Diving Physics
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Diving Physics
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Buoyancy
Pressure
The Gas Laws
Light
Sound
Heat
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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."
– Float (positive buoyancy)
– Neutral
– Sink (negative buoyancy)
• Displacement vs. density
– Weight of fresh water (62.4 lbs./cu.ft.)
– Weight of salt water (64 lbs./cu.ft.)
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Buoyancy
• Human factors affecting buoyancy:
– Fat
– Muscle
– Bone
• Equipment factors affecting buoyancy
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Buoyancy
• Controlling buoyancy:
–
–
–
–
–
BCD
Natural lung capacity
Exposure suits
weight systems
relaxation
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Buoyancy
Displacement
Float or Sink?
•Archimedes Principle
•Volume-Weight
Relationship
• Object weighs 128 lbs,
Displaces 2 cu ft
Sink or Float?
1 cu ft Sea Water = 64 lbs
(Fresh Water = 62.4 lbs)
Body Position
Neutral
Level
Float
Head Down
Sink
Head Up
• Object weighs 128 lbs,
Displaces 1 cu ft
Sink or Float?
• Object weighs 128 lbs,
Displaces 4 cu ft
Sink or Float?
Diver weighs 170 lbs and requires 22 lbs weight for neutral
buoyancy in sea water. What is the diver’s volume?
• Buoyancy Controlled by BCD/Weight
• Buoyancy May Change with Depth
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Pressure
• Defined as a force per unit area
– In US measured as pounds per square inch (psi)
• Atmospheric pressure:
– Pressure of 1 sq in of atmosphere (14.7 psi)
• Hydrostatic pressure
– Pressure due to weight of water
• .447 psi fsw
• .432 psi ffw
• Absolute pressure (psia)
– Hydrostatic plus Atmospheric
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Partial Pressure
• In a mixture of gasses, the proportion of the
total pressure contributed by each gas in the
mixture is called the partial pressure
• Atmospheric Air:
– Approximately 78% nitrogen, 21% oxygen
– Component Partial Pressures
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Pressure in Diving
1 Cu Ft Sea Water = 64 lbs
Atmospheric Pressure
The Weight
of Air
Pressure Changes
1 Atm for Each 33 ft
in Sea Water (34 ft
in Fresh Water).
1 Ft
1 Sq.. In
12 in
14.7 psi at
Sea Level
12 in
Base Area = 144 Sq. In
64 lbs/144 Sq. In = .445 psi / ft
(62.4 lbs/144 Sq. In = .432 psi / ft)
14.7 psi = 1 Atm
Sea Water Pressure Changes .445 psi per ft
(Fresh Water Pressure Changes .432 psi per ft)
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The Gas Laws
Define Relationship between 3 Factors
– Volume
– Temperature
– Pressure
A change in one will result in a measurable change
in the others
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Pressure-Volume Relationship
(Boyle’s Law)
Volume
Pressure
10
50
10
50
High
Large
Low
Small
P1V1 = P2V2
Closed, Flexible
Container
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Scuba Delivers Air at
Ambient Pressure/Density
Maintaining Constant Volume
Lungs Cannot Expand
Therefore, NEVER HOLD
YOUR BREATH on Scuba
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Temperature - Pressure - Volume
(Charles’ /Gay-Lussac’s)
A Direct Temperature-Pressure-Volume Relationship Exists.
A Change in Temperature Directly Affects the Volume of a
Closed Flexible Container.
A Change in Temperature Directly Affects the Pressure
Exerted by a Gas in a Closed Rigid Container.
The Pressure in a Scuba Tank Will Change by Approx. 5 psi
for Every 1 Degree Fahrenheit Change in Temperature.
3000 psi
SPG
70 Deg F
3150 psi
SPG
100 Deg F
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Partial Pressure
(Dalton’s Law)
Pt = PP1 +PP2 + …
Air: 78 % Nitrogen
21 % Oxygen
Sea Level - 1 atma
Air =
14.7 psia
Nitrogen +
11.61 psia
Oxygen
3.09 psia
33 fsw
66 fsw
99 fsw
2 atma
3 atma
4 atma
29.4 psia
44.1 psia
58.8 psia
23.52 psia
35.28 psia
47.04 psia
5.88 psia
8.82 psia
11.76 psia
297 fsw
10 amta
147.0 psia
117.60 psia
29.40 psia
Note: Pure Oxygen is Toxic at 2 atma (29.4 psia).
The Partial Pressure of the Oxygen Component
of Air at 297 fsw(10 atma) is 29.40 psia!
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Solubility
(Henry’s Law)
Gas Enters a Liquid in Direct Proportion to its Partial Pressure
If You Increase the Partial Pressure, the Amount of
Dissolved Gas also Increases
Reducing Pressure Too Quickly Allows Dissolved Gases to
Release Too Quickly and Form Bubbles: The BENDS!
DECOMPRESSION SICKNESS (DCS)
DCS Can Occur
When Diver Follows Tables
Prevention: Use the Dive Tables
CONSERVATIVELY!
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Light
• Diffusion
• Color absorption
• Refraction
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Light Properties
Sunlight is Diffused or Absorbed by
Particles Suspended in the Water by
Light Wavelength According to Depth
Surface
10
20
30
40 R
50
O
Y
60
70
G
80
B
90
100
110
I
V
Color Loss, Ambient Light
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Sight
Refraction, the Bending of Light Waves, When
Changing Transmission Medium, Causes Objects
to Appear Closer and Larger Underwater.
Actual Size
and Location
Perceived Size
and Location
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Sound
Sound Travels 4X Faster
in Water than in Air
Time Delay
Ears are
Calibrated
to Air
Sound Waves
You Can Hear, but
Can’t Determine Direction
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Temperature
Relative Body
Protection Rqmts
110
Lycra 90
1/8” Wet Suit 70
1/4” Wet Suit
Dry Suit 50
30
Heat Loss By
Conduction
Faster in Water
10
Temp F
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Heat
• Water is 800 times denser than air
–
–
–
–
Conduction
Convection
Radiation
Thermoclines
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Physics - Summary
Buoyancy
Use BC/Wt Belt to Regulate
Pressure
One Atm = 14.7 psi
Sound
Can’t Determine Direction
Sight
Gas Laws
Boyle - Volume
Dalton - Partial Pressure
Henry - Solubility
Gay-Lussac/Charles Temp/Pressure/Volume
Temperature
Proper Protection Essential
Objects Appear Closer/Larger
Light
Visibility/Colors Decrease
w/Depth
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