Diving
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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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Student Performance

• After reviewing this presentation you will be
  able to
• Define the different types of pressure
  measurements associated with diving
• Recognize the affect of a variety of
  environmental factors and explain how they impact
  the diver.
• List the Gas Laws associated with scuba diving.
• Explain each of the Gas Laws explaining how each
  pertains to scuba diving.
• Demonstrate your understanding of each of the Gas
  Laws by being able to solve problems germane to a
  scuba diving related scenarios.

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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 affected by 2 types 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 (equilibrium) with the forces at that
  depth.

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Atmospheric Pressure

• This 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 1 atm

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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 feet sea water (fsw)
  increases one atm for every additional 33 fsw
  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 feet fresh water (ffw)
  increases one atm for every additional 34 ffw
  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 14.7 ata

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Partial PressureDaltons 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/ft3 or g/cm3
• 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 (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.

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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 the English system 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 39.37 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 11.16 square meters.

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Volume

• Volume 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).

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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 180.4 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
• oF 460 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 273 K

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Temperature(conversions)

• To convert from Fahrenheit to Celsius
• C 5/9 X (oF 32)
• To convert from Celsius to Fahrenheit
• oF (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 dont by utilizing the proper
  exposure protection suit

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BuoyancyArchimedes 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 O2 79 N2
• Oxygen - O2
• 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

• Boyles Law
• Charles/Gay-Lussacs Law
• Daltons Law
• Henrys Law
• General Gas Law

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Gas LawsBoyles Law

• For any gas at a constant temperature, the
  volume of the gas will vary inversely with the
  pressure

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Gas LawsBoyles Law

• P1V1 P2V2
• P1 initial pressure surface absolute
• V1 initial volume in cubic feet
• P2final pressure absolute
• V2final volume in cubic feet

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Gas LawsBoyles Law

• Determine the volume (V2) of a 24 ft3 open
  bottom diving bell with at 66 fsw
• P1 1 ata
• V1 24 ft3
• P2 3 ata
• V2 (P1V1) / P2
• V2 (1 ata x 24 ft3) / 3 ata
• V2 8 ft3

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Gas LawsCharles/Gay-Lussacs 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 LawsCharles 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 LawsCharles Law Volume Change

• A balloon with 24 ft3 of air is lowered in water
  to a depth of 99 fsw. Surface temperature is
  80oF temperature at depth is 45oF. What will
  the volume of the balloon be at 99 fsw?
• V1 / V2 T1 / T2 (pressure remains constant)
• V1 volume initial T1 temperature initial
• V2 volume final T2 temperature final

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Gas LawsCharles Law Volume Change

• Boyles law determines that the balloon will have
  a volume of 6 ft3 at 99 fsw.
• Determine how temperature affects this volume.
• V2 (V1T2)/T1
• V1 Vol. _at_ 99 fsw 6 fsw V2 unknown
• T1 80o F 460 540 Rankine T2 45o F 460
  505 Rankine
• V2 (6 ft3 X 505 R) / 540 R 5.61 ft3

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Gas LawsGay-Lussacs 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 LawsGay-Lussacs Law Pressure Change

• A scuba cylinder contains 3000 psig (3014.7 psia)
  at 64oF. After being left in the Sun the reaches
  102oF. What is the new pressure?
• P1 / P2 T1 / T2 (volume remains constant)
• P1 pressure initial 3014.7 psia
• T1 temperature initial 64oF 460 524
  Rankine
• P2 pressure final unknown
• T2 temperature final 102oF 460 562 Rankine

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Gas LawsGay-Lussacs Law Pressure Change

• P2 (P1T2) / T1
• P2 (3014.7 psia X 562 R) / 524 R
• P2 3233.3 psia
• To convert to gauge pressure
• 3233.3 psia 14.7 psi 3218.6 psig

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Gas LawsDaltons 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 LawsDaltons Law

• Pt PP1 PP2 PP3, etc
• Pt Total Pressure
• PP1 Partial Pressure of first gas
• PP2 Partial Pressure of second gas
• PP3 Partial Pressure of third gas, etc

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Gas LawsDaltons Law

• What is the partial pressure of the nitrogen in a
  scuba cylinder filled with air to a pressure of
  2000 psig?
• PN2 Gas (decimal) X Pt
• PN2 .78 X 2000 psig
• PN2 1561.6 psig

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Gas LawsHenrys 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 LawsHenrys Law

• VG / V L oc P1
• VG Volume of a gas dissolved at STPD
• (standard temperature pressure dry)
• VL Volume of gas in the liquid
• oc Solubility coefficient at specific
  temperature
• P1 Partial pressure of that gas above the gas

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Gas LawsHenrys Law

• When a gas free liquid is exposed to a gas
  mixture, gas molecules diffuse into the solution,
  pushed by the partial pressure of each individual
  gas.
• Gas Tension as the gas molecules enter the
  liquid they add to a state of gas tension, a way
  of identifying the partial pressure of the gas in
  the liquid.
• Pressure Gradient - The difference between the
  gas tension and the partial pressure of the gas
  outside the liquid.

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Gas LawsHenrys Law

• When the gradient for diffusion is high combined
  with low tension and high partial pressure the
  rate of absorption into the liquid is high.
• As the number of gas molecules in the liquid
  increases, the gas tension increases until it
  reaches an equilibrium value equal to the outside
  partial pressure.
• Saturated - the point of equilibrium described
  above.

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Gas LawsGeneral 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
• Boyles Law

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Gas LawsGeneral Gas Law

• (P1 V1) / T1 (P2 V2) / T2
• P1 initial pressure (absolute)
• V1 initial volume
• T1 initial temperature (absolute)
• P2 final pressure (absolute)
• V2 final volume
• T2 final temperature (absolute)

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Gas LawsGeneral Gas Law

• A open diving bell with an air volume of 24 ft3
  is lowered to 99 fsw. Surface temperature is
  80oF and temperature at depth is 45oF. What will
  be the air volume of the bell at depth?

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Gas LawsGeneral Gas Law

• (P1 V1) / T1 (P2 V2) / T2
• or
• V2 (P1 V1 T2) / (T1 P2)
• P1 initial pressure (absolute) 14.7 psia
• V1 initial volume 24 ft3
• T1 initial temperature (absolute) 80o F 460
  540 Rankine
• P2 final pressure (absolute) 4 ata x 14.7
  psia 58.8 psia
• V2 unknown
• T2 final temperature (absolute) 45o F 460
  505 Rankine

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Gas LawsGeneral Gas Law

• V2 (14.7 psia)(24 ft3)(505 R)
• (540 R)(58.8 psia)
• V2 5.61 ft3
• The volume was reduced, due to the drop in
  temperature and the increase in ambient pressure.

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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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The Physics of Diving

• Review of the chapter
• Hydrostatic and atmospheric pressure both
  influence the diver.
• The density of the water, fresh vs. salt,
  affects the buoyancy of a diver.
• Depth affects the density of the gas being
  breathed thus affecting the rate of consumption.
• Measurements can be given in either the English
  System or the International System of Units.
• There are 5 different gas laws which explain the
  behavior of gases in a diving environment.
• Humidity, Light and Sound are all affected by
  the density of water