PADI Divemaster Course Chapter 9
Review Questions and Correct Answers
Why do you need a solid understanding of dive theory? ✅The more fluent you are with
dive theory, the more easily you can diagnose a problem, because you better grasp the
underlying causes.
A solid understanding of dive theory helps you: ✅Answer diver's questions, solve
problems that aren't easily predicted or predefined, understand relationships/causes
and effects in diving, function as a dive leader.
Why does water dissipate body heat faster than air does, and at what rate does it do
so? ✅- Heat energy is the motion of molecules, a denser substance has more
molecules that must be put into motion, so it absorbs more heat for a given volume
*Denser than air = more mass (molecules) for a given volume
*Water absorbs more heat than the same mass of air and most other natural
substances as it forms polar (weak) bonds between its molecules that require energy to
overcome, so water requires more heat energy to raise a given temperature
*20 times faster than air does
What affect does water's ability to dissipate heat have on a diver? ✅*Conduction:
transmission of heat through contact
*Convection: transmission assisted by the flow of a fluid; the fluid in contact with the
body becomes warmer, making it less dense causing the water to rise, so cooler water
flows in to replace it, creating a continuos cooling cycle
-Contributes to cooling when you sit still in water when you're not wearing an exposure
suit.
-Average diver will be dangerously cold in half an hour in 16C/60F water and only
survive an hour in 4C/40F water - 27C/80F is an enjoyable dive temperature without
insulation (you can stay in this indefinitely with a dry suit and thick undergarment) even
in exposure suits you continue to cool
What does light do when is passes from air into water, or vice-versa, and how does this
affect a diver? ✅It refracts (bending of light), the speed of light travels changes,
causing it to alter direction slightly unless its path is perpendicular to the surface
-Magnifies by a ratio of 4:3 (fish thats 4 meters away appears to be 3 meters away) 25%
closer or 33% larger
-Most affected when light passes from water to the air space in your mask or camera
housing
-Visual reversal: particles in water that resembles the haze we see in air over long
distances, so it looks farther away than it is (lol i cant see that far anyways)
,Why does sound travel faster in water than air, and approximately how much faster is it
in water? ✅Sound travels slightly 4x faster in water than in air
-Its a mechanical energy (pressure wave or p-wave) that moves through matter
-The denser a material the better sound travels through it - elasticity of a substance that
determines how well sound travels through it - more dense = better elasticity (except
carbon and lead)
-Water has more elasticity than air, which is why sound travels faster through it
How does the speed of sound in water affect hearing? ✅The brain interprets sound
direction based on the slight differences in intensity and time of the sound that reaches
the ear
-Underwater these differences are indiscernible, so you perceive sounds as being
directly overhead
Water dissipates body heat faster than air does, because it absorbs heat approximately
______ times faster than air does. ✅20
Because water dissipates body heat faster than air does, a temperature that is
comfortable and warm for an unprotected diver in air will, for an unprotected diver,
✅Quickly become uncomfortable and cool in water.
Light _________ when it passes through a substance of one density into a substance of
a different density. ✅Changes speed, changes direction. (a. and b. but not c.)
Due to refraction, underwater objects appear to be magnified by a factor of _______.
✅4/3
Sound travels faster in water than in air, because water is denser (has more elasticity,
to be more precise) than air does: ✅True
Sound travels approximately _____ times faster in water than in air. ✅4
Because sound travels faster underwater, it is ________ to determine the direction a
sound is coming from while diving. ✅More difficult.
What is meant by ambient, absolute and gauge pressure? ✅Ambient pressure:
surrounding pressure, the pressure that exists surrounding something at a defined
moment, can be expressed as ambient or gauge depending on purpose (diver at 10
meters/33 feet ambient 2 ata/bar absolute or 1 ata/bar gauge)
Absolute pressure: total pressure (needed to understand effect of pressure on the body
and gases) ends in "a"
, Gauge pressure: uses 1 ata/bar as the zero point, used to determine useable pressure
(why its the pressure reading for gauges, dive computers and camera housings) ends in
"g"
What is the relationship between changes in absolute pressure and a gas volume?
✅As you increase pressure, volume of a gas exposed to it decreases and as you
decrease pressure, volume increases (why you equalize as you descend and breathe
normally and release air from BCD as you ascend)
*BOYLE'S LAW - volume of gas in a flexible container is inversely proportional to the
absolute pressure. If you increase the pressure a certain amount, you will decrease the
amount proportionally and vice-versa.
What is the relationship between depth and gas density? ✅Gas density changes in
direct proportion to absolute pressure.
-as pressure increases on a gas and the volume decreases, the gas molecules squeeze
together into a tighter space, making the gas more dense
-use it to predict effect on gas supply, regulator delivers gas at an ambient pressure,
deeper you go the volume is more dense so each breath requires more from cylinder -
why tech divers use helium mixes below 50 meters/165 feet since it has a lower density
-full cylinder lasts one hour at 10 meters/33 feet or 2 bar/ata, would last tow hours at the
surface - 40 meters/132 feet or 5 bar/ata would only last 1/5th as at the surface or 24
minutes (120/5=24)
-use pressure-volume not pressure-density relationships to predict the changes in air
supply duration because we are comparing volumes of consumption not densities of
ocnsumption
What is the relationship of the pressure, volume and temperature of gas? ✅If you raise
a temperature of a gas, either the volume will increase (in a flexible container) or
increase (non-flexible container) - why you don't leave a full cylinder in a car on a hot
day, pressure increases and may blow out the burst disk
-as you heat a gas you give the molecules more energy so they move more vigorously,
they either spread out (flexible container) or impact each other and walls (non-flexible
container) with more force - amount of heat energy is the same but confined it to a
reduced volume causing the temperature to go up
-rapid drop in cylinder pressure after entering cool water, because gas in cylinder
cooled - ice divers: avoid long bursts on inflator hose, don't test purge regulator before
entering water, may freeze and cause free flow
If a diver is in fresh water at 10.3 meters/34 feet, what is the ambient pressure? ✅2
bar/ata
A diver in seawater at 20 meters/66 feet brings a balloon of air to the surface. Assuming
the balloon will not burst, its volume will be _______ what it was at depth. ✅Three
times
