DIVE THEORY STUDY GUIDE by Rod Abbotson CD69259 © 2010 Dive Aqaba
Guidelines for studying:
Study each area in order as the theory from one subject is used to build upon the
theory in the next subject.
When you have completed a subject, take tests and exams in that subject to
make sure you understand everything before moving on.
If you try to jump around or don’t completely understand something; this can lead
to gaps in your knowledge.
You need to apply the knowledge in earlier sections to understand the concepts
in later sections...
If you study this way you will retain all of the information and you will have no
problems with any PADI dive theory exams you may take in the future.
Before completing the section on decompression theory and the RDP make sure
you are thoroughly familiar with the RDP, both Wheel and table versions. Use the
appropriate instructions for use guides which come with the product.
Contents
Section One PHYSICS ………………………………………………page 2
Section Two PHYSIOLOGY………………………………………….page 11
Section Three DECOMPRESSION THEORY & THE RDP….……..page 21
Section Four EQUIPMENT……………………………………………page 27
Section Five SKILLS & ENVIRONMENT…………………………...page 36
,PHYSICS SECTION ONE
Light:
The speed of light changes as it passes through different things such as air,
glass and water. This affects the way we see things underwater with a diving
mask.
As the light passes through the glass of the mask and the air space, the
difference in speed causes the light rays to bend; this is called refraction.
To the diver wearing a normal diving mask objects appear to be larger and closer
than they actually are. About 25% larger and closer by a ratio of 4:3. (If the object
is actually 4m away it will appear only 3m away when viewed through a diving
mask and 25% bigger).
Turbidity (Bad visibility underwater) can cause the diver to perceive (think) that
objects are further away than they actually are because they obscured by
particles in the water. This phenomenon is known as visual reversal.
As light hits the surface of the water the light waves are scattered in all directions
this is why we get less light as we go deeper. The better the clarity of the water
and the higher the angle of the sun the more light penetrates. (This is why
photographers prefer to dive between 10.00am and 2.00pm).
Light is also absorbed as it travels through water; the shorter wavelengths
disappear first which in the spectrum is red. So the red colors are the first to
disappear and blue last.
Sound:
Sound travels four times faster in water than it does in air. This is because the
water is a denser and more elastic medium than air. 800 times more dense.
Because of this the diver’s brain perceives the sound as reaching both ears at
the same time. This means he cannot tell the direction the sound is coming from.
The sound seems to come from everywhere at once or overhead.
Although the dive cannot tell the direction he can tell whether the sound is either
closer or further away depending on its volume. Sound can travel very long
distances underwater.
Heat:
Water has a much higher heat capacity than air; this is its ability to draw heat
away from another object such as a diver. Water conducts heat away from the
diver twenty times faster than air for a given temperature. This is why a diver will
chill quickly without an exposure suit even in warmer water.
The diver loses heat by three different methods while underwater.
The first method is conduction which has most effect on the diver; this is caused
by the water drawing heat by direct contact with the diver or his suit.
The second method is convection; this is caused by the movement of the water
around the diver.
The third method is radiation which has the least effect on the diver; this is
caused by the diver radiating his body heat out to the water.
2
,Pressure:
Pressure is measured in bars or atmospheres (atm), essentially the same.
Gauge pressure is the pressure of the water at a given depth.
Absolute or Ambient pressure is the water pressure plus the atmospheric
pressure. (At sea level the atmospheric pressure is 1 bar/atm).
Pressure increases in sea water by 1 bar every 10 meters.
Pressure increases in fresh water by 1 bar every 10.3 meters.
To calculate gauge pressure in bar simply divide the depth of the water by 10 for
seawater and by 10.3 for fresh water.
Examples…
To calculate the gauge pressure at 37m in sea water.
37 ÷ 10 = 3.7 bar.
To calculate the gauge pressure at 16m in fresh water.
16 ÷ 10.3 = 1.55 bar.
To calculate the absolute/ambient pressure in bar simply repeat the above
procedure and then add 1 (providing you are calculating for sea level).
Examples…
To calculate the absolute pressure at 27m in sea water.
27 ÷ 10 = 2.7 + 1 = 3.7 bar.
To calculate the absolute pressure at 22m in fresh water.
22 ÷ 10.3 = 2.14 + 1 = 3.14 bar.
To calculate the ambient pressure at 40m in fresh water at an altitude where the
atmospheric pressure is 0.7 bar.
40 ÷ 10.3 = 3.88 + 0.7 = 4.58 bar.
Remember that in all questions in physics of diving (except if they just ask you for
the gauge pressure) you will use absolute pressure in your calculations.
3
, Pressure and volume:
For all intents and purposes you cannot compress a liquid or a solid by applying
greater pressure, but you can compress a gas as the molecules are further apart.
Boyle’s Law states that the volume of a gas is inversely proportional the
surrounding pressure on the gas.
So it looks like this…. Volume = _______1_______
Absolute Pressure
Examples….
If you take an inverted bucket down to 20m what would the volume of the air be
inside it?
Volume = ⅓
If a balloon contains 15 liters of air at the surface; what would its volume be if
taken down to 40m?
15 ÷ 5 = 3 liters.
If a balloon contains 7 liters of air at 30m and is then taken to the surface; how
much would its volume then be?
7 x 4 = 28 liters.
Remember, with volume, multiply by the absolute pressure if you go up and
divide by the absolute pressure if you go down.
If you move from one depth to another, it’s easiest to take it first to the surface
and then back down to the new depth.
Example….
You take a balloon containing 5 liters of air from 35m up to 15m, what would its
new volume be?
5 x 4.5 = 22.5 liters at the surface, then take it back down to 15m
22.5 ÷ 2.5 = 9 liters at 15m.
A balloon is sometimes referred to a flexible container. A scuba tank is referred
to as an inflexible container – a scuba tank does not change volume or the
amount of air it holds when changing depth.
4
Guidelines for studying:
Study each area in order as the theory from one subject is used to build upon the
theory in the next subject.
When you have completed a subject, take tests and exams in that subject to
make sure you understand everything before moving on.
If you try to jump around or don’t completely understand something; this can lead
to gaps in your knowledge.
You need to apply the knowledge in earlier sections to understand the concepts
in later sections...
If you study this way you will retain all of the information and you will have no
problems with any PADI dive theory exams you may take in the future.
Before completing the section on decompression theory and the RDP make sure
you are thoroughly familiar with the RDP, both Wheel and table versions. Use the
appropriate instructions for use guides which come with the product.
Contents
Section One PHYSICS ………………………………………………page 2
Section Two PHYSIOLOGY………………………………………….page 11
Section Three DECOMPRESSION THEORY & THE RDP….……..page 21
Section Four EQUIPMENT……………………………………………page 27
Section Five SKILLS & ENVIRONMENT…………………………...page 36
,PHYSICS SECTION ONE
Light:
The speed of light changes as it passes through different things such as air,
glass and water. This affects the way we see things underwater with a diving
mask.
As the light passes through the glass of the mask and the air space, the
difference in speed causes the light rays to bend; this is called refraction.
To the diver wearing a normal diving mask objects appear to be larger and closer
than they actually are. About 25% larger and closer by a ratio of 4:3. (If the object
is actually 4m away it will appear only 3m away when viewed through a diving
mask and 25% bigger).
Turbidity (Bad visibility underwater) can cause the diver to perceive (think) that
objects are further away than they actually are because they obscured by
particles in the water. This phenomenon is known as visual reversal.
As light hits the surface of the water the light waves are scattered in all directions
this is why we get less light as we go deeper. The better the clarity of the water
and the higher the angle of the sun the more light penetrates. (This is why
photographers prefer to dive between 10.00am and 2.00pm).
Light is also absorbed as it travels through water; the shorter wavelengths
disappear first which in the spectrum is red. So the red colors are the first to
disappear and blue last.
Sound:
Sound travels four times faster in water than it does in air. This is because the
water is a denser and more elastic medium than air. 800 times more dense.
Because of this the diver’s brain perceives the sound as reaching both ears at
the same time. This means he cannot tell the direction the sound is coming from.
The sound seems to come from everywhere at once or overhead.
Although the dive cannot tell the direction he can tell whether the sound is either
closer or further away depending on its volume. Sound can travel very long
distances underwater.
Heat:
Water has a much higher heat capacity than air; this is its ability to draw heat
away from another object such as a diver. Water conducts heat away from the
diver twenty times faster than air for a given temperature. This is why a diver will
chill quickly without an exposure suit even in warmer water.
The diver loses heat by three different methods while underwater.
The first method is conduction which has most effect on the diver; this is caused
by the water drawing heat by direct contact with the diver or his suit.
The second method is convection; this is caused by the movement of the water
around the diver.
The third method is radiation which has the least effect on the diver; this is
caused by the diver radiating his body heat out to the water.
2
,Pressure:
Pressure is measured in bars or atmospheres (atm), essentially the same.
Gauge pressure is the pressure of the water at a given depth.
Absolute or Ambient pressure is the water pressure plus the atmospheric
pressure. (At sea level the atmospheric pressure is 1 bar/atm).
Pressure increases in sea water by 1 bar every 10 meters.
Pressure increases in fresh water by 1 bar every 10.3 meters.
To calculate gauge pressure in bar simply divide the depth of the water by 10 for
seawater and by 10.3 for fresh water.
Examples…
To calculate the gauge pressure at 37m in sea water.
37 ÷ 10 = 3.7 bar.
To calculate the gauge pressure at 16m in fresh water.
16 ÷ 10.3 = 1.55 bar.
To calculate the absolute/ambient pressure in bar simply repeat the above
procedure and then add 1 (providing you are calculating for sea level).
Examples…
To calculate the absolute pressure at 27m in sea water.
27 ÷ 10 = 2.7 + 1 = 3.7 bar.
To calculate the absolute pressure at 22m in fresh water.
22 ÷ 10.3 = 2.14 + 1 = 3.14 bar.
To calculate the ambient pressure at 40m in fresh water at an altitude where the
atmospheric pressure is 0.7 bar.
40 ÷ 10.3 = 3.88 + 0.7 = 4.58 bar.
Remember that in all questions in physics of diving (except if they just ask you for
the gauge pressure) you will use absolute pressure in your calculations.
3
, Pressure and volume:
For all intents and purposes you cannot compress a liquid or a solid by applying
greater pressure, but you can compress a gas as the molecules are further apart.
Boyle’s Law states that the volume of a gas is inversely proportional the
surrounding pressure on the gas.
So it looks like this…. Volume = _______1_______
Absolute Pressure
Examples….
If you take an inverted bucket down to 20m what would the volume of the air be
inside it?
Volume = ⅓
If a balloon contains 15 liters of air at the surface; what would its volume be if
taken down to 40m?
15 ÷ 5 = 3 liters.
If a balloon contains 7 liters of air at 30m and is then taken to the surface; how
much would its volume then be?
7 x 4 = 28 liters.
Remember, with volume, multiply by the absolute pressure if you go up and
divide by the absolute pressure if you go down.
If you move from one depth to another, it’s easiest to take it first to the surface
and then back down to the new depth.
Example….
You take a balloon containing 5 liters of air from 35m up to 15m, what would its
new volume be?
5 x 4.5 = 22.5 liters at the surface, then take it back down to 15m
22.5 ÷ 2.5 = 9 liters at 15m.
A balloon is sometimes referred to a flexible container. A scuba tank is referred
to as an inflexible container – a scuba tank does not change volume or the
amount of air it holds when changing depth.
4
