The Physics of
Diving
NOAA Diving Manual
Fourth Edition
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.
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.
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
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.
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.
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.
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
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.
Density
Density can be defined as weight per unit
volume
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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.
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.
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
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.
Water
 Seawater:
– Contains almost every substance known.
– The most abundant chemical is sodium
chloride (common table salt).
– Seawater is a good conductor of electricity.
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
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
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
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.
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).
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
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
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).
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.
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
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
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
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”
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)
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.
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
Gas Laws
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Boyle’s Law
Charles’/Gay-Lussac’s Law
Dalton’s Law
Henry’s Law
General Gas Law
Gas Laws
Boyle’s Law
“For any gas at a constant
temperature, the volume of the
gas will vary inversely with the
pressure”
Gas Laws
Boyle’s Law
P1V1 = P2V2
P1= initial pressure surface absolute
V1= initial volume in cubic feet
P2=final pressure absolute
V2=final volume in cubic feet
Gas Laws
Boyle’s 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 = (1ata x 24 ft3) / 3 ata
V2 = 8 ft3
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.”
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
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
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.”
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.”
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
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
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.
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.
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.