Advanced Nitrox Student Manual

ADVANCED
NITROX
TDI Advanced Nitrox
Chapter 1: Introduction to Advanced
Nitrox and Technical Diving
Fundamentals
Topics Covered in this Chapter:
• Introduction
• The Technical Diving Mindset
• The Approach
• Foundational Skills
• Breathing
• Application to this Course?
• Review Questions

Introduction to
Advanced Nitrox
This Technical Diving International
course will help develop the mindset
and practical skills necessary to
optimize Nitrox breathing mixtures for
divers. After completing the course,
participants will be able to make
choices based on operational and
logistical concerns in order to best
suit their mission parameters and
personal needs.
The Advanced Nitrox course is the

first step in taking scuba beyond the
usual sport-diving applications. A
diver may simply want to have the
option to use oxygen for his safety
stops during recreational sport dives.
This may be the first stop on the way
to Decompression Procedures
through to Trimix, or this could be part of a closed-circuit rebreather
(CCR) course.
Regardless of the application or goals, the importance of this course

cannot be understated. The use of oxygen and oxygen enriched air
mixtures is critical to advanced applications in diving. This course will
arm students with the knowledge and understanding of the risks and
benefits associated with these gases. They will then be able to make
their own informed choices about optimal breathing mixtures for their
planned dives rather than having choices dictated to them.
Participants in this Technical Diving International course will learn the

use of EAN21 through 100 percent oxygen to a maximum depth of 40
metres or 130 feet. Dives will be made without creating situations in
which decompression is required. This course will build a foundation for
diving with a mindset and approach that will be new to even advanced
sport divers and prepares them for the challenges of open circuit
technical diving or CCR diving.
A basic nitrox course
allows divers to use
EAN mixtures of up
to 40 percent
oxygen. This course
will expand those
choices to mixtures
up to and including
pure oxygen. It is
designed to give
divers the freedom to
optimize their
breathing gasses.
The beauty of a diver
continuing his education is that each new step gives him added freedom
to make new and more choices. With these choices comes the duty to
master new information and techniques so that Advanced Nitrox divers
can take full advantage of their newfound knowledge and skills.
This text will guide students through the process of the course, both as
a reference and guide. A Technical Diving International (TDI) instructor
will supplement the information provided here with local examples and
additional information. More importantly, he will guide divers in the
application of this material to their diving practices.
Students will find it best to skim through this manual and then read the
study questions at the end of each chapter. Then, reread each chapter
carefully, keeping these questions and objectives in mind. Using this
study method, students will be able to answer the study questions
easily. The TDI instructor will provide plenty of additional practice
problems for any student who feels they need additional practice prior to
taking the final exam. Following the exam comes the most exciting and
important section of this TDI course: the application of what has been
learned through actual diving.
This course is just the beginning of
the progression into more advanced
applications in diving. Continuing
education beyond this course opens
up many more exciting dive
destinations and helps develop
stimulating new skills and capabilities
for divers. Each additional level of
TDI training opens new opportunities
and worlds filled with amazing
expeditions and adventures.
Regardless of which path graduates

from this course opt to take following
this training, they will need to
continue to apply and practice the
skills (both in dive planning and
execution) taught in this course to
truly master them. A diver is only as
good as he has trained himself to be.
A smart and conscientious diver never takes his ability to dive to the full
extent of his training for granted. It takes work and regular practice to
refine and fully master the essential basic skills of breathing, buoyancy,
swimming, trim and awareness. A smart and conscientious diver always
works to sharpen foundational skills until they become a natural and
constant part of his dive practice.
The Technical
Diving Mindset
This course is the first step in the
development of a technical diver.
Several definitions exist that try to
describe what technical diving is, but
at TDI we feel that any dive requiring
a complete dive plan and sets well-
defined limits deserves the label. A
number of graduates from this TDI
Advanced Nitrox course may not plan
to conduct deep, long, complex dives
or dives in an overhead environment
but their approach to the way they
prepare for a dive and how they
conduct themselves underwater will
have undergone a change. They no
longer will simply throw on their dive equipment, jump into the water and
mindlessly follow what their dive computer tells' them to do. No matter
what goals participants in this course have at the outset, developing a
technical diving mindset, where limits for the dive are based on gas,
equipment, experience and realistic objectives, will be critical.

The Approach
Technical diving is mission oriented. Some occasional sport divers may
ask: "I just want to dive for fun, why should I care about that?" At TDI,
we feel there are a number of reasons. By developing a technical diving
mindset, the vast majority of divers will have more fun because less of
their mental energy will go into thinking about the basics of diving. That
mental energy can be used to actually enjoy and see more on dives.
With a mission oriented approach and a technical mindset, elusive
wildlife, the smaller details of shipwrecks, and many other aspects of a
dive that went unnoticed before will be obvious. To justify the value of
making the shift in mindset and attitude, one might say that divers who
do so get more out of their diving.
For anyone moving into technical

diving, it is critical to develop this
mindset as early as possible in the
training progression. By working
through this Technical Diving
International course with a qualified
TDI instructor, students will learn how
to apply the methods to their dive
planning and execution.
The foundation of this mindset is that

diving is a vehicle to access
something distinct and special
underwater. Diving becomes the
means to other ends, not just a
matter of paying attention to the
procedures of diving. This requires
training that allows diving activities to
become automated. It is known as
automaticity training. In technical diving, the goal should be access to
some sort of target, be it a wreck, scientific project, cave or some other
goal beyond oneself. Even if the only goal is to enjoy the dive,
experienced technical divers find that having a focus that lies beyond
being able to perform the dive enhances the experience greatly and
allows them to accomplish much more with each dive.
All this begins with examining how the diver thinks about diving. The
diver will want to create a mental picture for himself of what it means to
be an elite diver. The TDI instructor will help with this. An elite diver is
one that is in control of their diving at all times, a diver that does not
allow things to happen by accident. Such a diver is aware of their impact
360 degrees around them at all times, including their impact on the
environment as they move. This diver is able to control their dive,
position in the water, buoyancy, breathing, and the manner in which
they move through the water without having to consciously think about
it. The diver may find they already do many of these things, and they
might begin to
realize that many
of the other divers
they have looked
up to in diving
are not as elite in
their performance
as they imagined.
Either way, the
diver will want to
develop a mental
picture of the
exemplary diver he
wants to become.
If the diver builds this mental picture, he will rise to the level of the
expectation he has set. If the diver becomes clear on the level of
performance he is working toward, he will continue to work toward the
ideal he creates in his mind. He will have something to work towards on
every dive, constantly refining his skills and learning how to apply them
to each type of diving he does.
Diving will become the way the diver accesses the underwater
environment. He will be able to do things with his diving without
exhibiting any changes in his performance simply because he is doing
something extra in addition to diving. Handling a camera or placing a
stage bottle should not cause the diver to see a reduction in his diving
performance.

Foundational Skills
The foundational
skills for all diving
are buoyancy
control, swimming,
trim and breathing.
All of these in
combination allow
the diver to dive
efficiently and
effectively. Each skill
set builds on the
mental foundation to
allow him to control
his diving at all times. These skills work in combination, but also have
unique aspects when taken alone. Breathing will be discussed in detail
in the next section.
Buoyancy control is the hallmark of an elite diver. A diver can quickly

recognize how good another diver is just by watching his buoyancy
control. The diver should make sure that he is able to appear to have no
change in buoyancy when picking up or placing equipment. This means
he needs to think ahead when he plans on picking something up or
dropping a stage. Remember that it takes a few seconds for changes in
buoyancy to take effect. Allow time for these changes to occur.
The diver should be able to make most of his buoyancy adjustments

through lung volume changes when he is not making big depth
changes. This will help him to feel his way through his dives. His first
reaction to the need to rise should be to inhale and slow the gas release
rather than trying to inflate his Buoyancy Compensating Device* (BCD)
or even worse, using his hands to cause upward movement.
Optimizing buoyancy control will help the diver master all the other
techniques that will be learned as the course progresses. Since
rebreather diving does not allow lung volume changes to affect
buoyancy like open circuit diving, the diver will want to make sure he
pays close attention to lung volume variances and maintain control over
his equipment for tight buoyancy control.
How the diver moves through the water becomes even more critical
when he adds more equipment. Being able to move efficiently through
the water is the mark of an elite diver. The TDI instructor will help the
diver refine and learn more ways of using his fins to provide this
movement through the water. The diver should not use his hands to
maneuver in the water. The diver's fins are big engines to help him
move; when he uses his hands it is like stepping on the brake while
trying to accelerate in his car. It is just not effective. The diver's hands
are for holding things or keeping out of the way.
Too many divers rely on their hands to make slight adjustments and
turns. The problem is that when the diver has things in his hands, he
then no longer has the ability to control his diving. Do not become
dependent on bad habits to correct for lack of control. Work on using
fins to make positioning and fine-tuning adjustments. This may be very
difficult for some. It is a fun exercise to see if the diver can keep from
using his hands for an entire dive, except to make adjustments to his
BCD. Give it a try; it might be more difficult than imagined.
(*The terms Buoyancy Control Device and Buoyancy Compensation

Device are both commonly used by the diving community.)
It is important to realize that the diver may need to slow down in order to
speed up. As divers add more equipment to their system, they actually
need to slow down in order to allow the water they are moving through
the time to move past them. Drag is a function of speed. The faster the
diver tries to move, the more critical drag becomes. He could reach a
point where his speed is
actually slower because he
is trying to move too fast.
Swimming should allow for
some glide after each kick.
Most divers forget about
this part of the swimming
cycle.
Learn how to flat turn. Use

fins to make turns. Even in
a stationary position, the
diver should be able to turn
using the big engine of his
fins instead of his hands.
Buoyancy in combination with breathing will help the diver's swimming
become even more effective. The diver can make these fundamental
skills work for him in combination. If he stays slightly negative on
descent and angles slightly head down, he will find the planing of his
body will help with his forward movement, allowing him to swim much
less. It becomes an exercise in how to swim less. The diver can test
himself to see if he can fin less and do more.
Streamlining and trim will play directly into your attempts to minimize
effort. Water is heavy. If the diver can reduce his exposure to the water,
he will use much less energy to move through it. Small things can make
a big difference. Make sure that all equipment is streamlined and tucked
in - no danglies. Not using
hands will keep them from
increasing drag; paying
attention to body position
will as well.
An ideal working position

while diving should be
horizontal in the water with
a slight arch to the back.
The arch in the back will
keep the diver from having
to strain his neck to be able
to see forward. A slight bend in the legs will help prevent the lift from
looking up and keep fin wash away from the bottom. If the diver swims
with a dramatic head up position, he greatly increases the area of his
body that contacts or "sees" the water as he moves forward. Even small
increases can double the amount of work it takes for him to move
through the water. Remember that water is heavy; the diver wants to
move as little of it as possible when he travels.
The diver will work with the TDI instructor to optimize their equipment
configuration. The easier it is for him to have a balanced position in the
water, the less work it will take to be an elite diver. Rebreathers can
change the center of buoyancy a great deal due to their counter lungs.
Make sure to adjust weights to compensate for shifts in buoyancy
throughout the breathing cycle. Do not discount the importance of good
streamlined trim while diving a rebreather; too many rebreather divers
do.
Breathing
Breathing is the primary skill from
which everything else evolves. It is the
cadence of the dive and will give the
diver the most immediate feedback as
to how his performance is progressing.
Mastering ideal breathing will have
benefits for all areas of diving and it will
become a critical survival skill as the
diver progresses into deeper diving.
Seventy percent of gas exchange

occurs in the lower third of the lungs.
Most people believe they breathe
correctly in life as well as on scuba.
Divers have all been breathing their
entire lives but most have not been
doing so correctly, especially when it
comes to breathing for scuba with the
increased gas densities and dead
spaces.
Ideal breathing will provide the best

gas exchange possible by allowing the
gas the diver breathes to spend time
where it needs to be for oxygen to be
absorbed. Divers have all heard
"breathe deeply and slowly," but few
have ever actually learned what that
really means.
When the diver dives, he will want to fill his lungs by drawing gas in
beginning from the bottom of his lungs and then letting it out from the
top. Fill from the bottom and empty from the top. Initiating breathing with
the diaphragm does this.
The diver wants to pull his diaphragm down and away as if he is trying
to have a potbelly, pulling the gas deep into the lungs. About half way
through this dropping of the diaphragm he will want to blend in the fill of
his chest, filling his lungs until they are comfortably full. He should then
pause and begin to release the gas slowly, thinking about keeping the
lower part of his lungs filled as long as possible. Placing his tongue
against the roof of his mouth will help slow the release of the gas. The
release of gas should take longer than
his inhalation.
This method is technically known as

diaphragmatically initiated breathing.
For our purposes, it will be called ideal
breathing. It can take a while to learn
how to breathe with the diaphragm.
Your TDI instructor will demonstrate.
The diver will want to work on

regaining ideal breathing when he finds
he is not doing it. Correcting his
breathing when he is outside of ideal is
almost as important to learn as how to
breathe properly. Once he has
mastered ideal breathing, he will be
able to recognize the instance when modifying ideal breathing is
necessary.
The control that comes with having good full breathing allows the diver
to recognize ways in which breathing can impact his diving. If he is
trying to maintain a tight hover, he will learn to modify his breathing to a
tighter control of volume, shortening the cycle. If he finds himself losing
control of his descent, he will begin to correct that by taking in a large
breath and slowing his release of gas and perhaps even cycling his
breathing more quickly, keeping lung volume high, until he is able to add
air to his BCD. The reverse holds true if he were to notice that he was
positively buoyant; he would cycle through his breathing, keeping a
lower lung volume. These adjustments to breathing will have large
impacts when done as a first step to correct breaks in performance.The
diver will find that by leading with his breathing he is able to adapt to
situations, reducing or eliminating their impact on his performance.
It is okay to deviate from ideal breathing when it is by choice. The diver

wants to be sure to spend the time to really learn what ideal breathing
is. He can only deviate by choice from what is ideal if he has mastered it
first. The goal is to have his actions every moment be his choice, not an
accident.
Diving a rebreather is different. If the diver is going to be diving on a
rebreather, this ideal breathing is not ideal. He will still want to begin his
breathing with his diaphragm, but he will want to make his breathing
cycle a bit shorter. The idea is to keep gas moving more often. He still
will want the gas to be drawn deeply into his lungs and dwell there, but
because he is not losing gas to the outside, his breathing rate can be
faster.

Application to this Course
Work on mastering
these four areas of
fundamentals:
buoyancy control,
swimming, trim, and
breathing. This will
help a great deal
once you begin to
use diving to do
other things. You will
find that your diving
gets better in any
application. Divers
now have a set of
fundamentals they
can work on at any time, regardless of what mode of diving they are
using.
The idea is that the basic diving skills should remain unchanged even
while performing other activities. This is the mark of an elite diver. The
diver must keep asking himself, "Am I able to do anything I choose to do
without affecting my diving performance?" This foundation will help
insure that you control your diving rather than having your diving control
you.
Work to optimize each fundamental skills set. The addition of new skills
and more equipment is likely to impact performance, if the diver has not
mastered the basics first. The diver must work with his TDI instructor to
begin to become an elite diver. It will payoff in all areas of his diving.

TDI Advanced Nitrox
Chapter 2: Diving Physics, Gas Laws
and Formula Work
• Oxygen
• Relevant Gas Laws
• Boyle's Law
• Dalton's Law
• Maximum Operating Depth, MOD
• Partial Pressure
• Equivalent Air Depth

Oxygen
This course is about oxygen and how to use this gas to optimize diving
and manage the risks and issues associated with its use.
Oxygen is a tasteless and odorless gas. It supports combustion and our

brains need it to stay alive. But, as with so many things, it has its costs
as well as its benefits to divers. It is naturally occurring in the air that we
breathe everyday in a concentration of 21 percent. Thankfully, most
friendly neighborhood gas suppliers are able to readily supply pure
oxygen on demand in gas or liquid form, making the lives of divers and
the staff at the local dive center even easier.
Oxygen is a diatomic molecule. This means is has two oxygen atoms

bonded together that make up the oxygen molecule we breathe. At the
temperature in our atmosphere, this molecule of oxygen is very stable
and it tends to react readily with many other compounds and tissues of
the human body.Discussion of the dangers of oxygen and how it
supports combustion will be covered later.
Within the body, oxygen brings life. Life does not last very long without
it, about four to six minutes for the average human being; double that if
you are a world-class freediver. Either way, it's still not very long.
Oxygen is critical for aerobic metabolism and supports almost every
bodily process. However, too little or too much of it can be a problem.

Relevant Gas Laws
There are really only two gas laws that need to be of concern for this
course. One is Boyle's Law and the other is Dalton's Law. Both are
named after the early chemical scientists who discovered them. It is not
important to know who these guys were; what matters is having an
understanding of the basic ideas that each was presenting.
Boyle's Law concerns itself with the relationship between pressure and
volume when temperature is constant. This law describes the inverse
relation between volume and pressure. Boyle's Law shows that during
decent, as a diver goes deeper and more gas is needed, volumes get
smaller. Boyle's Law is a fundamental topic presented in open water
diver training.
The golden rule of diving - Never hold your breath! - is tied to Boyle's Law.
Dalton's Law reflects the pressure of a gas in a gas mixture. It states

that each gas in a mixture will have a pressure that is directly related to
its fraction in the mixture. In other words, partial pressure of a gas is
equal to its fraction of the total pressure of the gas. Dalton's Law is the
law that allows the diver to calculate where and how much oxygen, or
any other gas for that matter, they can dive with. Divers are introduced
to the work and application of Dalton's Law in their Technical Diving
International basic nitrox course.
Boyle's Law allows a diver to calculate the pressure they will experience
at any depth, the amount of gas that will be consumed at that depth and
how much volume will increase if they ascend from one depth to
another. Pressure volume relationships tie directly to how pressure acts
on a diver as they progress through their dive.
Dalton's Law is used to calculate maximum operating depth, best mix,

and partial pressure calculations. It is also used for gas blending
calculations. How much of each breathing gas a diver absorbs is tied to
Dalton's Law. Diving exposures and decompression theory tie directly to
calculations made with this law.
Equivalent Air Depth, EAD, is a way for the diver to use any air dive
table to calculate dive profiles. Diving profiles and decompression
theory is tied to the partial pressure of the inert gasses a diver breathes
throughout their dive. The use of increased oxygen in a nitrox mixture
reduces the inert gas taken into the diver's body. So, diving while taking
up less nitrogen is physiologically the same as diving air at a shallower
depth. EAD allows the diver to calculate exactly what this depth would
be.
Formula
Work
Do not let the math
of diving intimidate
you. Physics is
simply a scientific
way to explain what
divers experience on
every dive. The
reality is that divers
are armed with all
they need to know in
order to do these
calculations. Being
able to make these calculations by hand is very important for developing
a sense of intuition about the numbers of diving. In more advanced
forms of diving this can literally be a survival skill. If there is an
emergency, the ability to be intuitive with the numbers allows the diver
to think on their feet.
There is a saying that goes "unless there is a number, the truth is not
known." Perfecting the math of diving is necessary but it doesn't need to
be intimidating. For any diving math problem, do not memorize
formulas. Most of what needs to be calculated can be thought through
logically. Take the problem diving before putting the numbers in.
Understand what is being asked and imagine what would be happening
in the water.
Boyle's Law
Boyle's Law speaks to the pressure volume relationship. An Advanced
Nitrox diver needs to be able to calculate breathing gas needs and other
aspects related to this law.
P1 V1 = P2 V2
Metric Example:
A diver diving to 35 metres will use gas how much faster than at the
surface?
35 m = (35 + 10) m / 10 m/ata = 4.5 ata

P1 = 1 bar
V1 = ?
P2 = 4.5 bar
V2 = 1
1 V1 = 4.5 (1)
V1 = 4.5 times more gas
Imperial Example:
A diver diving to 115 feet will use how much more gas than at the
surface?
115 ft = (115 + 33) ft / 33 ft/ata = 4.48 ata

P1 = 1ata
V1 = ?
P2 = 4.48 ata
V2 = 1
1 V1 = 4.48 (1)
V1 = 4.48 times the gas
Dalton's Law
Dalton's Law is the basis for most of the nitrox calculations.
Pg = Fg x P
Pg is partial pressure of the gas.

Fg is fraction of the gas.
P is total pressure.
The law can easily be represented by a T formula. A T formula is a

representative of an equation that allows for easy reference to visually
recognize what calculation needs to be made.
A simple saying can be used to remember the T formula. A pig flies over
a frog in a pond.
Pg / (Fg x P)
The T formula works by covering the item that needs to be calculated.

The remaining components are what need to be calculated for this
result.
These are the most common calculations that need to be done by the
Advanced Nitrox diver.
Best Mix
Best mix calculations are made to plan for breathing gas needs for a
specific dive. This course is about making use of optimum breathing
gasses for any dive.

Fg = Pg/P
Metric Example:
If a diver wishes to make a dive to 35 metres, what is the best mixture
forthis dive?
Fg = 1.4 bar / 4.5 bar

Fg = 31 percent
Imperial Example:
If a diver wishes to make a dive to 115 feet, what is the best mixture for
this dive?
Fg = 1.4 ata / 4.48 ata

Fg = 31 percent
Maximum Operating Depth, MOD

Maximum Operating Depth, MOD This calculation determines the
maximum depth to which a particular mixture can be dived. This is a
common calculation made when signing out a mix from a dive center for
their fill logs.
P = Pg/Fg
A diver wants to know how deep he can dive with a mixture of 29

percent oxygen nitrox.
P = 1.4 ata / 0.29

P = 4.83 ata / bar
Metric Example:
P = (4.83 - 1) 10 = 38.3 m
Imperial Example:
P = (4.83 - 1) 33 = 126 ft
Partial Pressure
This calculation is made to determine the partial pressure of a gas being
breathed by the diver. This calculation is necessary for oxygen
exposure calculations and narcosis exposure.
g = Fg x P
Metric Example:
A diver wants to know how much oxygen he will be exposed to if he
dives to a depth of 27 metres with EAN 33.
P = 27 m + 10 metres / 10 m/bar = 3.7 bar

Pg = 0.33 (3.7 bar) = 1.22 bar O2
Imperial Example:
A diver wants to know how much oxygen he will be exposed to if he
dives to a depth of 92 feet with EAN 33.
P = 92 ft + 33 ft / 33 ft/ata = 3.79 ata

Pg = 0.33 (3.79) = 1.25 ata O2

Equivalent Air Depth
This calculation allows for the use of any air dive table with any mixture
of Nitrox:
Metric Example:
If a diver dives to a depth of 30 meters with EAN 32, what is the EAD for
this dive?
Imperial Example:
If a diver dives to a depth of 100 feet with EAN 32, what is the EAD for
this dive?
TDI Advanced Nitrox
Chapter 3: Gas Physiology
• Hypoxia
• Oxygen Toxicity
• CNS Oxygen Toxicity
• Pulmonary Oxygen Exposure
• Single Dive versus Multiple Dives and Days
• NOAA 24-Hour CNS Limits
• Nitrogen Concerns
• Carbon Dioxide Toxicity
• Carbon Monoxide Toxicity
• Compliment Complex
Hypoxia
Oxygen is necessary for life; however, the partial pressure of oxygen
that can be breathed for prolonged periods of time is limited to a small
range. Too little oxygen is known as hypoxia. Air has 21 percent oxygen
in it. On the surface, this translates to 0.21 ATA of oxygen. Breathing a
pressure of oxygen less than 0.21 ATA of oxygen is breathing a hypoxic
mixture.
Trimix and rebreathers are usually the only places where a diver would face such a
risk. Partial pressure of oxygen below 0.16 ATA can prove hazardous. Levels below
0.12 ATA can prove fatal in just a short period of time. The smaller the pressure of
oxygen, the greater is the risk.
Breathing a gas absent (anoxic) mixture can shut down respirations

entirely. Regardless, if there is not enough oxygen in the body, the brain
will be impaired in its function and ultimately result in unconsciousness.
Closed circuit rebreathers carry with them the risk of progressing into
severe hypoxia if oxygen is not being added to the loop. In shallow
water, the risk is increased as partial pressures of oxygen can drop
rapidly. During ascents, this impact can be accelerated due to the drop
in ppO2 caused by the ascent.
Oxygen Toxicity
Oxygen Toxicity refers to the opposite issue the diver may face with
oxygen, that of hyperoxia. Just as too little oxygen can be dangerous,
too much can prove to be a problem as well.
If the diver breathes a partial pressure of oxygen above 0.5 ATA, the diver must
track oxygen exposure for each dive and for multiple dives throughout consecutive
days.
Sport scuba diving with air does not usually expose the diver to higher
levels for long enough periods of time to have to worry about tracking
oxygen exposures. However, in technical diving the exposure from air
does need to be considered as it could pose risk due to extended
exposures and the use of decompression gasses.
Generally, oxygen toxicity is considered to fall into two areas â“

short term high dose exposures and long term lower dose exposures.
Short-term high dose exposures affect the diver's risk of having the
most dramatic problem with diving mixed gasses known as Central
Nervous System Toxicity (CNS Toxicity). This is tracked as a
percentage of the allowed dose for each dive and the total of each day
of diving.
The other form of oxygen toxicity is long term lower dose related
problems. The diver tracks this exposure with the assistance of Oxygen
Tolerance Units, OTUs. These are used to track whole body /
pulmonary exposures.
CNS Oxygen Toxicity
The dive community recognizes a partial pressure of oxygen at 1.4 ATA
to be the maximum exposure allowed for the working portion of most
dives, whether they are sport dives or technical dives. An exposure as
high as 1.5 or 1.6 is acceptable for the decompression phase of a
technical dive and also for conducting short duration sport dives in calm,
warm conditions. But it is critical to always associate oxygen partial
pressure to exposure time to arrive at a CNS dose.
Tracking exposure to higher levels of oxygen is critical as the

consequences of having a problem with CNS toxicity can be very
devastating. Signs and symptoms of CNS oxygen toxicity need to be
recognized and quickly dealt with. Ignoring early signs of CNS oxygen
toxicity issues can result in the diver having a seizure. A seizure can
easily result in the diver drowning or embolizing as a result of an
uncontrolled ascent.
Beyond a seizure, there are several other signs and symptoms that may
present themselves during any issues with oxygen. An easy memory
device for remembering these signs and symptoms is the term
conVENTID.
Among US Navy divers, nausea has been the most common symptom
reported beyond convulsions. The problem is that a diver can have no
other sign or symptom other than a convulsion or progress so quickly
through any other symptoms that there is no time to take action prior to
a convulsion. Diving conservatively and well within the allowable limits is
critical for minimizing the risk of a convulsion. There is no resistance or
preparation that can be conducted to acclimate to oxygen exposure.
These limits should be carefully tracked and respected.
Particular partial pressures of oxygen carry with them exposure limits
that were set forth by the National Oceanic and Atmospheric
Administration, NOAA.
The exposures are a time dose concept: the higher the pressure of
oxygen to which the diver is exposed, the lower the allowable exposure
time available to the diver. This exposure is accumulated throughout the
dive. While diving open circuit, this exposure changes throughout the
dive and is calculated for each pressure of oxygen experienced. The
mixture the diver breathes is a fixed fraction of oxygen, but variable
pressure of exposure. While diving closed circuit, the rebreather
maintains a constant pressure of oxygen and therefore, the exposure
generally remains the same throughout the dive. The mixture the
rebreather diver breathes is fixed pressure, but variable fraction of
oxygen throughout.
The most critical calculation is that for CNS oxygen exposure, as the
results of having an issue in this area have the greatest risk of
immediate major consequences. The calculation is very simple. Time of
exposure is divided by total exposure allowed for each pressure of
oxygen to which the diver is exposed. This provides the decimal
equivalent of a percentage. This number can simply be multiplied by
100 to attain the actual percentage. All the percentages experienced for
the dive are then added up. This allows exposures at different pressures
to be accounted for by a number as a function of its percentage of the
total allowed. This makes comparisons easy no matter what pressure of
oxygen is being experienced by the diver.
For open circuit diving, generally the exposure is calculated for the
maximum depth of each phase of the dive and then for each
decompression stop. For a sport dive, the calculation would be based
on the maximum depth for the entire time of the dive. For a sport
rebreather dive, it would simply be the time exposed at the chosen set
point (pressure of oxygen) for the total time of the dive. Modern dive
computers that are capable of nitrox functions calculate this exposure in
real time, giving the diver all the
advantages of real time tracking of
oxygen exposure as they do for
decompression tracking.
It is wise to still plan dives by

calculating exposures and all other
aspects of the dive by hand or via
desktop software. In technical
diving, this is critical.
If a diver is exposed to 1.4 ata of

oxygen on a 40 minute dive, the
resulting exposure would be:
40 minutes / 150 minutes x 100 = 26.7 percent CNS exposure
In nitrox class, you simply added these percentages up throughout the

day of diving. At this level, accounting for a washout of oxygen or a
resetting of the clock is acceptable. It is generally considered that
oxygen exposure and a washout effect takes 90 minutes for half of the
exposure to be recovered.
So, if a diver completes a dive with 40 percent oxygen exposure and
has a surface interval of 90 minutes following that dive, the diver would
enter the water with 20 percent exposure beginning the repetitive dive.
This exposure needs to be accounted for of course. Oxygen exposure
washout is a way to more closely monitor the exposures being faced in
technical and rebreather diving, which tend to be higher than with sport
nitrox diving.
Pulmonary Oxygen Exposure

Whole body or pulmonary oxygen exposure is generally related to lower
dose and longer durations of oxygen exposure. Although the issues
associated with this form of exposure are fewer, they should still be
accounted for.
Oxygen breathed over 0.5 ata for prolonged periods of time will begin to
impact the tissue of the lungs and body. This form of oxygen toxicity is
due to the irritation and reactivity of oxygen with the body's tissues, in
particular in the respiratory tract. Exposure is measured in Oxygen
Tolerance Units (OTUs).
A diver suffering from pulmonary effects of oxygen will have some difficulty
breathing, pain on inhalation, dry unproductive cough, and soreness in the lungs
and/or throat.
This effect is cumulative. Once the process begins, it will not get better
until a break from higher pressures of oxygen is taken. If higher
pressures of oxygen are continued, the effects will worsen. It is highly
unlikely that a planned dive will exceed OTU limits in a single day; it is
far more likely that CNS limits would limit the exposure prior to OTUs
doing so, but the allowed exposure drops for repeated days of diving.
Also, tracking of this exposure becomes more important when diving
rebreathers because they allow for fixed pressures of oxygen to be
breathed for hours.
Oxygen Tolerance Units, OTUs, is the measure divers use to track long
term whole body oxygen exposure. The calculation is simple and is
based on time of exposure at a given pressure. There is no washout
accounted for with this calculation. The numbers are additive for each
day and from one day to the next.
Although the risks of Pulmonary Toxicity are minimal among technical

divers executing dives at the level covered in this program, it is good
practice to understand the possible effects of Pulmonary Toxicity and a
couple of methods to track its accumulation.
One OTU is the equivalent of breathing oxygen at atmospheric pressure

for one minute. One formula to work out the pulmonary dose in Oxygen
Tolerance Units is written:
OTU= Tx (0.5/(PO2 - 0.5)) -0.833
Where Tx represents Time in whole minutes and PO2 represents

Oxygen Partial Pressure in BAR or ATM. If you are familiar with the
workings of exponents and have a scientific calculator at hand, using
this formula delivers the most accurate results; however, for brevity the
following table may be used.
Example: To calculate the OTU loading for a dive for 60 minutes

breathing a bottom gas delivering an oxygen partial pressure of 1.3 bar,
and a 5-minute safety stop breathing a gas delivery 1.6 bar of O2
pressure. OTUs for the bottom portion of the dive equal 60 times 1.479
(which is 88.74 units). OTUs for the safety stop equal five times 1.928
(which is 9.64). Ascent time between the bottom and the safety stop
would be made with a decreasing oxygen partial pressure and so a
"fudge factor" can be used. One can half the distance between the
bottom depth and the depth of the safety stop; find the oxygen partial
pressure delivered by the mix at that average depth and multiply by the
number of minutes spent ascending. If the oxygen partial pressure is
less than 0.6 bar, disregard. Another method for dive planning at the
level covered by this course is to simply add 20 units to cover ascents
from any dive. This covers dives from a maximum depth of 45 metres at
a standard speed of 9 metres per minute (30 feet per minute) with
contingency for deep stops.
Single Dive versus Multiple Dives and

Days
Single Dive versus Multiple
Dives and Days A one-day
dose of 850 OTUs is thought
to reduce vital capacity by
approximately 4 percent,
which has been adopted
within the technical diving
community as an acceptable
risk. Of course, 850 OTUs is
an impractical outcome for a
normal day of diving since it
represents 850 minutes at
one bar or approximately 440
minutes breathing oxygen at
1.6 bar! Clearly, Central
Nervous System (CNS)
Toxicity and not Pulmonary Toxicity would be the concern.
For multiple day levels of Pulmonary Toxicity, the diving community as a

whole, and Technical Diving International specifically have arrived at a
total daily acceptable dose of 300 OTUs.
NOAA 24-Hour CNS Limits
An important limit for multiple day, multiple dive tracking of Central
Nervous System (CNS) Toxicity is the NOAA 24-hour limit. NOTE:
Tracking of NOAA's 24-hour limit does NOT allow reduction of dose
according to any half-time calculations. In other words, 24-hour values
for each dive are calculated and added together to discover the total
daily CNS toxicity levels.
Daily limit tracking is essential when multiple dives are planned and is
particularly important for divers doing trips where the first dive of day
two can easily be less than 12 hours after the last dive of day one (on a
live-aboard for example).
Nitrogen Concerns
Increased oxygen levels provide for a reduction in nitrogen levels when
diving nitrox. Optimizing breathing mixtures during this course and when
diving after the course will help make decompression and available
bottom time as optimal as possible.
Ideally, dives should be conducted with the deeper portion of the dive
occurring first. Saw tooth profiles or dives with many big swings in depth
should be avoided. There is no evidence that conducting deeper dives
after shallower ones adds risk, but careful consideration should be given
to dive planning and available bottom times.
There is some evidence that dives of longer duration provide some

protection against more serious decompression risk. Avoiding deep
spikes and bounce dives may provide protection against more serious
impacts from a decompression incident.
For no stop diving, the ascent is an area where diver performance can
lead to a better, more conservative profile. The Divers Alert Network has
found that the average ascent rate for divers after they have completed
their safety stop is 60 metres or 200 feet a minute. Remember that the
dive is not over until hours after the diver is back on the surface. Adding
a surface delay prior to exiting the water, if possible, may also provide
added benefits.
The use of nitrox does not eliminate the need to plan available bottom
times based on nitrogen. The risk of having a decompression incident is
the same if nitrogen exposure levels equal those of an air dive.
Increasing oxygen will provide longer available bottom times over air,
but remember the need to still "plan your dive and dive your plan."
Being conservative and planning for contingencies is part of responsible
diving.
All dives are decompression dives, though some do not require stops
and some do. The risk of decompression illness is always present. All
divers should plan for the eventuality that they may be faced with such a
DCI event by:
• Being trained in first aid and oxygen administration.

• Ensuring that emergency oxygen is available for all dives.
• Carrying diving insurance.
• Continuing your education in rescue diving.
• Planning for decompression incidents on all your dives.
Remember, being prepared can turn a not-so-good situation into a not-

such-a-big-deal situation.
Besides the risk of decompression

illness, nitrogen can also become
narcotic at depth. If a diver plans a
dive with a limit of 1.4 ata for oxygen
exposure, planning for nitrogen's
narcosis exposure is equally
important. The generally accepted
range for nitrogen narcosis exposure
is between 4.0 and 5.21 ata of N2.
Some divers may choose less of an
exposure and some may choose
more. There is no set rule for accepted
nitrogen exposure from a narcosis
standpoint. Each diver must look at the
planned activity, previous experience and training, the dive
environment, and any other factors that may be complicated by
increased narcosis. Diver performance also plays a big role in the
impact of narcosis on a dive.
Narcosis is characterized by the suppression of mental activity of the
brain. Often spoken about as a drunken feeling, narcosis will generally
amplify the existing state of mind of the diver. If the diver is nervous,
that feeling will worsen. If the diver is relaxed, narcosis will tend to make
them feel even more relaxed. Task fixation and lack of general
awareness can complicate dive plans that require a clear head. For
dives in overhead environments (cave, wreck and ice), 4.0 ata N2
should be considered the maximum narcosis exposure that is
acceptable. This can be true for cold and dark waters as well. Some
divers may choose to have even less exposure for highly detailed dive
plans or complicated missions.
Oxygen is thought to carry with it narcosis properties as well, perhaps even slightly
greater than that of nitrogen. The easy rule of thumb is to not dive nitrox deeper than
you would dive with air, assuming all other limits are respected.
Carbon Dioxide Toxicity

A great deal of research has begun to point to carbon dioxide, CO2, as
playing a major role in complicating narcosis issues as well as oxygen
toxicity and decompression problems. Carbon dioxide is the resulting
byproduct of human metabolism. When we take in oxygen the body
uses the oxygen for metabolic processes; when we exhale, carbon
dioxide produced by our metabolism is transferred to the lungs and
leaves.
For open circuit diving, carbon dioxide is thrown out with our exhaled
breath to the environment. However, if the diver is not breathing
properly, is overexerting, has poorly performing equipment or is
retaining CO2, the resulting effects can be
dramatic.
Human respiration is keyed to CO2 levels

in the blood. If CO2 is high, the body is
going to begin to create a desire to
breathe more rapidly. This desire often
does not lead to better breathing
techniques, so the resulting breathing only
increases CO2 levels, creating a feeling of air starvation that can lead to
panic.
For rebreather divers, the exhaled gas of the diver is recaptured and
CO2 is chemically scrubbed out of the loop. A failure of the scrubber
can increase CO2 levels artificially. Generally, the onset of feeling air
starved is more rapid and can be quite profound in its effect. It is not a
comfortable feeling.
Carbon dioxide also poses a narcosis risk. CO2 is more narcotic than
nitrogen and often has a more dramatic narcosis, creating less pleasant
effects as compared to that of nitrogen. This narcosis can act in addition
to any other narcosis present and amplify the effect.
Carbon dioxide also increases decompression risk, and complicates

and increases oxygen toxicity risk. Basically, having too much carbon
dioxide present in the body is a bad thing. As dives get deeper and work
levels go higher it is critical to use the best possible equipment and train
to maintain ideal breathing throughout any workload. If diving on a
rebreather, scrubber packing and duration logging are critical steps to
reduce risk. Everything should be done to minimize exposure to
increased carbon dioxide levels at all times while diving.
Carbon
Monoxide
Toxicity
Carbon Monoxide, CO, is the
byproduct of incomplete
combustion. Generally, the
way a diver becomes
exposed to CO is from a bad
gas source such as exhaust
becoming entrained in the air
intake of a compressor or
combustion occurring within
the compressor itself. The effects of CO toxicity can be profound, as CO
binds to the red blood cells of the body with a much greater affinity than
oxygen. CO toxicity is essentially metabolic suffocation.
Carbon monoxide toxicity can be characterized by headaches, nausea,
vomiting, altered level of consciousness, and in very severe cases
cherry red lips and nail bed. The latter is not likely to be seen in a diver.
If a gas supply tastes funny or if, while diving, the sense of not feeling
well becomes greater throughout a dive or at greater depths, it is worth
considering the quality of the breathing gas. When CO toxicity is
suspected, end the dive immediately. Even better to catch the issue
prior to entering the water. Severe CO toxicity can result in
unconsciousness and even death. A reputable gas supplier is the best
insurance against issues with bad gas.
However, even the best facility can have a compressor failure or a damaged filter. It
is a good idea to always smell and taste a breathing gas prior to entering the water.
Compliment
Complex
There is some evidence that
part of the reason
decompression illness issues
are so complicated is
because the body reacts to
the disorder as if the body is
being attacked more so than
when it has sustained an
injury. This is in part why it is
now referred to as an illness.
When bubbles form in the
body, the body appears to have an immune response to this foreign
body within it. The body then attacks the gas bubbles as if they are an
illness. This creates an immune cascade that can occur throughout the
body, not just at the point where the bubbles have occurred. Thus, the
presence of even a small number of bubbles can create an even larger
whole body reaction.
The movement of platelets and thickening of the blood when an incident

occurs can complicate the body's ability to deal with decompression
stress. Staying well hydrated, being in good health, and avoiding the
problem to begin with need to be a focus. Part of the acceptance of risk
in diving is being prepared for issues and working to prevent them.
The body's immune system seems to play a role in most of what has
been discussed in this chapter. The best measure of protection from this
response is to avoid its activation. The only sure way of doing so is to
be conservative and keep the risks in mind. The simple act of hydration
following a dive may mitigate developing issues.
TDI Advanced Nitrox
Chapter 4: Making Nitrox Work and
Dive Planning
• Computer Generated Dive Tables
• Personal Dive Computers
• Programmable Dive Computers
• Carrying a Backup Computer
• Planning Software
• Gas Planning
• Respiratory Minute Volume (RMV)
• Oxygen Planning
• Nitrogen Limitations
• Thermal Considerations
Computer
Generated Dive
Tables
Computer generated dive
table programs are very
common. These programs
allow detailed dive planning.
However, these programs
are limited in that they
provide exact information for
the profile entered; they do
not allow for any flexibility or modification. Learning to use the software
and making sure to integrate conservative procedures into diving
planning are critical.
Personal Dive Computers

Personal dive computers now allow multiple gas mixtures and greater
flexibility in dive planning. They also allow dive plans to be modified in
real time during the actual dive. It is important to make sure that any
decision made to modify an existing dive plan is done so by taking into
account all of the dive plan parameters originally planned for. During "no
stop" sport diving, these decisions are easier to make. During technical
dives, these decisions based on the dive computer could create issues
with other areas of the dive plan. For example, extending bottom time
could require more decompression than the gasses carried would allow.
Programmable
Dive
Computers
These dive computers are
programmable. Some
interface with a personal
computer and others use
controls on the dive
computer. Some even allow
for computer-based software
to be brought into the dive
computer. It is very important
to make sure that the gas
mixture being dived is
correctly entered into the
dive computer. Most of these
computers now allow for the
user to increase the level of conservatism the computer will use.
Because dive computers provide real time information based on the
actual dive profile of the diver, there is not much room for pushing the
limits of the computer. When the computer reads zero remaining bottom
time, the diver is at the maximum exposure allowed by that computer. It
is important to plan on leaving some room in the dive profile to account
for safety.
It is also important to understand that simply because a dive computer

provides a readout of a possible dive, that does not mean the profile
should be dived. Dive computers are simply a planning tool. They are a
very valuable tool, but they are not a replacement for common sense.
Each diver must learn how to use all of the features of the computer
they choose to dive. Many dive computers now provide a great deal of
information. It is important to read the user manual and become familiar
with all of the computers functions.
Carrying a Backup Computer
It is important to carry a backup for every planning tool and gauge. For
technical diving they are required. Matching the backup computer to the
primary computer for decompression model and function is a good idea.
For sport diving, a dive computer failure simply ends the dive. However,
it also ends the diving day if no backup was in use. It is a cheap
investment to carry an additional computer to backup the primary when
on an extensive dive trip in order not to lose a day of diving.
Planning Software
Sport dive planning is usually pretty casual.
Most divers check their gas supply and
follow their computer. Entering the technical
level of dive training should bring with it a
higher level of dive planning. This does not
have to take a great deal of additional time.
However, working dive planning by hand
and gaining a familiarity with the numbers is
a very good way to build an intuitive sense
for how different decisions will affect the
numbers. Planning software is a great tool
to aid in planning, but it is not a substitute for understanding where the
programs get their numbers from and executing calculations on your
own. The skills you practice and perfect will provide a very good check
for recognizing a potential problem when a program has produced
numbers that do not make sense.
For technical and rebreather diving, dive planning takes on an even

more detailed form. Whatever type of dive is being conducted, until a
number is secured the diver does not truly know what is needed for the
dive. A set system for dive planning should be developed so no step is
neglected or omitted. Dive planning must account for all gas
requirements, oxygen limits and calculations, nitrogen limits and a
strategy for tracking the dive, thermal considerations, and any other
relative aspects that are needed in order to make the dive.
Gas Planning
For sport diving, gas

requirements are
relatively simple.
However, with the
added available no
stop dive time, gas
planning becomes
more important as
gas supply is more
likely to limit the
duration of the dive.
Most divers simply
follow a basic plan
for ensuring they
return to the exit of the dive with enough gas to conduct a safety stop
and safely make it out of the water. If there is an issue with gas supply,
the diver can cut the dive short at any time because it is no stop diving.
It is important to note that it is not a good idea to cut a safety stop short
simply to come back to the boat with a required amount of gas
remaining in the cylinder. If the diver has cut into their reserve of gas, it
is better to conduct a proper safety stop and be in trouble with the dive
staff rather than cut short or not do a safety stop. It is best to simply
follow the plan for both.
Respiratory Minute Volume (RMV)

For technical and rebreather diving, gas planning takes on a greater
importance. Running out of gas on a technical dive can cause injury or
worse. For a rebreather diver, running out of oxygen without being
aware of it can lead to unconsciousness. So, proper gas calculations
become critical. Even a sport diver can benefit from understanding and
performing these calculations. For open circuit diving, the first issue
becomes understanding how fast a diver consumes the gas they
breathe. It is easy enough to figure out.
The process is known as Respiratory Minute Volume, RMV. This is a fancy phrase
for the amount of gas the diver moves in and out of their lungs in one minute.
This number will change depending on the equipment used, the

temperature of the water, the amount of diving that has been done by
the diver, and whether it is the working portion of the dive or during
decompression. Gaining data from as many different applications as
possible will aid in dive planning a great deal.
RMV can be calculated from data collected

in the pool or a real dive. The diver simply
swims at a constant depth for a set period
of time. Some dive computers that are air
integrated can provide real time data as
well. RMV is calculated as a volume of gas,
so pressure used needs to be converted
into volume of gas per minute used. The
cylinder baseline can be used to make the
conversion.
Cylinder baseline is a way to figure out the

constant for each cylinder. It is calculated as
volume per unit of pressure. In metric countries this is easy, as most
cylinders are known by their wet volume. So, wet volume multiplied by
fill pressure will give the volume of gas in the full cylinder. For imperial
countries, the volume of the cylinder is not usually recorded on the
cylinder. So, when a cylinder is purchased, it is important to record the
true volume of the cylinder and the pressure required for that volume to
be present.
Metric Example:
11 L cylinder fills to 200 bar.

11 x 200 = 2200 L of gas in the full cylinder
Imperial Example:
80 ft3 cylinder is full at 3000 psi

Cylinder baseline is 80 / 3000 = 0.0267 cu ft/psi
RMV is figured out by looking at the pressure of gas breathed over a

period of time, corrected for depth and converted to volume used at the
surface.
Metric Example:
A diver uses 15 bar of gas while swimming at 10 m for 10 minutes,
diving with an 11 L cylinder that is full at 200 bar. What is this diver's
RMV?
15 bar x 11 L = 165 L gas used at depth
Use Boyle's Law to convert the consumption to surface volume.
165 L / 2 bar = 82. 5 L surface equivalent / 10 min = 8.25 L/min RMV
Imperial Example:
A diver uses 250 psi of gas while swimming at 33 feet for 10 minutes
diving an 80 ft3 cylinder that is full at 3000 psi. What is this diver's
RMV?
Cylinder baseline = 80 ft3 / 3000 psi = 0.0267 ft3/psi

250 psi x 0.0267 ft3/psi = 6.675 ft3 used at depth
(33 ft + 33) / 33 = 2 ata pressure at depth
Use Boyles Law to convert consumption to surface equivalent volume.
6.675 / 2 ata = 3.3375 ft3 / 10 min = 0.33 ft3/min RMV
Knowing how much gas is breathed is critical for planning dives. As

ideal breathing is
mastered, these
numbers will likely drop.
While technical diving,
these numbers must be
maintained as
increases in breathing
parameters can literally
kill a diver since they
cannot simply surface
at any time. Adding in
some conservatism is a
good idea. For the sport
diver, being able to
calculate gas consumptions will allow for more complete dive planning
and increased peace of mind. It also gives valuable feedback about
performance.
This is also critical for rebreather divers in order to calculate bailout

needs. Open circuit bailout is the ultimate solution to a catastrophic unit
failure. RMV calculations for rebreather bailouts should be conducted
while diving the rebreather with the bailout system that will be used.
This can be done for open circuit bailout as well as semi-closed bailout
modes. It is normal to find that RMV is much higher after coming off
closed circuit to open circuit. This is why it is critical to master ideal
breathing and be able to move from one mode of diving to another with
minimal drop in performance. However, being conservative in bailout
planning needs is the best choice.
RMV provides the ability to know exactly how much gas will be used for
a dive. In sport diving it is simple to calculate total gas usage for a dive
by depth, adjusting RMV for the maximum depth of the dive for the
duration of the planned dive. The result will be the total gas needed by
the diver.
Metric Example:
A diver plans a dive to 30 m for 50 minutes. The diver has an RMV of 20
L/min. What is the total gas needed and the pressure that will be
breathed from an 11 L cylinder?
20 L x (30 m + 10 m) / 10 m/bar = 80 L RMV at depth

80 L/min x 50 mins = 4000 L total gas needed for the dive
4000 L / 11 L/bar = 364 bar needed for the dive in the cylinder
Imperial Example:
A diver plans a dive to 100 feet for 50 minutes. The diver has an RMV of
0.4 ft3/min. What are the total gas needed and the pressure that will be
breathed from an 80 ft3 cylinder that is full at 3000 psi?
0.4 ft3/min x (100 ft + 33) / 33 ft/ata = 1.61 ft3/min at depth

1.61 ft3/min x 50 min = 80.6 ft3 total gas needed for the dive
80 ft3 / 3000 psi = 0.0267 ft3/psi cylinder baseline
80.6 ft3 / 0.0267 ft3/psi = 3019 psi needed for the dive
Both examples show the importance of figuring out gas needs as

neither dive could be conducted with a single cylinder. These
calculations assume maximum depth for the entire dive, so further
refinement can be made by planning multilevel dives at known times
and calculating gas needs for each level of the dive and then simply
adding up all the segments of the dive. Also, extra gas should be
planned for to conduct ascent and safety stops and to return to the
shore or boat with some reserve to deal with unforeseen issues.
Dive buddies should make sure that their gas needs match. For sport
diving, check to insure that gas supply will allow for the same plan to be
dived by both divers. For technical diving, it is critical to ensure that
there is enough gas to support both divers in the event of catastrophic
gas loss. For rebreather diving, it is critical for bailout needs and
reserves to assist dive buddies. Oxygen supply needs will be discussed
in your rebreather course.
Gas reserves are for the diver that carries them.
THERE IS NO EXCUSE FOR RUNNING LOW OR OUT OF GAS!

It is that diver's choice to lend assistance to another diver or their dive
buddy. Gas planning should include consideration for gas loss; it is
NEVER acceptable to run out of gas simply because of bad planning.
This is why knowing the numbers is critical.
Oxygen Planning
It is necessary to calculate and plan
for CNS exposure and OTUs. These
numbers should be within the
acceptable limits. If diving for multiple
days, daily OTUs must be tracked
and limits obeyed. Oxygen exposure
is one of the most critical aspects of
dive planning.
Nitrogen
Limitations
This course concentrates on no stop
diving within the recreational sport
diving envelope. Dive profile planning
should be conducted before each dive via dive computer, computer
based software or dive table. Breathing gas choices should be made to
optimize breathing mixtures to extend no stop bottom times within each
dive.
Dive tables are a traditional way to plan for nitrogen exposure. Any dive
table can be used with nitrox through the use of the EAD concept,
reviewed earlier. Dive table use is limiting as it assumes that the
deepest depth of the dive is maintained for the whole of the dive. If a
dive table is used, it is important to understand all of the assumptions
and rules for its use. The instructor will review dive table use if they will
be utilized in the course.
Thermal Considerations
Recreational no stop diving

allows for the dive to end at
any time. However, planning
for thermal exposures can
be critical to enjoying the
diving day. In technical
diving, running out of heat
can be as critical as running
out of gas. Make sure that
thermal considerations
based on the water
temperature, number of
dives in the day, thermal
protection, weather, depth,
personal thermal characteristics, and mission needs are part of the
planning process. Thermal protection takes on greater importance when
heat loss is increased due to cold weather and water temperature. It is
also important to be aware of heat stress and becoming overheated
prior to diving.
TDI Advanced Nitrox
Chapter 5: Equipment, Dive Protocols
and Common Gas Mixing Procedures
• 40 Percent Rule
• Compressor Cleaning
• Cylinder Cleaning
• Regular Air Use in an O2 Cleaned Cylinder
• Pre-dive Checklists
• Diving Gasses
• START Checks
• Bailout Procedures
• Partial Pressure Blending
• Continuous Blending
• Membrane Separation System
• Premix
• Molecular Weight
40 Percent
Rule
Most of the industry
recognizes that any regulator
may be used with nitrox
mixtures up to 40 percent
oxygen concentration. No
special procedures need to
be used for diving with these
mixtures. However, TDI
recommends that all
cylinders used with nitrox
mixes be oxygen cleaned. Cylinders should be properly marked and
dedicated for nitrox so as to not create confusion about what is in them.
Compressor Cleaning
Compressors should not be considered okay to use on mixtures up to
40 percent. Because of the heat and
functioning of the compressor, a
compressor working with any mixture
other than air should be properly prepared
to do so.
Cylinder Cleaning
Equipment being used with mixtures over
40 percent oxygen must be properly
cleaned and prepared for oxygen service.
Also, the material the regulator is made of
should be compatible for this use.
Titanium and aluminum regulators should
never be the choice for this application.
Cylinders must be cleaned and properly
labeled. A trained and properly equipped
service technician should conduct cleaning procedures.
The technician will first prepare the equipment for cleaning. Once the
equipment is in the best shape it can be in, it will be cleaned of all
hydrocarbons. This is usually a two-step process of cleaning and
confirming the absence of hydrocarbons, then progressing to higher
levels of cleaning. Several tests are run to confirm the absence of
hydrocarbons. Once the parts of the equipment are confirmed to be
clean, then the technician will replace parts with oxygen compatible
parts where applicable. All lubrication is done with non-hydrocarbon
lubricants. The technician then reassembles the equipment.
The equipment is the cleanest it will be after service. It is very important

to keep the equipment free of contamination between services. If the
equipment is exposed to contamination, it is very important to have it
serviced again prior to use.
The use of oxygen and high oxygen concentration mixtures does carry
some increased risk of equipment related issues. Always turn valves on
slowly. Make sure to maintain the level of cleaning present in dedicated
gear. Never put cleaned equipment back into use when it has been
exposed to non-compatible gasses. These items must be cleaned
again.
Cleaned cylinders should only be filled or topped off with oxygen compatible air.
Generally, this air is called hyper
pure or modified grade E air. It
goes through additional filtration to
insure that very little hydrocarbon
content is present.
Regular Air Use in an O2 Cleaned
Cylinder
If a cleaned cylinder is used with regular air it needs to be cleaned prior
to use with mixtures over 40 percent oxygen content. Any cylinder that
is going to be used for partial pressure blending must be cleaned for
such use because the introduction of high oxygen concentration
mixtures prior to topping with leaner mixtures is common.
START
Checks
Stress analysis and
mitigation prior to the
dive is especially
important when
diving with a new
team or new
equipment
configuration.
S = S Drill - Out of
Air drill and Bubbles
checks
T = Team - Buddy equipment checks
A = Air - Gas matching
R = Route - Entry / Exit and planned path underwater
T = Tables - Depth, Duration, Waypoints and Schedule
Descents should be conducted as a team and be a time when divers

prepare for optimum performance during the bottom phase of the dive.
Ideal breathing and a performance mindset should be the focus. This is
also a time when observation of the dive buddy's kit is critical. This is
the final go or no go moment in the dive. Descents need to be done at a
reasonable pace to allow for equalization and avoid the build up of
carbon dioxide from over working. The team should level off in advance
of the bottom and buoyancy should be locked in prior to the working
phase of the dive, to optimize performance and visibility.
Ascending from the dive is a critical performance and safety concern.

Ascent rates should be slow and allow for safety stops. During this
course, gas switches may be conducted. Switching to richer oxygen
mixtures during safety stops can add benefits to off gassing. However, it
does require the use of additional cylinders. This can complicate
logistics and affect the streamlining of the diver. It also dictates that the
diver build in protocols in case he cannot make the switch. It is not
unreasonable to make use of oxygen during the final safety stop in
order to maximize off gassing during this critical safety procedure.
Safety stops have become mandatory in diving. Further investigation

has suggested that more than one delay in a no stop dive ascent could
provide additional benefits to off gassing and thus lower DCI incidence.
The use of intermediate safety stops in addition to one conducted at 4
metres or 15 feet are useful. Some dive computers have adopted these
strategies. A general rule of thumb has been developed suggesting an
additional safety stop for deeper dives at half of the bottom depth for a
minute or two, with additional stops progressing to the shallow stop; this
may help lower incident rates and
improve safety.
For a dive to 30 metres or 100 feet,

the diver would begin their first safety
stop at 15 metres or 50 feet. This
stop would be made for one to two
minutes. The next stop would be
made half way between there and the
shallow stop for another minute or
two. The traditional shallow safety
stop would also be conducted for
three to five minutes with extensions
beyond that being made if desired.
The use of richer mixtures at the
shallow stop could be utilized as well.
Equipment checklists should include all equipment necessary for the

dive operation. This is to include but not be limited to scuba units,
exposure protection, snorkeling equipment, dive computer(s),
accessories, cylinders and appropriate gasses, and specialty items.
This list should be reviewed prior to the dive trip and departure.
Self and buddy checks occur directly prior to entering the water. These
include safety checks for the diver and their buddy. Each diver should
confirm the function of their buddy's equipment and use this time to
familiarize themselves with their buddy's equipment if there is any item
that is new to them.
Bailout Procedures
A diver should carry enough gas to make a direct ascent to the surface
with a shallow safety stop from any dive. For the open circuit diver, this
should be accounted for in gas planning with predetermined departure
pressures for different depths.
For rebreather divers, bailout procedure must account for enough open
circuit breathing gas to make a safe ascent from the deepest portion of
the dive to the surface with a safety stop. Gas consumption calculations
should take into account less than ideal breathing parameters and real
data should be secured, having gone to open circuit from the rebreather
in order to make more accurate decisions in planning for bailout. Most
rebreather divers will not have ideal breathing after immediately
switching to open circuit.
Bailout calculations for rebreathers will be conducted as part of the

rebreather course.
Common Gas Mixing Procedures

Gas blending is not complicated and is
integral to nitrox diving. Taking a TDI
Blending Course is a great way to
expand abilities and prepare the diver to
make their own breathing gasses.
Partial Pressure Blending
The most common mixing procedure that will be encountered in the field
is partial pressure blending. This is the simplest method for blending
from an equipment standpoint. However, this method does require the
use of oxygen or high oxygen mixtures richer than 40 percent, which
mandates that all equipment be cleaned for oxygen service.
This method of blending is most commonly done by adding oxygen to a

cylinder and then topping the mixture with air in order to blend the
desired mix. This method is labor intensive and slower than other
methods. It requires slower fill rates and careful observation of
procedures.
Continuous Blending
This method of blending mixes the nitrox prior to the gas being
compressed by a modified compressor. The desired blend is confirmed
prior to the mixture entering the compressor to be pumped into a dive
cylinder or banks. This method usually uses pure oxygen and ambient
air. This method is less labor intensive, but is more equipment intensive.
For mixtures under 40 percent oxygen, cylinders do not require cleaning
for oxygen service; however, TDI strongly recommends that all cylinders
be O2 cleaned since this method may not always be available.
Membrane Separation System
This method of blending is similar to continuous blending except the
method of nitrox production is different. A membrane is used to remove
nitrogen from air. The membrane is able to differentiate between the
size of the oxygen and nitrogen molecules in air and with each pass is
able to remove more and more nitrogen. This produces nitrox mixtures
that can then be passed through a compressor for filling a cylinder or
banks. This method also allows cylinders to be used that have not been
cleaned for oxygen service; however, just as with continuous blending,
TDI strongly recommends that all cylinders be O2 cleaned since this
method may not always be available.
Premix
Premix requires no blending or
minimal blending. A gas supplier
delivers premixed blends of nitrox.
This gas can be pumped directly into
cylinders or used to achieve the
desired mix. This method can be very
useful if the same mix is desired for
all diving needs but can be wasteful
and more expensive than other
methods of blending. This method
allows for the use of cylinders that
have not been cleaned for oxygen
service.
Molecular Weight
This method is generally reserved for
mixing large quantities of gas. Large gas suppliers use the weight of the
component gasses to make an exact mix based on the weight of the
gas introduced to the pressure vessel. This method is rarely used in the
field or at a dive center as it requires the use of scales and can be
tricky.
TDI Advanced Nitrox
Chapter 6: What is Next and Using
Buhlmann Air Decompression Tables
• What's Next
• Suggested Reading
• Using Buhlmann Air Decompression Tables
• Final Exam
What's Next?
Advanced Nitrox is only the first step in your journey towards achieving
the freedom to make more advanced dives. This course may be a
stand-alone course or part of a combined course. It is important to
remember that if more advanced diving is desired, additional training
must be secured prior to conducting those dives. Staged
decompression dives are not part of this course. Enrolling in the TDI
Decompression Procedures Course is a great way to progress into
deeper and longer dives as well as gain additional opportunities to apply
what has been learned in this course.
This course is required for or included in most rebreather courses.

Closed circuit diving is a great way to help extend available bottom
times and gas supply to allow for greatly lengthened no stop diving. The
added benefit of silence allows the diver to more easily approach
marine life and is an added bonus to any dive.
Never attempt to conduct dives that require stops or take the diver
beyond what is learned in this course without further training. It can be
tempting to give it a try as what is learned in this course provides some
understanding of what might be required. It is very important to receive
proper training in the complex procedures required to properly conduct
these dives.
Beyond entry-level technical training, Extended Range and Trimix Diver

courses are great ways to access even deeper and more complex
diving. For rebreather divers, technical training can be taken with the
rebreather as well. Technical diving is a great adventure and anyone
with the correct mindset and proper approach can enjoy the challenges
and rewards of continuing their dive education in this way. The
foundation that is built in this course will serve the diver well as they
advance to more complex dive plans. More importantly, the diver can
constantly perfect skills and be better on each dive than they were on
the last.
Suggested Reading
• Deep Diving, Revised: An Advanced Guide to Physiology,
Procedures and Systems, Bret Gilliam, 1995 Aqua Quest
Publications, Inc.
• Diving Physiology in Plain English, Jolie Bookspan, 1997,
Undersea and Hyperbaric Medical Society.
• The Practice of Oxygen Measurement for Divers, J. S. Lamb,
1999 Best Publishing Company.
• NOAA Diving Manual: Diving for Science and Technology,
Fourth Edition, James T. Joiner (Editor), 2001 Best Publishing
Company.
• Peak Performance: Mental Training Techniques of the
Worldâ s Greatest Athletes, Charles A. Garfield, 1985
Warner Books.
• If you can find it: Oxygen and the Diver, Kenneth Donald, 1995
Best Publishing Company.
Using Buhlmann Air Decompression
Tables
Please click here to download the Buhlmann Air Decompression Tables.
Note: A full size version of the TDI dive tables also appears in the
appendix of the course manual, and are available for download here.
These tables are valid for use for dives conducted from sea level up to
elevations of 700 metres (2,300 feet). Dives conducted at altitudes
higher than 700 meters are beyond the scope of these tables and
require specialize planning and training (SDI Altitude Diver Specialty).
Using the Buhlmann Air Decompression Tables for non-decompression
dives is very easy and anyone familiar with working " paper" tables from
other algorithms such as the U.S. Navy tables will recognize the
methodology. To plan a dive, follow the left-hand column to the planned
depth in metres or the nextDEEPER depth. Next step is to find a time
that corresponds to the planned time the divers will be spending at
depth (bottom time). Choose the actual planned bottom time or a time in
minutes that is the next LONGER time from the list of times shown for
the chosen depth. The "decompression" stop(s) required by the table for
the dive can be found by reading from left to right along the row that
corresponds to both depth and time of the planned dive and are shown
in whole minutes. The depth of the required decompression stops is
found at the head of each column (reading left to right: 12 m, 9 m, 6 m,
3 m. The Rep. Group (repetitive dive group) is found in the column next
to the 3 m stop. The total ascent time is shown in the right-hand column.
The ascent speed for all dives using the Buhlmann Air Decompression
Tables is 10 metres per minute.
Example One:
A dive to 20 metres for 20 minutes. Find 20 metres in the depth column.
There is no 20 metre entry so the next deeper depth is 21 metres. Now
in the 21 metres section find 20 minutes. There is no entry and the next
longer bottom time is 22 minutes. Following the row across to the right,
we find the dive requires a one minute stop at 3 metres. The Rep Group
is C and total ascent time (total in-water time) should be 31.1 minutes,
which the majority of divers would round up to 32 minutes. This dive is
within the NDL for these tables.
Example Two:
A dive to 39 metres for 10 minutes. Find 39 metres in the depth column.
Now find 10 minutes in the 39 metre depth section. Following the row
across to the right, we find the dive requires a one minute stop at 3
metres. The Rep Group is D and total ascent time (total in-water time)
should be 14.9 minutes, which the majority of divers would round up to
15 minutes. This dive is close to the NDL for these tables.
Planning a repetitive dive

The first step is to find how much residual nitrogen remains after a
surface interval from a previous dive. To do this, transfer Rep Group
from previous dive to Table 2: Buhlmann Repetitive Letter Group Table.
Find entry for Rep Group in the outlined rectangles (A, B, C, etc.).
Surface intervals in minutes is shown in the horizontal row
corresponding to the Rep. Group. Find the actual surface interval in
minutes or the next LONGER time from the times shown. Follow the
column up to the new Rep. Group.
For example:
Following a surface interval of 30 minutes, a Rep Group D becomes a
Rep Group A. Table 2 also shows the surface interval time necessary to
"clear" residual nitrogen following a dive and the necessary minimum
surface interval until a diver can fly with "acceptable risk," although TDI
suggests a minimum of 24 hours surface interval before flying even on a
commercial flight.
To complete the "decompression" planning aspect of a dive, it is

necessary to convert the Rep Group letter after the planned surface
interval into minutes of residual nitrogen time and add this time to the
next diveâ s planned bottom time.
To do this, use Table 3 Residual Nitrogen Time in Minutes. Find the

new Rep Group (from Table 2) in the left-hand column. Find the
maximum depth of the new dive in the top column. Find the intersection
of the horizontal row and vertical column and this gives residual nitrogen
in minutes. This time MUST be ADDED to the planned actual bottom
time for the repetitive dive and this equivalent bottom time be used on
Table 1.
Example:
A Rep Group A diver planning a dive to 24 metres will "carry over" 11
minutes of bottom time. This time must be added to the planned bottom
time, which in this example is 15 minutes. In this case, the residual
nitrogen would make this dive's equivalent bottom time 26 minutes.
Table one tells the diver that his dive will required a two-minute stop at 3
metres and carry an E Rep group at the dive's conclusion.
PLEASE NOTE: The Buhlmann Tables mandate a minimum of a one-minute stop at

3 metres for EVERY dive. Any dive shown to have a stop time in excess of a
standard safety stop is a staged decompression dive and is beyond the scope of a
TDI Advanced Nitrox certified diver, and requires special planning, training and
equipment (TDI Decompression Procedures).
Almost There!
You have almost completed this TDI online academic program. Just a
few more steps and you will be finished and ready to complete your
training with your instructor.
SDI/TDI/ERDI offers many different paths for divers and the dive
professional alike. Is sport diving leadership or public safety diving
calling to you? There is nothing holding you back. You will have the
opportunity to discuss your options in more detail when you meet with
your Instructor to complete your program.
Please remember the you will need copies of the following documents
to submit along with the course completion form that will be sent to you
via email once you have satisfactorily completed the final exam. You will
submit these forms and the course completion form to your Instructor.
• Proof of appropriate diver and/or TDI certification

• You may also be required to pay an additional fee for the
practical/skill development portion of your course.
Your Instructor will also have you complete an additional medical

statement, and may at his or her discretion have you complete an
additional written and/or oral examination.
On the following exam, you may be asked questions pertaining to the
US Navy Dive Tables. If you need to download the TDI Dive
Tables, please click here.
Please direct any questions regarding the program to your instructor,

and good luck on your final exam!

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ADVANCED

The Advanced Nitrox course is the

Regardless of the application or goals, the importance of this course

Participants in this Technical Diving International course will learn the

Regardless of which path graduates

For anyone moving into technical

The foundation of this mindset is that

Buoyancy control is the hallmark of an elite diver. A diver can quickly

The diver should be able to make most of his buoyancy adjustments

(*The terms Buoyancy Control Device and Buoyancy Compensation

Learn how to flat turn. Use

An ideal working position

Seventy percent of gas exchange

Ideal breathing will provide the best

This method is technically known as

The diver will want to work on

It is okay to deviate from ideal breathing when it is by choice. The diver

Oxygen is a tasteless and odorless gas. It supports combustion and our

Oxygen is a diatomic molecule. This means is has two oxygen atoms

Dalton's Law reflects the pressure of a gas in a gas mixture. It states

Dalton's Law is used to calculate maximum operating depth, best mix,

35 m = (35 + 10) m / 10 m/ata = 4.5 ata

115 ft = (115 + 33) ft / 33 ft/ata = 4.48 ata

Pg is partial pressure of the gas.

The law can easily be represented by a T formula. A T formula is a

The T formula works by covering the item that needs to be calculated.

Fg = 1.4 bar / 4.5 bar

Fg = 1.4 ata / 4.48 ata

Maximum Operating Depth, MOD

A diver wants to know how deep he can dive with a mixture of 29

P = 1.4 ata / 0.29

P = 27 m + 10 metres / 10 m/bar = 3.7 bar

P = 92 ft + 33 ft / 33 ft/ata = 3.79 ata

Breathing a gas absent (anoxic) mixture can shut down respirations

Generally, oxygen toxicity is considered to fall into two areas â“

Tracking exposure to higher levels of oxygen is critical as the

It is wise to still plan dives by

If a diver is exposed to 1.4 ata of

40 minutes / 150 minutes x 100 = 26.7 percent CNS exposure

In nitrox class, you simply added these percentages up throughout the

Pulmonary Oxygen Exposure

Although the risks of Pulmonary Toxicity are minimal among technical

One OTU is the equivalent of breathing oxygen at atmospheric pressure

OTU= Tx (0.5/(PO2 - 0.5)) -0.833

Where Tx represents Time in whole minutes and PO2 represents

following table may be used.

Example: To calculate the OTU loading for a dive for 60 minutes

Single Dive versus Multiple Dives and

For multiple day levels of Pulmonary Toxicity, the diving community as a

There is some evidence that dives of longer duration provide some

• Being trained in first aid and oxygen administration.

Remember, being prepared can turn a not-so-good situation into a not-

Besides the risk of decompression

Carbon Dioxide Toxicity

Human respiration is keyed to CO2 levels

Carbon dioxide also increases decompression risk, and complicates

The movement of platelets and thickening of the blood when an incident

Personal Dive Computers

It is also important to understand that simply because a dive computer

Generally, oxygen toxicity is considered to fall into two areas â“