1 Author: Mark Ellyatt – Technical Instructor Trainer - Inspired Training

Technical 

SeriesDeep 

Explorer 

Author: Mark Ellyatt – Technical Instructor Trainer 

1

Dear Diver 

We are delighted you have selected ITDA for your 

continuing diving education. From the outset we have 

dedicated ourselves to producing the best training manuals 

and support materials in the industry. 

From our roots in the forefront of technical diver training you 

rest assured that years of common sense diving know how 

will be imparted through our programmes. To this end we 

have associated with the finest minds in technical diver 

training to create manuals like this one that will convey 

information in an easy to read format that will be a pleasure 

to work with, and become your reference guide for later 

reflection 

Our authors are also recognised experts in their specific 

topics. So when you thumb through an ITDA manual you will 

have the confidence of knowing that author has spent 

thousands of hours underwater and in academic discussion 

to create a text that reflects direct “hands on” practical 

experience, rather than theoretical idealism 

We also recognise that technology changes rapidly in 

today’s society and any suggestions for ways to improve our 

materials are welcomed at all times. 

We want you to get the best possible experience from your 

Deep Explorer course and your instructor. Please let us 

know how we can better serve you. All of us at ITDA would 

like to extend our appreciation to you for your business and 

the time that you will invest in our training programmes 

www.ITDAhq.com 

2

Deep Explorer 

Contents 

Foreword. Touring Bandit Country 

Chapter 1. How Deep’s Deep? 

Chapter 2. Helium – Friend or Foe 

Chapter 3. Physiology you should know 

Chapter 4. Equipment for the job 

Chapter 5. Dive planning 

Chapter 6. What comes next 

INTERNATIONAL TECHNICAL DIVING ASSOCIATION 

TEL: +34 971481978 

Apartado Correo 8 

Ciutadella de Menorca 07760 

Espana 

www.itdahq.com email to : info@itdahq.com 

Copyright © Mark Ellyatt 2007 

The right of Mark Ellyatt to be identified as the author of this work has been asserted 

by him in accordance with the Copyright, Designs and Patents Act 1988 

3

Touring Bandit Country 

By starting on the road to technical diver you took on a complete 

list of new challenges. You have clearly embraced the mindset 

and discipline to function safely in deeper water; Congratulations. 

To get this far you have shown a desire and an ability to go 

beyond mainstream technical diving. Deep Explorer Training will 

help you further develop your confidence and obtain the 

knowledge and skills necessary to function within the realms of 

safety at depths that have often killed the unprepared many times. 

You will no doubt appreciate and understand the dangers and 

implications of advanced decompression diving but please review 

all the information related to Technical Nitrox level diving as little 

explanation to terminology or buzzwords will be found within 

these pages short of the glossary at the end. There will be no 

cosseting during training. If you continually exhibit skills unsuited 

to deep water with its life changing implications your instructor 

will not lie to you but instead advise and subsequently certify you 

to dive to depths better suited to your current comfort zone. You 

will be encouraged to practise before re-attending Deep Explorer 

training. Please be aware – deep exploration diving is not for 

everyone. Don’t think of this as failure but simply as deferred 

success! 

The Mod 1 training course is the first in the line of Deep Explorer 

diver training and will impart the knowledge and skills to the user 

necessary for diving into the Twilight Zone with Helium based 

breathing gases to a maximum depth of 70metres (240ft). Open 

Circuit and or Closed Circuit equipment may be used 

Mod 2 training will further your understanding of deep trimix dive 

planning and allow you to dive with confidence into the Dark zone 

as deep as 100metres (330ft) using either open circuit scuba or 

closed circuit apparatus. 

Mod 3 training is the pinnacle of Deep Explorer training. Diving as 

deep as 130metres can have immediately life shortening / 

changing consequences and the risks are often higher than the 

rewards. Training will attempt to impart problem solving and 

planning skills commensurate with this ‘Exploration Diver’ level 

experience course. Your instructor will help you progress through 

this advanced training in a comfortable and safe method. Your full 

attention is required and you will be expected to demonstrate 

good judgement and sound diving skills throughout. 

MOST IMPORTANTLY…ENJOY! 

4

As with all successful ventures, solid foundations are 

mandatory. Before venturing deeper and deeper under the 

water, divers must ensure they are confident and 

comfortable at every stage and with all new topics, both 

academic and practical. 

The material contained within this manual is designed to 

supplement classroom and practical in-water instruction by 

an authorised Deep Explorer Instructor. Your Trainer will 

verify your skills and explain any topic in detail. Simply 

reading this text may not make you a exploration diver 

regardless of your diving experience or certification level. 

“Limits” can never take into account differing physiologies, 

capabilities, training and experience within. Therefore, the 

limits in this manual should be considered purely as 

guidelines. Every diver should evaluate their own personal 

parameters that make them unique and set their own 

individual limits well within the guidelines of common sense. 

Dives outside recreational boundaries can be a very 

hazardous undertaking. The sometimes hostile underwater 

environment will put to the test your training and calm the 

moment you enter. Please treat every advanced level and 

formal decompression dive with respect and maintain the 

attention to detail you will come to learn on completion of this 

course. 

Divers who obtain advanced level training should understand 

the following additional responsibilities: 

After certification is earned, you must strive to maintain the dive fitness 

as well as the physical fitness displayed during this training course, 

throughout your diving adventures. When embarking on technical 

dives, only proceed when you have spent time practicing your skills 

thoroughly beforehand. Special care should be taken with regard to 

buoyancy control, especially when using dry suit systems. When using 

new equipment such as reels and surface marker buoys or rebreather 

equipment, practice until totally proficient before use during 

mandatory decompression stop dives. 

Practice Diminishes Risk…seek the guidance of an 

experienced technical instructor if you need additional skill 

development. Even a few weeks out of the water can 

degrade your self rescue abilities dramatically. 

5

The aim of this training course is to familiarise divers with 

procedures and equipment that will improve understanding 

and hopefully performance during advanced level scuba 

dives using Helium. Academic sessions may be instructor- 

led or self-study but be prepared always, both physically and 

emotionally. Your equipment needs to work perfectly on 

aggressive dives…inspect it and learn how it works: 120 

metres down is the wrong place to notice something isn’t 

working! 

4 dives are required for each level or depth progression 

The Pre requisites experience for Deep Explorer training 

are a minimum of : 200 dives for Mod 1. 

300 dives for Mod 2. 

500 dives for Mod 3 

Mod 1 max depth 70m (240ft) (min 55m) 



Technical Nitrox (or equivalent) needed to start Mod 1. 

Mod 1 before starting Mod 2. Mod 2 (or equiv) before 

starting Mod 3. Instructor may allow a combination of 

Modules commensurate with the attending diver’s ability 

BEFORE attending training all divers must 

be at least 21 years of age with medical less than 3 months old. 

Stay Dive Fit…Stay Dive Safe 

6

Chapter 1. How Deep’s Deep? 

Depth alone is not an indicator of a dive’s complexity. Simply 

bouncing down to 120metres for a few minutes is of course 

extremely dangerous but can be accomplished reasonably 

safely by an appropriately experienced diver wearing a fairly 

standard equipment rig such as twin 15litre tanks and 2 side 

mounted 12litre deco tanks. However, a dive to even 80 

metres for 30 minutes may prove a logistical nightmare 

considering the decompression times. Recently the technical 

dive scene has shown a slow exodus to CCR units more and 

more, especially on the deeper dives. CCR units promise 

some intoxicating gas durations when working properly but 

these advantages are sometimes deadly, as sadly the 

average user simply sees them as some form of unfailing 

magic carpet with scant regard to realistic open circuit bailout. 

CCR definitely has its uses, but divers need to be a little 

more realistic when considering bail-out options, quite often 

the ubiquitous 2 ali 7 side-mounts give them a snowballs 

chance in hell when the doodoo hits the fan! 

Depth is relative. An unqualified diver just below the surface 

breathing from scuba equipment may be in great peril but a 

highly experienced Mixed Gas diver may be comfortable and 

relatively safe even at depths beyond 150 metres (with the 

right plans and support of course). Divers embarking on 

Deep Explorer training will have dived as deep as 60metres 

breathing air in a variety of conditions. Deeper air divers will 

always have to cope with narcosis and will have adapted 

their technique to best function at these depths. With Helium 

in the breathing medium, help is at hand. Adding Helium to 

the mix will minimise nitrogen narcosis and help limit the 

effects of Oxygen toxicity. Also, small amounts of Helium will 

even lower the work of breathing of your favourite regulator 

and subsequently lower Carbon Dioxide levels in you, further 

adding comfort. 

However, Helium has its drawbacks. Helium rapidly diffuses 

throughout the body and forms bubbles more readily than 

nitrogen. This means that slow ascent speeds MUST be 

maintained and a series of deeper decompression stops 

performed to avoid getting Helium induced decompression 

sickness. Another issue is that Helium transports heat very 

7

effectively. Breathing high Helium concentrations will make 

you cold, possibly even hypothermic. Helium shouldn’t be 

used to inflate your drysuit as it is a very poor insulator too. 

All breathing mixtures must even be checked for 

compatibility to ensure that deco gases don’t cause 

complications when switched too. The higher the Helium 

concentration the more serious the issues. All diving below 

80metres can add significant complexities especially when 

bottom times run over 30 minutes…only fools rush in. 

The purpose of these training courses is to firstly explain the 

procedures involved in the safe use of Trimix or Heliox 

mixtures in moderate depths and then cover the best 

practises to use when taking these mixtures very deep using 

either open circuit and closed circuit equipment and even 

combinations of the two. 

Breathing mixtures containing Helium have been used very 

deep. Free swimming divers have used Trimix deeper than 

300 metres once or twice with some form of success and 

many times as deep as 180 metres even adding a degree of 

predictability and reliability. 

Depths below 150 metres provide a unique experience in that 

they give the diver a virtual rollercoaster ride for often the 

first time regarding physiology. Debilitating cold and HPNS 

can make these exposures more than unpleasant but the 

rewards gained after successful completion can be almost 

tangible. Will you be able to function in these depths? Only 

time will tell…remember the competition is between you and 

the environment with the loser paying a heavy price - take it 

slowly and enjoy the ride! 

8

Commercial divers have used either Heliox, Trimix or 

Hydreliox (Hydrogen/Helium/Oxygen) mixtures as deep as 

701 metres in controlled environments and deeper than 600 

metres in the open sea. Divers should not confuse 

commercial diving with technical diving. Commercial divers 

have a massive infrastructure to control every working 

moment, the ascents and descents are completely controlled 

to speeds as slow as 1 metre per hour. Technical divers 

however are usually completely self sufficient, with their 

personal abilities and choices determining the success or 

outcome of every dive. Technical divers will never work for 

hours at extreme depth like their commercial diver cousins 

do every day. Similarly, few commercial divers will have the 

skills or even the desire to throw their bodies into relatively 

deep water with little or no support like a ‘techy’. 

Safe decompression takes very different forms. Pictured above is multi 

million pound saturation system, used by commercial divers with almost 

guaranteed success. Pictured below is a simple surface marker buoy. 

Beneath the red marker buoy is a technical diver decompressing, safe in 

the knowledge that the same physiological exchanges are taking place 

as in the top picture. The scuba diver below the bag has trained in 

decompression techniques and has the equipment and knowledge to 

execute technical dives safely and reliably. 

9

Chapter 2. Helium – friend or foe 

There is no doubt about it Helium is necessary for safer deep 

diving particularly below 60metres. Too little or none and 

nitrogen narcosis will affect your performance. Similarly, 

insufficient Helium could cause elevated po2’s that can have 

disastrous side effects. During your previous Trimix training 

you will have hopefully seen how to derive the ideal trimix, let 

us recap this information with traditional parameters. 

SELECTING A TRIMIX 

Before choosing your gas mixtures for a mixed gas dive, you 

need to know: 

• You should not exceed a PPO2 of 1.4-1.6 bars. 

Each gas mixture should maintain a PPO2 of 1.4 bars or less for 

the working portion of the dive. Nitrox mixtures reaching 1.6 

bars PPO2 can be used during the decompression phase as 

long as the total Oxygen CNS percentage does not exceed 

100% at the end of the dive. 

• Your required Equivalent Narcotic Depth (END). 

Once you have your required END, the gas mixture to be used 

can be selected in association with the 1.4 bar Oxygen limit. 

• The depth at which you will operate, and the time you wish to 

spend there. 

This will govern both the final selection of bottom mix and the 

amount of gas that must be carried to maintain a credible 

reserve. 

Finding Your Equivalent Narcotic Depth. or END 

Your END is the comparative depth on air at which you are 

entirely in control of narcosis, even under stress. You find the 

partial pressure of nitrogen at this depth by using the Dalton's 

Law formula: 

PPN 2 = FN 2 x P. For example, an END of 40 metres would give a 

PPN 2 of : 0.79 x 5 = 3.95 bar. 

3.95 ata/13 ata = 30% Nitrogen 

10

Lets Plan the ideal Trimix for a 120metre dive (13 ata) 

First let us find the Oxygen % 

Fg = P02 / Pata where Fg is the optimum o2 % 

1.4 / 13 ata = 10.7% rounded to 11% 

The Oxygen concentration in our trimix should be no more than 

11% 

************************************* 

Next, lets find the END, assuming an END no greater than 40 

metres and the same maximum depth of 120 metres using 

another method. 

AND SO; 

Desired END = 40 metres +10 metres 

------------------ X 0.79 (N2) 

Actual Dive Depth = 120 metres + 10 metres 

Desired END = 50 metres (absolute) 

------------------------------- X 0.79 (N2) 

130 metres (absolute) 

=.3846 X 0.79 OR 30% (Nitrogen) 

So we have 11% Oxygen & 30% Nitrogen…subtract both from 

100% and the balance is Helium… ie 59% easy! 

The Ideal Trimix for this dive is 11/59/30 (o2/He/N2) many 

divers write this mixture as TX 11/59 (writing N2 is optional) 

Dives to extreme depth are often limited to short bottom 

times. Po2’s can be higher than 1.4 but usually not higher 

than 1.6. 

END values for the open water can be higher than those used 

for any overhead environment. If venturing deeper use an 

END that you have used often and comfortably before. Dives 

in pleasant conditions can use END’s as high as 60metres 

but those performed in less attractive environments for 

11

example bad visibility or less than 18 degrees centigrade 

water temperature may use END’s as low as 40 m (very low 

END’s in cold water can cause hypothermia) 

Deep diving in overhead environments or using CCR 

equipment can use END’s as low as 20 m! END figures for 

CCR have special considerations regarding CO2 production 

due to scrubbers both near exhaustion and/or dived beyond 

manufacturers guidelines. Elevated CO2 can exacerbate 

inert gas narcosis and Oxygen toxicity. CO2 is produced by 

workload and/or over-dived or exhausted scrubbers but also 

lime over-packing etc. CCR must have low END values and 

also low po2’s. 

There are many factors to consider when selecting the 

various bottom mixes and deep decompression gases, for 

example. 

Decompression Obligations 

Gas Volume Logistics 

Environment Temperature 

Overhead Environment 

Inert Gas Counter-Diffusion 

HPNS 

OC vs CCR 

COST 

The key to successful deeper diving is balancing all the 

various physiological and logistic parameters, for example. 

It is important not to switch to gas mixtures that will cause a 

jump in END value unless the outcome of the switch has been 

checked using Decochek software. Gas mixes that cause a 

jump in END can cause complications relating to inert gas 

counter diffusion (ICD) (more later) With ICD in mind many 

deep trimix dives must have a selection of tanks that allow 

the necessary drop in Helium content at every gas switch 

during the ascent phase. Ascent gases on open circuit must 

drop Helium content carefully to allow a gentle off gassing 

gradient. As Helium content drops the reduction is made up 

by additional Oxygen, Nitrogen must never be increased by 

more than 5%. Reducing Helium is necessary on open circuit 

equipment as it allows efficient decompression and 

minimizes breathing potentially chilling gas mixtures. 

12

Deep Trimix divers should note that especially low END 

values and therefore high Helium fractions can have 

disastrous consequences if used without regard to 

hypothermia and ICD. Let past experience guide your END 

choice, not internet forums. Heliair is term associated with 

Trimix, the two terms are interchangeable. Heliair is simply a 

blending technique 

Heliair is simply making trimix by mixing air with Helium. No 

Oxygen is introduced during blending so generally the END’s 

are high relative to the po2 at depth. Divers select a bottom 

mix primarily by po2 suitability. With Heliair the ideal po2 

(1.4) generally gives an END of approximately 55metres. This 

level of intoxication may be fine for bounces in open water 

but could prove troublesome in an overhead environment. 

Heliair is very often used in very deep diving with 

experienced users or shallower in ideal conditions. 

Heliair mixes can be added to non o2 clean cylinders and can 

be remixed again and again. Realistically any trimix can be 

remixed as only the Oxygen fraction of the mix is important in 

open water. 

Below is a list of Heliair mixes and appropriate fill pressures 

REQUIRED CYLINDER PRESSURE (PSI / BAR) 

O2HeN2 1.4 bar 1500 2000 3000 4000 150 bar 210 bar 230 bar 300 bar 

H 18/14/68 220’/66 210 286 429 571 29 30 32 42 

E 17/19/64 240’/72 285 381 571 762 30 40 44 57 

L 16/24/60 255’/76 360 476 714 952 36 50 56 72 

I 15/28/57 275’/83 470 571 857 1143 42 60 65 84 

A 14/33/53 300’/90 495 667 1000 1333 50 70 77 99 

I 13/38/49 320’/98 570 762 1143 1524 57 80 88 114 

R 12/43/45 350’/107 645 857 1286 1714 65 90 100 129 

This chart give the pressure of Helium to add to an empty cylinder to produce the desired heliair 

mixture. Select the cylinder pressure in psi or bar from the top row, and move down that column 

until the required mix is reached. All these mixes have an END of 55m (180’) 

13

Other popular Heliair mixes – all expressed as o2/He/N2 – MOD’s @ 1.4bar Po2 

10/50/40 MOD 130m 

8/62/30 MOD 165m 

7/67/26 MOD 190m 

6/70/24 MOD 220m 

5/76/19 MOD 270m 

4/80/16 MOD 340m 

To Blend these mixes simply take 

FHe and multiply by Final pressure 

for He bar. Top to Final Pressure with 

air. Eg 6/70/24. 0.70 (FHe) X 230bar = 

161 Bar He. Fill Helium first, slowly. 

Check pressure and top to 230bar. 

For mixes other than the standard mixes use the following 

formula. The example has 14% Oxygen - 33% Helium 

Final Pressure (FP) - FP x Fo2 (wanted) 

-------------------------- 

Fo2 topping mix 

Or 230bar - 230 x .14 

------------- 

.21 

230bar – 153bar = 77 bar Helium to add then 153 bar air 

77/230 = 33% He therefore Heliair 14/33/53 

WHATS MY NEW E.N.D? 

After you’ve blended analyse for Fo2 and Fhe. If the Fo2 is 

correct – its all good. If not maybe re-blend or recalculate. 

Actual E.N.D values can be calculated with: 

FN2 (in mix) 

--------------- X (Desired Depth +10m) - 10m 

0.79 (n2 in air) 

OR: 

.53 

------ X (85m + 10m) - 10m = 53m END 

.79 

14

Chapter 3. Physiology you should know 

D EP DIVING 

The deeper we dive, the more concerned we must become 

about the physical and physiological effects of gases at depth, 

about the effect that the constant exposure to temperature and 

pressure has on our bodies, and about its effect on our normal 

bodily processes which are after all, simply chemical reactions. 

THE MAIN PROBLEMS 

The main problems of deep diving could be quickly summarised 

in column form : 

Oxygen Toxicity 

Nitrogen Narcosis 

Decompression Obligations 

C02 Toxicity 

Thermal Exposure 

HPNS / Isobaric Counter Diffusion 

Gas Logistics 

Equipment 

None of these should be treated in isolation. All of them are 

integrated in some form or another, increasing the complexity 

and the seriousness of deep diving. 

NITROGEN NARCOSIS 

Nitrogen narcosis is the result of inert gas absorption acting as 

a depressant on the central nervous system. Nitrogen Narcosis 

is easily compared to anesthesia 

Narcosis appears in most divers when the partial pressure of 

nitrogen in the breathing gas exceeds about 4 bars, and 

increases in severity with increasing pressure until it is virtually 

incapacitating in most case by a partial pressure of 7 - 8 bars. 

On air, 4 bars PPN 2 is reached at about 40 metres, while 7 - 8 

bars PPN 2 represents a depth of about 80 - 90 metres. 

15

Symptoms 

Symptoms vary from one diver to the next, and indeed in the 

same diver from one day to the next. The physiology of 

narcosis is complex, and the contributing factors are many. 

The symptoms include : 

• Disorientation 

• Memory Loss 

• Dizziness 

• Personality change 

• Euphoria (in good conditions) 

• Depression (in poor conditions) 

• Un-coordination 

• Numbness 

• Tunnel vision 

• Memory Loss! 

• Wah Wah hearing effect (advanced) 

• Loss of consciousness (ultimately) 

Avoiding Narcosis 

Although it is physiologically not possible to completely avoid 

the effects of nitrogen narcosis on deeper air dives, it is 

possible to reduce or control them to a degree. The only way to 

avoid them completely is not to go deep or use a Trimix or 

Hyper-Oxic Trimix 

• Acclimatise. If you plan to make a series of deeper air dives, 

build up to them slowly, with regular dives to increasing 

depth. Even a gap of two or three weeks will do much to 

lose this acclimatisation, and it should not be confused with 

simple familiarity with depth. 

• Familiarise. Make sure you are absolutely familiar with your 

equipment and all the environmental parameters of the dive 

location and dive plan. Narcosis is a mood enhancer, and is 

irrevocably intertwined with stress. The more things that 

stress you on a dive, the more “narked” you will feel. 

• Be physically and mentally fit. The better you feel, the less 

narcosis will affect you, all other things considered. Reduce 

all effort at depth to a minimum, to reduce the inert gas 

concentration in your tissues. 

16

• The harder you work, and the heavier you breathe, the more 

narcosis will affect you. Carbon Dioxide build up caused by 

exercise or poorly tuned regulators is known to exacerbate 

narcosis and Oxygen Toxicity. A good night’s sleep and a 

smooth boat ride can also do wonders for Narcosis! 

• Use appropriate gas mixtures. If you really want to go deep 

diving then obtain further training and use the appropriate 

gas mixtures, ie: Trimix 

• 0 - 50 metres (0 - 165’) : Nitrox / Air (Open water) 

• 0 – 60 metres (0 – 200’) : Cave/Wreck HyperOxic Trimix 

• 60 - 75 metres (160 - 250’) : OC or CC Trimix / Heliox 

• 75-150 metres (250’ - 500’) : Open or Closed circuit 

Trimix / Heliox 

• 150 metres (500’ +) Open Circuit Trimix or OC/CCR 

Hybrid 

Human beings were designed to work at a pressure of around 

one atmosphere, and the greater we abuse our design 

specifications, the more chance we have of malfunctioning or, 

breaking down completely. 

In this case it is a primary constituent of the gas we are 

breathing which is inappropriate. To reduce the effects of 

nitrogen narcosis, we can reduce the partial pressure of the 

gas by exchanging or diluting it with an alternative - for 

example, Helium. To avoid nitrogen narcosis completely, we 

must remove nitrogen from the breathing mixture. 

Predisposing Factors 

There are many things that predispose you to narcosis, and 

also to many of the other effects of pressure. Remember that 

few of these sit in isolation, and most of them contribute to 

more than one potential physiological problem. 

17

The more you can do to manage the below list of predisposing 

factors, and to reduce their effects before or during the dive, 

the better you will be able to manage nitrogen narcosis. 

To minimise Narcosis …avoid the following 

Cold 

Drug Abuse 

Speedy Descents 

Heavy Workload 

Tiredness 

Poor physical condition 

Stress / Anxiety 

Poor equipment 

OXYGEN TOXICITY 

Breathing Oxygen at a PO 2 of more than 1.6 bar places a diver 

at extreme risk of Oxygen toxicity. Oxygen Toxicity has many 

symptoms sadly they do not appear in any ascending order, in 

fact the first symptom you may encounter is as likely to be the 

most disastrous as the least. 

The most serious symptom of Oxygen Toxicity is full body 

convulsions similar in appearance to those observed during 

epileptic seizures. Should a diver enter a convulsive state 

underwater it is highly unlikely that the scuba regulator will 

remain in place. In addition it is very likely that loss of buoyancy 

control will occur with the added risk of uncontrolled ascent or 

descent. Risk of drowning is highly likely. 

Breathing air, a 1.6 PO 2 is reached at 66 metres (218’). For 

technical diving, a PPO 2 of 1.4 is the recommended maximum, 

which equates to an air depth of 57 metres (190’). Oxygen 

toxicity is not only depth related, but the risk increases with 

time spent at depth. Use of the appropriate toxicity tables, e.g. 

18

The NOAA Oxygen Exposure Table, is essential to ensure that 

provocative exposures do not take place. 

OXYGEN TOXICITY 

VISUAL Problems such as temporary loss of vision, tunnel vision 

EARS Tinnitus 

NAUSEA Sick to stomach feeling 

TWITCHING of Chest/Rib muscles and or Lip Twitching 

IRRITABLE Depression or Anxiety / Doom Radio 

DIZZYNESS vertigo / un coordination 

CONVULSIONS unconsciousness then drowning 

During previous enriched air training you were no doubt 

introduced to the above Oxygen Toxicity symptoms. While it is 

of academic interest to review the symptoms, the information 

will not stop the symptoms occurring or prevent them 

escalating. Divers should not try to analyse or second guess the 

above symptoms in themselves or others while underwater.. 

Remember the following: If any symptom manifests itself at 

depth or during decompression stops, simply change breathing 

gas to a lower fraction of Oxygen (if possible) and or ascend 

shallower. 

To minimize the likely hood of Oxygen toxicity, advanced and 

technical divers should abstain from any “diet” products for 

extended periods pre dive, be they drink or food form. 

Artificial sweeteners / E numbers are reported to be CNS 

exciters, and have been linked to Oxygen Toxicity incidents. 

Many non prescription and prescription drugs particularly 

nasal decongestants contain ingredients known to increase 

occurrences of Oxygen toxicity, consult a diving physician 

for advice on mixing medications and diving, or better still, 

put off diving until your ailment improves. 

Technical dives should be attempted by people on a “level 

playing field “. Divers should be fit, regularly doing aerobic 

exercise. They should avoid cigarettes /alcohol for months 

19

efore a deeper dive and have no history of drug abuse. If a 

technical diver benefits from diet fizzy drinks then they 

should exercise until they don’t! 

Technical divers can reliably avoid Oxygen toxicity symptoms 

by ensuring that Oxygen exposures remain well within 

established limits as described within the NOAA P02 and 

Oxygen Exposure Time Limit Table as outlined below. 

P02 Single Exposure Limit 24 Hour Exposure limit 

ATA Minutes Hours Minutes Hours 

0.6 720 12:00 720 12:00 

0.7 570 9:30 570 9:30 

0.8 450 7:30 450 7:30 

0.9 360 6:00 360 6:00 

1.0 300 5:00 300 5:00 

1.1 240 4:00 270 4:30 

1.2 210 3:30 240 4:00 

1.3 180 3:00 210 3:30 

1.4 150 2:30 180 3:00 

1.5 120 2:00 180 3:00 

1.6 45 0:45 150 2:30 

Use the above chart to find dive time limits for each P02 used. 

When a single exposure limit is reached you must extend you 

surface interval to 2 hours. If a daily exposure limit is reached a 

full 12 hours must elapse to allow sufficient time for your CNS 

levels to normalise. 

CNS (Central Nervous System) toxicity can be approximately 

measured throughout multiple dives using the CNS percentage 

exposure chart. 

20

P02 Single 

Dive 

Limit 

24 

Hour 

Limit 

5 

mins 

10 

mins 

15 

mins 

2 0 

mins 

21 

25 

mins 

0.6 720 720 1% 1% 2% 3% 3% 4% 5% 6% 6% 7% 8% 9% 

0.7 570 570 1% 2% 3% 4% 4% 5% 6% 7% 8% 9% 10% 11% 

0.8 450 450 1% 2% 3% 4% 6% 7% 8% 9% 10% 11% 12% 13% 

0.9 360 360 1% 3% 4% 6% 7% 8% 10% 11% 13% 14% 15% 17% 

1.0 300 300 2% 3% 5% 7% 8% 10% 12% 13% 15% 17% 18% 20% 

1.1 240 270 2% 5% 7% 8% 11% 13% 15% 17% 19% 21% 23% 25% 

1.2 210 240 2% 4% 6% 9% 12% 14% 17% 19% 20% 24% 26% 28% 

1.3 180 210 3% 6% 9% 11% 14% 16% 20% 22% 24% 26% 28% 30% 

1.4 150 180 4% 7% 10% 13% 17% 20% 23% 27% 30% 33% 37% 40% 

1.5 120 180 5% 9% 14% 17% 21% 25% 30% 33% 38% 43% 46% 50% 

1.6 45 150 11% 23% 33% 45% 55% 67% 78% 89% 100% - - - 

To use the table and calculate your CNS % amount, line up your 

time at depth with your corresponding P02. e.g 60 minutes @ 

1.4 PO2 yields 40% CNS loading 

The percentage should be added to any repetitive dive values 

for cumulative CNS total. 

CNS toxicity build up is thought to have a half life of 90 minutes. 

If 90 minutes is spent between dives you may halve your 

current CNS exposure. There is no credit for surface intervals 

of less than 90 minutes. This simplifies the process and adds 

conservatism during short surface intervals. 

30 

mins 

35 

mins 

40 

mins 

45 

mins 

50 

mins 

55 

mins 

60 

mins

CONVULSIONS 

If someone convulses in the water whilst with you, there is little you can do 

about it until the convulsions cease and the diver goes limp. If depth and 

gas reserves are a concern the afflicted diver should be taken shallower. 

Convulsing divers neither breathe nor breath-hold during the convulsions. 

Pull the afflicted diver (ideally face down) by their tank valve or even their 

hand towards the surface or to an anchor rope for a more controlled 

ascent. Try and replace a breathing regulator as soon as is practical. The 

convulsions may last several minutes. Rebreather divers should be given 

an open circuit regulator containing bottom mix, buddies should be 

mindful of closing the rebreather loop before removing the mouthpiece. 

While the convulsions are taking place the diver’s larynx is locked in 

spasm and water is unlikely to be inhaled. Once the convulsions cease 

however, the build up of Carbon Dioxide in the diver’s lungs will cause 

rapid and possibly deep inhalations, and at this stage the diver is likely to 

drown, so try and replace the regulator at this point and treat from here on 

as an unconscious breathing diver. When the convulsions stop, the diver 

will likely regain consciousness but will remain dazed and confused for 

several minutes. Maintain low po2’s for at least 25 minutes during the rest 

of the ascent. 

Unfortunately, statistics show that divers who convulse at 

depth have very little chance of survival. 

Do all you can to avoid the problem in the first place. 

PULMONARY OXYGEN TOXICITY 

Generally, we would gloss over any references to Pulmonary 

Oxygen Toxicity during technical dive training as it simply does 

not have any bearing outside commercial saturation diving. 

However, extreme technical diving can expose the diver to 

often excessive CNS% in the 200-300% range. As CNS values 

escalate above 100% we can expect the lung linings to become 

irritated and start to swell. Swollen lungs become less efficient 

at exchanging gas. In extreme cases the affected lungs may 

impede decompression and lead to Decompression Sickness. 

Air Breaks are the standard means to moderate CNS exposure 

and pulmonary toxicity during recompression therapy and this 

has sadly filtered down into technical courses as a must do. 

Clearly, lungs that are suffering from irritation caused by high 

po2 over 20 minutes will not return to post-insult condition with 

22

just 5 minutes of low po2. In addition divers MUST NOT switch 

from potential +10 END (100% o2 at 6m) to -6m (.21 at 6m) 

instantaneously. Such dramatic jumps in END at or near the 

decompression ceiling may trigger ICD complications. To deal 

with potentially high CNS% figures noted during dive planning, 

divers should consider the following Po2 MANAGEMENT 

methods. 

Drop Deco po2 to 1.4-1.5 max at 9m 

Drop Deco po2 to 1.3-1.4 max at 6m 

Keep the 3m stop on at least 80-100% O2 for a final washout 

Switch to deco mixes at 1.5 po2 depths rather 1.6po2 from 21m 

While the above steps may add time to the deco the ultimate 

goal is to surface without DCS. Fast decos at high po2 often 

end with DCS for no ‘apparent’ causes. Try the above when 

CNS% goes above 200% - Your lungs may thank you for it! 

CARBON DIOXIDE 

Carbon Dioxide (CO 2) is a contributory factor to various forms 

of gas toxicity, including its own. While we can reduce the 

insertion of additional amounts from the atmosphere into our 

breathing mixture by appropriate compressor filtration, we 

cannot eliminate it, because we produce it ourselves. It is a 

waste product of our metabolism. For every litre of Oxygen we 

physically consume, we produce about 0.8 litre of Carbon 

Dioxide. During a dive, and especially during elevated po2 

dives, the increase in dissolved PPO 2 acts as a slight barrier to 

the blood’s ability to transport dissolved CO2 to the lungs for 

expulsion. The potential for Carbon Dioxide retention is 

increased by skip breathing, restrictive equipment and other 

poor diving practices, and can lead to a considerable increase 

in the effects of other dissolved gases. Some individuals have a 

tendency to be Carbon Dioxide retainers, and these individuals 

may be particularly susceptible to the effects of a variety of gas 

toxicity problems (including acute Oxygen toxicity and 

decompression illness) as a result. 

Divers using CCR equipment in deeper water (>50m) run the 

risk off increasing CO2 exposure due to the extra workload 

placed on the scrubber. The deeper the dive the more at risk 

23

from Co2 poisoning the diver becomes. CCR manufacturers 

state maximum times for scrubber durations at various depths, 

this should not be ignored. For these reasons CCR divers 

should run a low set-point (

VASODILATION 

Carbon Dioxide is termed a vasodilator. Excess CO 2 dilates the 

blood vessels, which in turn allows more dissolved gases to be 

carried throughout our system, and to the brain. 

An excess of CO 2 can therefore increase the chance of O 2 

toxicity, nitrogen narcosis, decompression sickness and 

hypothermia, unless steps are taken to avoid it. 

Treatment 

As with all gas poisoning, the first step is to remove the toxic 

source. Because excess CO 2 is usually a result of poor 

breathing in some way or another, this is best done by taking 

deep, flushing breaths and relaxing. Reducing the depth may 

help a little, though buoyancy should be controlled, and all 

work should be ceased. 

If the diver is actually unconscious, he or she must be removed 

from the water and resuscitated if necessary. Give 100% 

Oxygen if available, though it may not immediately reduce all 

the symptoms, especially the headache. Carbon Dioxide build 

up can take many hours to subside even breathing Oxygen 

As with all victims of unconsciousness or gas toxicity, continue 

to monitor breathing and circulation, and seek formal medical 

aid as soon as possible. 

DECOMPRESSION ILLNESS 

Widely known as the Bends, Decompression illness is a 

condition all divers should plan to avoid. This can be a little 

difficult in practice. Decompression Illness describes a 

plethora of injuries caused by being immersed in a hyperbaric 

environment and then returning to one atmosphere or less. 

There are many methods for arriving uninjured at the surface, 

some procedural and some physiological. 

Decompression illness refers to both Decompression Sickness 

(DCS) in its many guises Types I,II,III,IV which are caused by 

inert gas complications, also DCI includes the various forms of 

Arterial Gas Embolism (AGE). DCI may manifest itself on 

surfacing, or several hours later, but even while the diver is still 

under the water. 

25

First Aid for any of the various forms of Decompression Illness 

is the same, administer the highest percentage of Oxygen 

possible, and make an effort to re hydrate a conscious patient. 

Basic First Aid procedure comes above all else and that is… 

Maintain an Airway 

Maintain a cycle of Breathing for the patient 

Maintain a rhythm of Circulation 

Remember the ABC’s 

There are many empirically tested procedures designed to 

minimize the chances of getting DCI but many people still get 

the bends in one form or another and the numbers rise steadily 

each year! Divers should be especially careful with dive tables 

that seem to be at odds with historically tested dive data 

Unfortunately, even following an acceptable set of rules and 

procedures to the letter one can still end up a patient in a 

recompression chamber. 

What rules are out there? 

Ascent Speeds: 10 metres per minute or 18 metres per minute 

or a variable with depth option, Deep Stops. 

Dive Tables: US Navy, US Navy modified, Buehlmann, DCIEM, 

AB2, VPM, VPMB, VPMBE, RGBM, Haldanean, Neo Haldanean, 

Dissolved Gas, Bubble Models, Gradient Factors. 

Last Stop at 3m or 4.5m or 6m or 9m? Gradient 30/80 or 20-100 

Breathing Gas: Nitrox Trimix Heliox Heliair or plain old AIR 

With all the choices above, each with a huge set of rules, a 

panel of experts and an internet forum dedicated to supporting 

the method/madness, it’s hard to separate the useful from the 

marketing hype. Even if you follow the rules and guidelines 

there are still some physiological causes for DCI that lurk in the 

shadows! 

26

The following physiological modifiers for DCI are many and to 

some extent preventable with best practice… 

Dehydration, Hypothermia, Inappropriate Workload, 

Patent Foramen Ovale (PFO). 

In virtually every case of Decompression Illness that requires 

recompression therapy, a modifier that is generally evident is 

DEHYDRATION. Lack of proper hydration causes circulatory 

inefficiency and decompression complications. 

Inexperienced Divers tend to have a combination of Rapid 

ascents combined with Dehydration, while Technical Divers 

completing decompression dives display both dehydration and 

rapid ascents, but also Inappropriate Exercise levels such as 

holding on to ascent lines for extended periods without 

alternating hands or Exertion at depth or using Liberal 

decompression planners. 

If you suspect that you or a buddy could be experiencing DCI 

symptoms or have made a rapid ascent it would be wise to 

immediately breathe 100% Oxygen and attempt to rehydrate. 

27 

A picture of a resort based 

recompression chamber. 

Divers engaging in 

Decompression or other 

Technical level diving 

should obtain insurance 

coverage that does not 

exclude such diving and 

includes recompression 

chamber treatment fees, 

which can be excessive. 

Enquire about 

recompression facilities 

available at your intended 

dive destination, or 

browse the internet for 

information.

A common situation that sadly exists amongst experienced and 

inexperienced divers alike is DENIAL. Some divers still attach a 

level of stigma to admitting to symptoms of decompression 

illness. There is no weakness in getting the bends, consider it a 

sports injury if the cause is not obvious. Decompression Illness 

often has a cause that can be removed with understanding. 

Better hydration or slower ascents and a more conservative 

decompression plan can minimise the likelihood of a repeat 

ride in a recompression chamber. 

Types of DCI 

Divers wishing to advance to technical diving levels should 

make themselves fully familiar with the physiology of 

decompression illness, a review of the common symptoms 

occasionally should ensure familiarity 

DECOMPRESSION ILLNESS : 

Pain / Sensitivity in or near Joints Weakness in limbs 

Purple / Red Blotchy Skin Rash General Body Pain 

Lack of Coordination Headache 

Tingling Sensation (anywhere) Numbness 

Loss of Bladder Control Nausea 

Extreme Tiredness Vertigo / Vestibular 

Dizzyness Mental Confusion 

Itching Skin Cold/Shock 

As you can see, the symptoms for Decompression Sickness are 

many, however the first aid procedure for any symptom is the 

same…breath Oxygen at the highest concentration available. 

Breath the Oxygen until there is no more, do not simply stop 

when your symptoms improve, as they are highly unlikely to not 

return! Try to drink fluids with the aim of rehydration. Water or 

Juice is preferred over Tea or Coffee. Refrain from drinking 

alcohol. 

28

Any diver experiencing unusual symptoms after diving is 

strongly recommended to seek advice from a physician who is 

knowledgeable in diving physiology 

Do not delay breathing 

OXYGEN… 

Second opinions are not 

required! 

HPNS or High Pressure Nervous Syndrome 

High Pressure Nervous Syndrome is a physical manifestation of 

a high pressure gas gradient across cerebral tissue 

compartments noticeable while breathing Helium and 

descending to extreme depth. It is exacerbated by rapid 

pressurisation to depths of over 150 metres but may appear at 

depths of between 120 and 200 metres (400-650’), depending 

on the speed of descent and the fraction of Helium, also to a 

degree, the physiology of the diver. Some divers, for reasons 

not fully understood, appear to be more prone to HPNS than 

others. 

The symptoms of HPNS include muscle tremors, drowsiness, 

loss of appetite, nausea, dizziness, vertigo, difficulty in 

concentrating, and visual disturbances, such as spots or 

patterns breaking up the diver’s field of vision. Some of these 

symptoms are common to several forms of gas toxicity or 

physiological stressors (e.g. dizziness, nausea, loss of 

concentration) and could be confused with nitrogen or Oxygen 

toxicity. It is quite common however for divers to confuse HPNS 

symptoms with mild shaking caused by cold water and 

breathing chilling breathing mixtures. 

In commercial diving, the effects of HPNS are reduced by slow 

and staged pressurisation, and by adding small amounts of 

nitrogen to "relax" tissues. Divers are pressurised to 

approximately 10-11 bars (90-100 metres) and held there for 

several hours for tissue saturation to take place, and the gas 

gradient to equilibrate. Pressurisation is then resumed, and 

the dive halted again after a further increase in pressure, for 

the process to repeat itself. The transit to the “bottom” may 

thus take many hours, far longer than is possible on open or 

29

closed circuit SCUBA, with an attendant decompression lasting 

several days due to the complete saturation of the divers’ 

tissues with the inert gas mixtures involved. 

To reduce the effects of HPNS, small amounts of nitrogen may 

be used in the mixture to “relax” different tissues 

compartments and so reduce certain of the side effects, 

notably the muscle tremors that are typically the earliest and 

least controllable of the effects. The tremors are thought to be 

caused by differential dissolution of gases into the tissues of 

the myelin sheath surrounding the nerves, causing the nerves 

to locally spasm. HPNS has been likened to a Neural Hail 

Storm. 

At depths of up to 120 metres HPNS is unlikely to be a problem, 

though in general, the greater the depth, the more the chance 

of the syndrome appearing. On the rare occasions that opencircuit 

divers have descended to greater depths, trimixes 

containing between 15-25% of nitrogen are thought to have 

contributed to the partial controlling (though not the 

elimination) of HPNS. Divers experiencing HPNS can try to stop 

or at least slow the descent for a moment if time and depth 

allow. The pause in pressurisation may ameliorate the 

symptoms. If symptoms do not subside the diver must abort the 

descent, before it becomes impossible to do so. Extreme deep 

divers often run high END’s (60-70m) to soften HPNS combined 

with relatively slow descents in the deep water and relatively 

fast descents earlier in the descent. 

Isobaric Counter Diffusion (ICD) 

Isobaric Counter Diffusion seems very fashionable on the 

internet at the moment. With OC and CCR Divers getting 

deeper for longer than ever before, once rare ICD accidents 

are occuring. With formal decompression ceilings getting in 

the 50 metre range (not deep stops), traditional gas 

switching methods and funky dive software could leave you 

less than healthy. 

ICD...occurs every time you switch from a light to heavy gas 

e.g. gas switch from Trimix to Nitrox. Gas switches from 

Trimix to Nitrox or even Heavy Trimix to Lighter Trimix 

typically cause a jump in END (equivalent narcotic depth) 

also. People do these switches all the time without getting 

(noticeably) injured. However, when certain conditions arise 

e.g. the gas switch occurs at a decompression ceiling AND 

30

the jump in END is sufficiently large, then horrendous injuries 

can and have occurred. Rarely though will the ICD risk be 

noticeable unless the dive profile is greater than say 

80metres for 30minutes i.e a very lengthy decompression 

with fast tissue deco stops. The severity of the injuries will 

reflect the current tissue controlling the ascent ceiling, for 

example: 

Deep gas switches generally impact fast tissues, particularly 

the vestibular apparatus. IEDCS (Inner Ear Decompression 

Sickness) examples have been recorded numerous times at 

deep stop gas switches where the new mix contained 

insufficient Helium and a resulting in a jump in END. IEDCS is 

the most well known symptom with its debilitating extreme 

vertigo and vomiting. However, any jump in END at or near 

an ascent ceiling may cause DCS somewhere. 

Jumps in END can occur during air breaks, back switching or 

gas switches. If the END changes dramatically at or near a 

decompression ceiling...BEWARE. Shallow gas switches 

(shallower than 21 metres) can cause similar consequences 

in slower controlling tissues. Slow tissues are less sensitive 

to jumps in END - they bubble, but you don't often notice it. 

Several divers have made panicked gas switches after a 

rapid ascent. Rapid Ascents will bring decompression 

ceilings much deeper and possibly even send them below the 

next ‘planned’ stop depth. 

Bubble Model generated dive plans used during Extreme 

Deep/Long exposures (100metres for 25minutes etc) will 

actually keep fast tissue ceilings very close to the diver by 

virtue of their excessive deep water deco stops. Contrary to 

their original postulates – ascent ceilings will not disappear 

however many deep stops you do! Experience has shown 

that doing too many deep stops will not speed up shallow 

decompressions. Older Bubble Model software use will put 

divers in very vulnerable situations as depth and time 

increases. 

On larger dives...END’s should remain constant or be relaxed 

very slowly. Steve Burton suggests, as a rule of thumb, a 

maximum 5% drop in Helium for every 1% increase in 

nitrogen (at or near a ceiling), especially if ascent speeds 

may be questionable. To know for sure what the biggest drop 

31

in Helium is permissible, simply run the dive-plan through 

DecoChek, still the only real-time and complete deco plan 

analysis program. 

Below is a screen grab from DecoChek highlighting ICD 

warnings. The dive profile is effectively 110 metres for 25 

minutes using a CCR. The image shows ICD warnings 

triggered by possible OC bail-out switches from a strong 

trimix bottom mix to various weaker trimix’s also air or nitrox. 

The planned Bailout gases contain insufficient Helium that 

cause a jump in END at the ceiling. All ICD risks are 

highlighted with a Yellow flashing icon. The program 

suggests the minimum Helium content to add thereby 

removing or minimising any Narcotic Jumps. DecoChek 

allows OC and/or CCR formats and will optimise any dive plan 

before you dive it. In the examples below you will see that the 

closer the stops come to the ‘ceiling’ the less END latitude is 

allowable. 

CCR divers can experience considerable risk when switching 

from the unit to ill conceived OC bailout gases. The risk is 

higher than the OC user as typically CCR users use far 

greater fractions of Helium in their bottom mix. The stronger 

32

the Helium in the bottom mix, the more care is required when 

switching to bailout mixes containing less Helium. CCR 

divers are recommended to take a cylinder of Bottom Mix or 

other gas that is breathable at maximum depth. In an 

emergency where ICD risk is unknown, switch to potentially 

high END mixes either very deep (far below stop ceiling) or 

very shallow (above 21metres where slower tissues are 

controlling) 

An illuminating IEDCS paper is circulating on the NET by Doctors Doolette and 

Mitchell. The text suggests the physiological processes behind IEDCS and the impact 

of counter diffusing gases on IEDCS. The paper makes loose recommendations as to 

gas switching protocol. Divers embarking on aggressive technical dives should obtain 

a copy or get a large team of support divers to hold them while they vomit 

disorientated on the up-line. 

IMMERSION DIURESIS 

Immersion diuresis is a condition causing frequent urination 

particularly a concern for long decompressions after ultra deep 

dives. If the diver remains vertical in the water during stops the 

pressure difference in the upper and lower torso will cause the 

body to turn plasma volume to water in an effort to lower 

hypertension. This water is expelled by frequent urination that 

left unchecked can dangerously dehydrate the diver. Technical 

divers should aim to consume at least 1 litre of (ideally warm) 

fluids for every hour spent decompressing. To minimise the 

pressure differences over the body during decompression, 

divers are encouraged to maintain a horizontal trim during the 

stops. In-water rehydration is always recommended. 

33

Chapter 4. Equipment for the job 

All advanced divers should strive to be self sufficient, there 

comes a time however that the decompression obligation 

cannot be fulfilled by personally wearing all the tanks 

required. Back-up tanks can be staged in caves or even 

quarries but the open water is far more dynamic and the risk 

of losing them too high. Support divers are key in any 

advanced and successful dive-plan. 

Many have made the decision to switch to CCR in the 

mistaken belief that this gives them more gas. While the unit 

functions correctly you may have plenty of gas but after 

malfunction often the naïve or inexperienced CCR diver has 

insufficient or inappropriate bailout and problems arise. Both 

OC and CCR divers need to utilise support teams when 

decompression times are longer than 2hours. Also CCR 

divers generally do not carry any reserves for team mates 

often preferring to ride lady luck. Successful extreme diving 

should not factor a great deal of luck into the plan. Easy 

access and well thought out redundancy is the key to sucess. 

Self sufficiency is developed both mentally and by 

familiarization AND practice with back-up kit. Other 

specialised equipment that improves the likelihood of 

success as you know is a spare mask, back-up depth/time 

recorders, deco-plans plus any additional cutting equipment 

such as knives or trauma shears. Of course a means of 

ensuring contact with the surface via an ascent line is 

preferred but the skills needed to improvise a new up-line 

with either surface marker buoy (SMB) or lift bag device as a 

back up is mandatory. Carrying this stuff is only beneficial if 

you can reach it and use it! 

Self sufficiency is not about taking as much equipment as 

can be physically attached to you! Self sufficiency and self 

reliance are abilities that should be practiced often and 

refined always. 

Self sufficient divers should still utilise the buddy system if possible. The deep 

ocean is not a game or a dare. Astute divers use redundant equipment systems 

combined with the skills and experience needed to make appropriate decisions 

striving to minime the impact of potentially challenging situations. 

34

Regulators for Deep Water 

Divers need to look past glossy adverts and dubious CE 

markings, to identify products that employ sound 

engineering instead of features that are seldom benefits. 

Most Scuba regulator designs are the result of a battle, on 

one side a desire to offer adequate performance coupled 

with a go faster stripe, on the other, the costs involved in 

manufacture. Although metal is the ideal material for most 

bits of breathing equipment, it is truly amazing to see how 

manufacturers like to shave down the metal in favour of 

cheaper plastic, after all jaw fatigue really is the number one 

danger underwater! 

There are many points to consider when purchasing a diving 

regulator, the ones I am about to explain, don’t often make 

the magazines equipment review criteria. If you are 

swimming at 5metres in the Caribbean and plan to do that 

forever, then it is safe enough to believe all the marketing 

hype. However if cold water and a little depth, combined 

with some air sharing, maybe even some future training are 

on the horizon, then the following information could prove 

useful. 

Regulators should be made of 

metal. Most first stages are made 

of brass that has a light chrome 

coating, however, virtually all 

second stages are injection 

moulded plastic. Plastic does 

have a couple of benefits, firstly 

its light weight making it ideal for 

travelling by airplane. Secondly 

its cheap, both price and quality. 

Metal second stages are far more likely to resist free flow 

due to thermal concerns. Metal second stages are far more 

resilient, lasting many more years, and servicing cycles. 

Another minor benefit to metal second stages is they attract 

condensation, this can offer a moister breath, especially 

useful with super dry nitrox. Plastic second stages have a 

reputation for being lighter in the mouth, but this is more the 

result of hose angle from the first stage or simply the position 

of the tank relative to the jaw. 

35

First stages can be made of brass, monel, titanium and its 

alloys, or aluminium. 99% of regulators are constructed of 

brass, with 1% or less made of titanium. Titanium regulators 

are very light, very tough, very expensive. Low weight can be 

useful if you travel with heavy luggage. Titanium regulators 

cannot be used with enriched air. Finally, aluminium in any 

form is best avoided within breathing regulators as anyone 

who has witnessed them dissolving in sea water due to 

galvanic action will confirm. 

Any engineer familiar with high pressure gas delivery will 

advise keeping the connecting hoses between stages as big 

“internally” as possible. Most high performance regulators 

use ½ inch diameter fittings on the primary hose. Typical 

(low) performance equipment uses 3/8 inch hose fittings, this 

smaller size has the negative effect of causing intermediate 

pressure drop, which makes the regulator lose smoothness 

or stutter and dramatically increases breathing resistance, 

causing Carbon Dioxide build up with the ensuing head 

aches or even blackout. The divers primary (½ inch port) 

regulator should be donated to the stressed out diver, to 

keep the air sharing process as comfortable and smooth as 

possible 

Hose Diameter - Intermediate Pressure Drop 

Environmental seals are next on the agenda. Do they help 

prevent free flow? Who knows…? Free flow can be caused by 

various situations. The Environmental seal simply prevents 

water from entering the first stage close to the working 

parts. The reasoning behind sealing the first stage is, that by 

36

keeping the cold water away from the HP seat area, this may 

minimise freezing. I think any novice diver knows that the air 

leaving the first stage gets very cold, more likely is this air is 

colder than the surrounding water temperature. The 

coldness is due to reverse adiabatic compression (Joule- 

Thompson effect). To see for your self, open a full tank valve 

and notice that the tank valve quickly covers with ice as the 

air escapes and expands. 

Environmental Seals are popular, mainly as they often 

associated with high performance and high quality 

equipment, this is mainly a marketing device to justify a 

higher price tag. Environmental seals keep water from 

circulating within the first stage, and this actually lowers the 

first stage temperature, thereby encouraging icing and 

possible free flow. As technical divers dive deeper and 

deeper it is important to know another highly undesirable 

feature of some environmental sealed first stages. 

Intermediate pressure amplification, or I.P boost. 

Looking through promotional literature, it is easy to see 

manufacturers that claim their regulator actually breathe 

easier as you dive deeper. This may have a limited benefit for 

divers who breathe very heavily and intend diving to 

recreational depths. These regulators offer a lower work of 

breathing as you get deeper and deeper, if you are breathing 

regular compressed air and diving to the limits of air diving, 

then you may notice the regulator getting easier and easier 

to breath. However, this breathing assist feature will become 

a full free flow beyond a certain depth. I have dived using 

environmentally sealed regulators on hundreds of deep 

dives, the free flows have affected, in some instances ALL of 

my 5 or 6 regulators! This feature is not beneficial to 

technical divers going to Trimix depths, therefore It’s a little 

surprising to see manufacturers marketing this system to 

deep divers. These regulators give perfectly satisfactory 

performance for shallow divers. This is purely a concern for 

deeper divers, and for very deep dives can 

have disastrous outcomes. 

The Environmental seal stops water from 

entering the first stage. If water (or its 

hydrostatic effects) does not manage to 

reach the main diaphragm, the regulator 

looses ambient pressure compensation. An 

37

un-compensated regulator becomes harder and harder to 

breathe as depth increases. Manufacturers used to add a 

liquid (silicone oil or even alcohol) between the 

environmental seal and the diaphragm but this was messy 

and makes the first stage incompatible with enriched air. 

Without the oil, another means of thrust transfer must be 

added. 

Certain manufacturers simply transfer thrust with a plastic T 

piece. It’s the shape of the T piece assembly with its typical 

mushroom shape that causes the undesirable increased 

intermediate pressure. The process behind the design uses 

the Thumb tack principle…a wide surface area providing the 

pushing power to a smaller end. This big to small change in 

surface area will give an increase in pressure that very 

effectively causes the pin to punch through hard surfaces. In 

this case the larger top of the T piece, compared to the 

smaller diameter of the spring carrier causes a gradual 

increase in intermediate pressure over ambient, resulting at 

some depth in unstoppable free flow. If free flow arrestors 

are in use, the hose itself will blow! Many divers will have 

noticed free flow problems with Auto Airs and Air 2’s and the 

like, when used with environmentally sealed first stages. The 

problem always worsens with depth…ideal. 

Environmental Seals – good news for reg 

salesmen…bad news for would-be deep divers! 

38

Intermediate Pressure Boost Chart for 

Environmentally sealed First Stages 

Amb Load Tx I.P.Boost 

Depth(msw) Pres. (+)Thrust(Kg) (bars-abs) I.P.Pressure(rel) 

0 1 0.00 0.00 9.50 

10 2 5.65 1.20 9.70 

20 3 11.31 2.40 9.90 

30 4 16.96 3.60 10.10 

40 5 22.62 4.80 10.30 

50 6 28.27 6.00 10.50 

60 7 33.92 7.20 10.70 

70 8 39.58 8.40 10.90 

80 9 45.23 9.60 11.10 

90 10 50.88 10.80 11.30 

100 11 56.54 12.00 11.50 

110 12 62.19 13.20 11.70 

120 13 67.85 14.40 11.90 

130 14 73.50 15.60 12.10 

140 15 79.15 16.80 12.30 

150 16 84.81 18.00 12.50 

160 17 90.46 19.20 12.70 

170 18 96.11 20.40 12.90 

180 19 101.77 21.60 13.10 

190 20 107.42 22.80 13.30 

200 21 113.08 24.00 13.50 

From the I.P Boost row, you will see the current hose pressure at the corresponding depth. 

Many seemingly ‘balanced’ second stages have a factory set yield pressure resulting in super 

light work of breathing or even free flow as shallow as 80metres. The matter is made worse by 

running higher than normal intermediate pressures, although IP should not be arbitrarily 

lowered without serious thought to the work of breathing (WOB) in shallower depths. High 

WOB means higher Carbon Dioxide and even accelerated breathing rates. 

Divers should not use environmentally sealed first stages for extreme 

deep dives (i.e below 180 metres) 

39

I’m not being pedantic by explaining these regulator 

problems; I’m just saddened by the ever increasing technical 

diver accident numbers. Some of these accidents may have 

been prevented by better understanding the equipment. I 

have been lucky to survive many underwater equipment 

related calamities; the equipment I use now has been chosen 

to meet very sound engineering principles, not simply a 

marketing budget. 

Finally, we should talk about 

whether the regulator should have a 

“down stream” second stage valve 

system (99% do) or an “up stream” 

design. The benefits of the down 

stream are simple reliability. Servo 

assisted - Up Stream second stages offer an extremely low 

work of breathing, but need lots of engineering know how to 

achieve this. Lots of engineering may work well in a less life 

continuing setting but when you are underwater, simplicity is 

often the key to survival. Many deep technical divers in the 

past have found their “up stream” regulator free flowing 

because the scuba tank pressure became too low to keep 

the system operating properly! Imagine…you are ascending 

up from a deep dive with dwindling gas supplies, maybe even 

gas sharing…the last thing you need is a free flow on top of 

everything else. Traditional down stream second stages will 

continue to work at far lower tank pressures than upstream 

or servo assisted regulators, this is vital. Another reason to 

avoid upstream valves is their tendency to gush on initial 

pressurisation. This blast of air needed to balance the 

internal servo mechanism will make it impossible to feather 

the tank valve in an attempt to breathe from a free flowing 

regulator. Possible lung rupture may occur! 

When deciding on which diving regulators to purchase, 

consider the future and were it may lead you. Regulator 

designs differ from model to model, but some clear winners 

are easily identified with a few hours of homework. The long 

and sometimes painful road I have travelled through over 

thousands of deep dives led me to a new regulator model in 

2003, The Mares MR22 first stage and Abyss second stages 

are ideal dance partners for a tango with the deep! On my 

last dive to 313m the combination performed totally reliably 

meaning I could focus on all the other problems! 

40

Doubles / Twinsets 

A popular way of ensuring a redundant 

air supply for technical dives is by 

connecting two tanks of the same 

large size. Twin tanks are available in 

many different cylinder sizes from twin 

7 litre tanks right up to twin 21 litre 

tanks. Dual scuba tanks are often 

connected via a manifold system, 

allowing the use of both tank contents 

from either regulator attached. Very 

deep dives need very big tanks. 

Double 15 litre tanks are considered 

too small when dives below 100metres for more than 

15minutes are planned. 

The optimum manifold system has an isolator valve mounted 

centrally that allows cylinders to be isolated in the event of 

major manifold damage (i.e. cave/wreck meets tank valves at 

great speed) or, the slightly more likely scenario of burst disk 

plugs failing (where fitted.) 

Many a divers’ pub banter is about tank neck O-ring failure 

draining the tanks in seconds, this is highly unlikely unless 

the valves themselves fall out with the O-rings! 

The most acceptable valve system in use for technical diving 

is the DIN system, it gives a more reliable regulator to valve 

attachment and by its captive O-ring design is resilient to 

knocks from an overhead environment. The regulators 

should be set up similar to the H valve system. One 2 nd stage 

per 1 st stage and one low pressure inflator from each 1 st 

stage complement a single contents gauge. Hoses from 

regulators are better routed downwards to minimise damage 

from sharp surfaces and collisions on dive boats 

Right side 1 st stage 

carries “long hose” 

regulator plus 

alternate inflator hose 

for dry suit 

41 

Left side 1 st stage 

carries spg hose and 

“short hose” regulator, 

also Inflator hose for 

wing/bcd

Reels and Surface Signalling Equipment 

Diver’s who dive in areas swept by a current or, who are 

conducting dives that involve a staged ascent, should be 

comfortable in deploying a surface marker buoy from a reel. 

An exception could possibly be diving in a cave system that 

allowed decompression while still confined within an 

overhead environment. 

Having a line that stretches from you to a buoy at the surface 

has many advantages. Firstly it shows people who may be 

following your progress from a dive boat, your approximate 

position. Secondly it gives a means of maintaining depth 

while conducting decompression stops. 

During this training course the deployment of reels and 

surface marker buoys (SMB) / lift bags will be practised 

extensively. Effective SMB deployment is your connection to 

the topside cavalry. 

There are as many designs of diver reels on the market as 

masks or fins. Many reels have a particular purpose, sadly 

this seems to be a unknown science to even the most 

seasoned professional diver. 

Reels are available that have 

been designed for the particular 

purpose of marker buoy 

deployment, but the vast 

majority of reels in divers 

possession are the type best 

used for deployment during the 

exploration of overhead 

environments. An example of a 

SMB reel will have a trigger to 

control the free wheeling of the 

Trigger 

REEL 

with SMB 

attached 

line spool. Pulling the trigger will allow the reel to freewheel 

freely. To add tension the trigger is released, this will stop 

the line paying out. This function is highly desirable as proper 

deployment will need the progress of the expanding SMB to 

be halted momentarily during its trip to the surface. 

Each time the SMB is stopped during its ascent, it will 

attempt to achieve the shortest path to the surface…a 

42

straight line. Left to its own devices the SMB / lift bag will 

ascend to the surface at unpredictable angles and will have 

used considerable more line than the linear distance to the 

surface. 

Reels that are designed 

for horizontal line laying 

have no trigger 

mechanism, and rely on 

friction wheels to simply 

slow their progress. 

Adjusting the friction 

wheel will do much to 

slow the free wheeling of PRIMARY REELS 

the reel, but cannot be 

relied upon to cease the line spool turning entirely. Line 

laying reels are often called Exploration or Primary reels and 

contain huge quantities of line, often in excess of 90m (300’). 

In the hands of advanced 

users, Exploration reels 

BACK UP REEL 

can be used for SMB 

deployment, but are far 

more difficult to deploy 

predictably. Purpose 

built reels designed for 

bag deployment are the 

perfect tools for the job. 

SPOOL 

Other designs of reels 

incorrectly used for SMB deployment are “back up” reel 

types (smaller Primary styles with 40m approx line) and 

Spools, which have no moving parts, being just the line 

bearing device. Spools are commonly used as Jump reels for 

joining passageways in Cave diving activities. Similar to 

“Back up” reels, Spools can be used for SMB deployment 

with extensive practice, but they are definitely not the 

weapon of choice or common sense. 

Just like reels, the type of lift bag or SMB you use can affect 

the outcome of a successful dive. Historically, lift bags have 

been used to give the diver a temporary up line to the 

surface, but SMB’s have clear advantages. 

Lift bags do have superior lifting abilities, but this ability does 

give the lift bag an inappropriate shape for providing 

optimum visibility due its low profile at the surface. 

43

Consider this: The height of the marker buoy 

above the surface is inversely proportional to 

the delay in finding you! 

Surface Marker Buoys should be at least 5’ (1.5m) tall and 

brightly coloured. Bright Orange or Yellow have colours have 

been proven to give the best contrast against an ocean back 

drop. Lift bags that stick up from the sea to an acceptable 

height in even the calmest seas 

would have to be in excess of 100 

50lb 

LIFT 

BAG 

lbs lift capacity. Lift bags of this 

size are very unfriendly to use 

underwater. Open ended lift bags 

tend to spill their air on arrival at 

the surface, but this can be 

minimised by pre attaching a small 

weight to the base. Surface Marker 

Buoys should be of a closed design 

with an over pressure valve. The 

closed SMB will not spill at the 

surface and is far more reliable at 

staying inflated at the surface with 

the additional benefit of superior 

height and visibility even in moderate seas. The picture 

above shows a 50lb (20kg) lift bag used as a surface marker. 

The picture right shows a 

comparison between a surface 

marker buoy and the 50lb lift bag. 

The surface marker is far easier 

to see at distance because of its 

clear height above sea 

advantage. Wreck divers that 

routinely decompression dive, 

are often advised to return to the 

anchor line rather than drift 

decompressing in the open sea 

or enter the lottery that is lift bag 

deployment. Boat Captains are 

very wary of the outcomes of 

relying on divers to send up lift 

bags or surface marker buoys. Sea searches of often many 

square miles are frustrating and time consuming but 

44

thankfully quite avoidable with the right equipment and 

practiced technique. 

Can you see the needle in the haystack below? 

The yellow dot on the landscape above is only 100m (330’) 

from the dive boat and the sea conditions are almost ideal. 

Imagine the scene with the sea a bit rougher and the 

captains one-eye looking through your bags rather than his 

telescope! 

Safe technical diving does not have to be an OXYMORON 

Any dives outside of the recreational envelope should be 

taken seriously. Although deeper diving seems to attract 

divers preferring dark coloured equipment, every now and 

again a diver lost at sea relies on a bright coloured piece of 

equipment to bring rescue faster. 

Conditions underwater can often include bad visibility and 

low light levels. These very conditions can reduce contrast to 

a point were hoses fade into wetsuits. 

Imagine needing a dive buddies assistance in poor visibility, 

a bright exposure suit and bright fins and a brightly coloured 

long breathing hose would seem highly desirable… 

What Do You Think? 

45

The theme running throughout all 

successful, technical dive 

planning is minimising discomfort 

and drama during sometimes 

very challenging situations. With 

sufficient planning and practice 

the most challenging projects can 

be safely realised. 

Picture right shows a diver 

prepared to give assistance in the 

most expeditious fashion. The 

bright orange wrapping over the 

Long hose clearly identifies the 

breathing source. The Yellow 

second stage cover offers a 

superior contrast in poor visibility. 

Wing inflator and deflator buttons 

are brightly coloured for easy access 

In demanding situations the adage of Every Little Helps can 

make the difference between an inconvenience and tragedy. 

Use of brightly coloured and tall 

surface marker buoys can get you 

spotted quickly, once back on the 

boat you can eat all the sandwiches 

and technical diving cakes! 

When buying a long hose or even an 

Octopus hose for the first time, 

consider a 

brighter hose 

material. They 

don’t cost any 

more and 

might save 

someone a lot of anxiety. A Bright 

hose wrap can give dark hoses an 

instant make over and protect them 

from abrasion and ozone damage. 

46

Wings and Harnesses 

There is always great debate on the best style of wing and 

harness to wear. A back mounted wing allows better trim and 

a face down position while underwater, jacket style BCD’s 

don’t make this so easy. When technical diving you should be 

mindful of buoyancy changes as depth increases. The wing 

or buoyancy compensator you wear must have ample 

buoyancy to counter act the heavy tanks you wear and the 

inevitable loss of buoyancy from your exposure suit. Dive 

boats and wrecks are sharp places; many times a diver has 

punctured a dry suit or buoyancy compensator at the wrong 

moment! Redundant buoyancy is necessary. This can take 

the form of single air cell buoyancy compensator and dry 

suit, or for the wet suited diver, a dual bladder wing would be 

advisable. 

Many advanced divers adopt a harness and back plate 

system together with a wing style bladder. The harness and 

back plate take the weight of the dual tanks easily, leaving 

the back wing to inflate freely behind them. A debate about 

“bungeed” or not “bungeed” wings is often quite spirited. 

Either wing when used properly, offers a safe system. The 

bulking of the wing when bungeed may indeed add a small 

amount of drag, but the bungeed wing allows less air 

migration giving more balanced buoyancy. Adding strips of 

car inner tube to the inner wing bag can be very effective at 

preventing punctures too. 

Redundant Wing System 

47 

Back plate and Harness

Most pieces of dive equipment will be attached to you by a 

clip of some sort. Some clips have preferred uses, some are 

multiple use. I think you could find whole websites dedicated 

to clips used by tech divers, which is a little sad, but hey 

each to their own! 

The photo below shows many of the popular clip styles in use 

by technical divers. 

The Double Ended Piston has many uses from attaching 

small accessories like knives or back up lights to stage/deco 

tanks. As it has two ends, It is highly unlikely to cause a 

problem should one end Jam. 

The Standard Piston Clip is popular as it comes in many sizes 

and has a swivel joint at the base. The larger sizes are useful 

for stage tank attachment use and smaller sizes great for 

back up lights or even your loose 2 nd stages. 

Butterfly Clips were designed to help clipping stage bottles 

onto D rings easier. Just pushing the clip against a stainless 

D ring will cause it to open. However with use the clip looses 

functionality and becomes as stiff to use as any other type. 

Gate Clips are sometimes referred to as Suicide Clips. This 

nickname came about by the clips inherent ‘Line attracting’ 

48

abilities; any line of small enough diameter will pass into the 

clip as if by magic. While this clip is useful for easy 

stage/deco bottle removal and replacement, it should be 

treated cautiously if used during line laying exercises. 

Mini Clips are perfect for SPG attachment and any loose 

second stages that need to parked for anytime. The finger 

rest makes equipment removal very easy. 

It’s a good idea to purchase good quality brass or stainless 

steel clips. The piece of diving equipment that you would like 

to attach will have the clip attached by cable tie or nylon 

string. Some divers prefer that the clip is attached to 

accessory by something easily broken like an O ring, but 

small cable ties do the same job and have fewer components. 

Talking of “line magnets”, one of the most unusual recent 

inventions used by technical divers involves using long metal 

springs as fin straps. The spring is usually (on the cheap 

ones) partially covered by plastic hose or cloth. These spring 

heel straps find reel lines and discarded fishing line like a 

beach clean-up session! 

If you feel your original fin straps are in some way hampering 

your performance, replace the straps with bungee cord or 

purchase a pair of fins with Bungee Heels Straps by design. 

Bungee last almost forever, is cheap and easily replaced, 

and lines do not gravitate towards them… 

49 

Spring Heel Straps should 

be treated with utmost 

care around lines

While we are on the subject of fins let us address some 

flipping technique! 

A diver needs to be able to support the entire weight of the 

equipment using his wing and/or drysuit system. When using 

heavy tanks a dual-bladder wing is mandatory as a drysuit in 

itself will never safely balance a deep water dive rig. Even 

lightweight tank users should consider a double wing as in 

the event of wing failure long periods spent on the surface 

can be very uncomfortable or even impossible in an over 

inflated drysuit simply trying to stay afloat or heads up. 

Drysuits are intended to keep the wearer dry-ish not support 

heavy equipment either in-water horizontally or at the 

surface vertically. Recreational divers may use drysuits to 

maintain buoyancy, but this is generally in combination with 

lightweight single tanks and in most cases also supplement 

their buoyancy control with a bcd at the surface. 

A good indicator of buoyancy control is trim or body position 

in the water. Equipment should be arranged so that the diver 

can float motionlessly horizontally. No additional upward fin 

or hand thrust to keep seemingly ‘neutral’ is necessary. The 

fin techniques shown below can only be performed with 

perfect poise and balance underwater. Even with huge tank 

configurations you will be expected to perform these fin 

strokes using your primary AND secondary sources of 

buoyancy. 

Scene 1: Shows Poor body 

position/bad trim. Head high and 

feet low, Fin wash directed 

downwards compensates for bad 

buoyancy. 

Scene 2: Showing good Modified 

Flutter kick position. 

Fin wash directed backwards. 

Body position shows ideal Trim 

with horizontal position 

50

Scene 3: Showing sub step of the 

Frog Kick. The Frog Kick is useful 

inside areas of heavy silt but no 

current. 

Scene 4: Showing sub step of Frog 

Kick. Fins are wide apart ready for 

next step of bringing fins together, 

as Scene 3 

Good trim while moving underwater is a sign of superior 

buoyancy control. Using the correct fining technique for the 

moment is a sign of experience. There are several different 

types of propulsion technique; some situations may call for a 

combination of all of them. 

Shuffle kick: Sometimes called the modified flutter kick, this 

kick style involves flexing the legs below the knee only. 

Keeping the thighs inline with the body, knees bent at 90 

degrees, fin movement comes from moving fins back and 

forth down to the midline and back. Fin wash should be 

directed backwards. This fin kick is best used inside a wreck 

or when ambling close to the bottom taking photos or video. 

Frog Kick: This kick is a little tricky to perfect, but involves 

kicking with both fins at same time, with the fin wash being 

directed inwards and backwards, a bit like a frog kicking! 

The frog kick is a popular kick with cavers and when 

mastered, very little effort is needed. Using this kick 

effectively needs perfect buoyancy control. It is not much 

use if currents are present 

Pull and glide: This technique is very useful for moving 

against a moderate current or in very confined areas. Simply 

using finger tips, pull gently and propel yourself forwards, 

51

you may have to kick fins additionally if the current is strong. 

Remember that when pulling yourself in narrow areas such 

as a shipwreck, watch the position of your legs and fins to 

avoid contact with the wreck. Using arm muscles to pull or 

stabilise, instead of legs is a good way to avoid overexertion. 

You will practice these different methods of propulsion 

throughout this course. Different fin techniques are very 

beneficial, especially if you decide that Cave or Wreck diving 

appeals to you. 

Good buoyancy control saves energy too! 

52

Chapter 5. DIVE PLANNING 

Dive planning takes many forms, from deciding which gas 

mixtures to be used, the decompression obligations that will 

be incurred and the gas volumes needed to complete the 

stops. If we know how much we breath in a minute (RMV or 

SCR) then we can begin to plan dives more accurately, 

although the astute diver will always take sensible reserves. 

Firstly, all tech divers must plan to take sufficient gas 

volumes to complete the dive with sensible reserves 

remaining. While often not the ideal reserve for all examples, 

the rule of thirds is adequate for most open water deep 

diving projects with regard to back-gas volumes. Gas 

volumes for decompression tanks may need to be doubled to 

cover various ‘what-ifs’ and a minimum of 7litre tanks used. 

10,12,15 litre tanks are often used as decompression tanks 

for extreme dives. 

Take enough gas, don’t just fill what you have and think its 

enough! 

53

So, lets plan a dive, 120metres for 15mins on the bottom. The 

easiest way to plan technical dives is to run the desired 

depth and time through PC based decompression software. 

There are many titles to choose from, some crap, some 

great. Technical divers must verify that their deco software 

is the latest version and that they are using it with 

parameters to give adequate decompression. The following 

examples were all derived from the Decochek Optimiser 

program. 

The gas volumes for 

this dive look like this: 

This is a lot of gas! 

The above dive has 4 gas mixtures but spread over how 

many tanks? We have 5204 litres without any reserve of 

bottom mix. A reserve of one third must be added so: 

5204 x 1.5 = 7806 litres 

54

7806 divided by 230bar = 34 litres of capacity needed, so 

triple 12 litre tanks are required of course double 18’s will 

work too. If you take less, learn to breathe water. 

This dive has a lot of deep water deco using trimix 21/30, 

roughly 5969 litres. How many tanks is this? We need a 

reserve…if we use thirds the volume swells to 8954 litres, if 

we took double then its 11962 litres! Okay lets imagine the 

third extra is enough. 8954 / 230bar = 39 litres of tanks or 

double 15’s and a 10litre and that’s only if it goes right! 

Here we can start to analyse the deco and try to make it 

more efficient. The 21/30 deco mix volume is more than the 

bottom mix volume so it may make sense to add another tank 

either deeper or shallower to help spread the deco load. 

Adding an extra deco 

tank gives a shorter 

overall deco time and 

allows a drop in Helium 

within ICD parameters 

The extra deco tank in Gas List 2 helps spread the 

decompression gas volumes more evenly. The reason we 

take the extra volume is to afford us the extra decompression 

time required if we are forced to go back to the previous 

deco gas in the event of regulator failure. 

In this case of regulator failure a suitable and easy to 

remember fix is to revert to previous gas and if still below 

50metres add 50% to the prescribed deco stop time. In the 

water if you have a multi-mix dive computer simply update 

the inspired gas in the computers gas list to recalculate the 

deco schedule. 

55

All bail-out options should be run through software as part of 

your dive planning to see the effects of missing the correct 

deco gas. Extreme depth dives performed alone need only 

take 1/3 rd extra gas volume per tank. Dives performed with a 

buddy should take double the required gas volumes. The 

outcome of taking insufficient gas to cover the various bailout 

scenarios could be the bends or it could be death, take 

enough and if not, plenty as its only you riding the chamber 

In the event of regulator failure, ascending while adding an 

extra 50% time will get you shallower within the realms of 

safety. A diver practised in swapping first stages underwater 

will want to consider this to get back on track especially 

considering the current gas may not last until the next 

change! Swapping regs underwater is straight forward if the 

first stages utilise the same connection system ie Yoke or 

DIN. All decompression tank regulators should use the same 

method of attachment to allow easy switching. 

Periodically inspect all tanks and regulators throughout the 

descent and bottom time. 

Keep all spg’s pressurised but the tanks turned off. 

Avoid all servo assisted 1 st or 2 nd stages…there are few 

benefits to these regulators and many potential problems 

most starting or ending with catastrophic free-flow. 

Take sufficient gas for all contingencies. CCR users may 

swap twinsets for suitable rebreathers but they must take 

sufficient OC bail-out including at least one cylinder of 

bottom mix. 

56

DECOMPRESSION PLANNING 

DecoChek Dive Plan 

Depth Arrive Stop Leave Mix 

0m 00:00 TMX 21/30 

120m 00:06 00:15 00:21 TMX 11/60 

66m 00:27 00:01 00:28 TMX 21/30 

54m 00:30 00:01 00:31 TMX 21/30 

51m 00:31 00:02 00:33 TMX 21/30 

48m 00:33 00:01 00:34 TMX 27/20 

45m 00:34 00:02 00:36 TMX 27/20 

42m 00:36 00:03 00:39 TMX 27/20 

39m 00:39 00:03 00:42 TMX 27/20 

36m 00:42 00:04 00:46 TMX 27/20 

33m 00:46 00:04 00:50 TMX 27/20 

30m 00:50 00:06 00:56 TMX 27/20 

27m 00:56 00:07 01:03 TMX 27/20 

24m 01:03 00:09 01:12 TMX 27/20 

21m 01:12 00:04 01:16 TMX 50/10 

18m 01:16 00:09 01:25 TMX 50/10 

15m 01:25 00:12 01:37 TMX 50/10 

12m 01:37 00:17 01:54 TMX 50/10 

9m 01:54 00:24 02:18 TMX 50/10 

6m 02:18 00:18 02:36 100% 

3m 02:36 00:36 03:12 100% 

0m 03:12 Surface 

This dive plan requires an ascent speed of 10 metres 

per minute from the bottom to the first gas switch. 

Clearly this is a lengthy dive and will need several 

support divers to ensure a drama free outcome. 

Another option is the staging of any extra 

decompression tanks on the down line (not 

recommended in an ocean environment 

To wear all the tanks needed here is to risk 

entanglement and confusion not to mention possible 

complete loss of buoyancy control if buoyancy control 

systems malfunction. 

This dive will require 2 x 18 litre tanks of bottom mix 

1 x 12 litre of 21/30 

2 x 12 litre of 27/20 

1 x 12 litre of 50/10 (minimally) 

1 x 10 litre of 100% 

57

It is recommended that a diver carry no more than the 

double 18’s plus the 12litre of 21/30 and the two 12 litre 

tanks of 27/20. This configuration is fairly easy to swim 

with and allows the user to get shallow enough 

autonomously to depths were support divers can assist 

without fail and in comfort 

Let us plan the same dive using CCR equipment with 

appropriate bail out. Setpoint used is 1.25po2. 

DecoChek Dive Plan 


0m 00:00 CCR 0.70 

120m 00:06 00:15 00:21 CCR 1.25 

60m 00:28 00:01 00:29 CCR 1.25 

57m 00:29 00:01 00:30 CCR 1.25 

54m 00:30 00:01 00:31 CCR 1.25 

51m 00:31 00:02 00:33 CCR 1.25 

48m 00:33 00:01 00:34 CCR 1.25 

45m 00:34 00:02 00:36 CCR 1.25 

42m 00:36 00:03 00:39 CCR 1.25 

39m 00:39 00:02 00:41 CCR 1.25 

36m 00:41 00:03 00:44 CCR 1.25 

33m 00:44 00:04 00:48 CCR 1.25 

30m 00:48 00:04 00:52 CCR 1.25 

27m 00:52 00:05 00:57 CCR 1.25 

24m 00:57 00:05 01:02 CCR 1.25 

21m 01:02 00:07 01:09 CCR 1.25 

18m 01:09 00:08 01:17 CCR 1.25 

15m 01:17 00:10 01:27 CCR 1.25 

12m 01:27 00:13 01:40 CCR 1.25 

9m 01:40 00:16 01:56 CCR 1.25 

6m 01:56 00:21 02:17 CCR 1.25 

3m 02:17 00:33 02:50 CCR 1.25 

0m 02:50 CCR 1.25 

Comparing the two plans you first notice that the CCR 

plan is 22 minutes shorter total dive time. This time 

saving coming from the maintained set-point which can 

give deco benefits but also a risk of pulmonary toxicity if 

taken to extremes. Long Decompressions (>3hrs) on 

CCR may benefit from switching to open circuit 

decompression from 6metres in an attempt to avoid 

exhausted scrubber complications. 

58

In addition to the CCR unit the diver must wear 1 x 

Bottom Mix 12 litre tank in case of CCR failure at depth 

plus the same 3 x 12litre OC deco mixes as the totally 

OC diver. CCR may offer some monetary savings 

regarding gas costs (these can be considerable if 

regularly using high Helium %’s) although unit purchase 

and upkeep plus additional travel freighting costs 

should be factored into any purchase equation. 

The author has used with great success the Pelagian 

DCCCR for many mid-range deep dives. The modular 

nature of this rebreather makes it very easy to 

incorporate into a twin set as the photos indicate. The 

benefits of combining CCR with OC twin-sets give 

considerable added redundancy to the CCR with of 

course the gas economies of the CCR. One fill of trimix 

8/65 in this setup has allowed 7 dives between 90 and 

147metres leaving more than 170bar pressure still after 

all of the dives combined! Future deeper dives will test 

the suitability of this system…stay tuned. 

Of course appropriate open circuit decompression 

tanks are always worn with the above unit. 

Always dive with completely filled tanks to help deal 

with unforeseen emergencies. Any spare breathing gas 

gives you additional thinking time…you never can tell 

when you will need it. 

59

A slate plan for a 120metre for 15minutes dive plan with 

appropriate contingency should include the following 

information 

DecoChek Dive Plan – Primary Slate 


0m 00:00 TMX 21/30 

120m 00:06 00:15 00:21 TMX 11/60 

Travel 00:21 - - 00:27 TMX 11/60 

66m 00:27 00:01 00:28 TMX 21/30 

54m 00:30 00:01 00:31 TMX 21/30 

51m 00:31 00:02 00:33 TMX 21/30 

48m 00:33 00:02 00:35 TMX 21/30 

45m 00:35 00:03 00:38 TMX 21/30 

42m 00:38 00:03 00:41 TMX 21/30 

39m 00:41 00:01 00:42 TMX 32/20 

36m 00:42 00:03 00:45 TMX 32/20 

33m 00:45 00:04 00:49 TMX 32/20 

30m 00:49 00:05 00:54 TMX 32/20 

27m 00:54 00:06 01:00 TMX 32/20 

24m 01:00 00:07 01:07 TMX 32/20 

21m 01:07 00:04 01:11 EAN 50 

18m 01:11 00:09 01:20 EAN 50 

15m 01:20 00:12 01:32 EAN 50 

12m 01:32 00:16 01:48 EAN 50 

9m 01:48 00:23 02:11 EAN 50 

6m 02:11 00:17 02:28 100% 

3m 02:28 00:34 03:02 100% 

0m 03:02 Surface 

Next, we will look at an adjusted stop plan for lost deco gas. 

Technical divers must carry two depth and time measuring 

devices on all decompression dives in addition to two copies 

of the dive-plan either on slates or in laminated form. 

The ‘Bail-out’ plan overleaf contains more information still 

and may be used instead of simply doubling-the-deco-still 

breathing-previous-gas mix idea or relying on an updateable 

mixed gas dive computer. These plans are Cut from the 

Decochek program and Pasted in a text editor for editing and 

subsequent lamination. I would optimistically wear the 

primary plan on my wrist and stow the bail-out plan 

separately in my drysuit pocket. 

60

DecoChek Dive Plan BAIL-OUT PLAN 

LOST GAS – NEW ARRIVAL TIME 

Depth Arrive Stop Leave Mix - T 21/30 - T 32/20 -EAN 50 -100% 

0m 00:00 TMX 21/30 

120m 00:06 00:15 00:21 TMX 11/60 00:06 00:06 00:06 00:06 

Travel 00:21 00:06 00:27 TMX 11/60 

66m 00:27 00:01 00:28 TMX 21/30 00:28 00:27 00:27 00:27 

54m 00:30 00:01 00:31 TMX 21/30 00:35 00:30 00:30 00:30 

51m 00:31 00:02 00:33 TMX 21/30 00:38 00:31 00:31 00:31 

48m 00:33 00:02 00:35 TMX 21/30 00:41 00:33 00:33 00:33 

45m 00:35 00:03 00:38 TMX 21/30 00:45 00:35 00:35 00:35 

42m 00:38 00:03 00:41 TMX 21/30 00:50 00:38 00:38 00:38 

39m 00:41 00:01 00:42 TMX 32/20 00:56 00:41 00:41 00:41 

36m 00:42 00:03 00:45 TMX 32/20 00:57 00:44 00:42 00:42 

33m 00:45 00:04 00:49 TMX 32/20 01:00 00:49 00:45 00:45 

30m 00:49 00:05 00:54 TMX 32/20 01:05 00:54 00:49 00:49 

27m 00:54 00:06 01:00 TMX 32/20 01:11 01:01 00:54 00:54 

24m 01:00 00:07 01:07 TMX 32/20 01:18 01:10 01:00 01:00 

21m 01:07 00:04 01:11 EAN 50 01:28 01:21 01:07 01:07 

18m 01:11 00:09 01:20 EAN 50 01:34 01:24 01:17 01:11 

15m 01:20 00:12 01:32 EAN 50 01:45 01:34 01:29 01:20 

12m 01:32 00:16 01:48 EAN 50 02:01 01:47 01:46 01:32 

9m 01:48 00:23 02:11 EAN 50 02:21 02:05 02:10 01:48 

6m 02:11 00:17 02:28 100% 02:51 02:31 02:44 02:11 

3m 02:28 00:34 03:02 100% 03:13 02:50 03:01 02:46 

0m 03:02 Leave 3m stop @ 03:57 03:29 03:40 03:45 

The above plan shows the primary plan with the additional 

modified new arrival times dictated by a loss of a deco gas. 

Once a deco gas is lost then you simply stay at your current 

depth until you can travel to arrive at the next depth 

according to the modified runtime. Consider swapping a 

regulator 1 st stage change to reclaim lost gas, ideally not 

below 100m. Feathering tank valves from free-flowing regs 

wastes gas and cannot be performed if using upstream 2 nd 

stages! 

Remember that the next deco gas can be safely breathed as 

deep as 2.0bar Po2 if necessary. The above contingency plan 

only covers the loss of a single deco gas…if you lose more 

than one, it probably wasn’t your day to begin with and you 

should have stayed in bed! 

61

On a serious note, multiple deco gas failures could be 

handled semi-effectively by breathing what you can until the 

last breath before switching to the next semi-hot (high po2) 

deco gas. Remember…you cannot breathe water and bolting 

to the surface likely means death through embolism. 

However, minimising deep water deco rather than blowing 

huge chunks of it away can temporarily fix seemingly giant 

problems. Getting to a depth where you can summon help via 

support divers is key. Advise your support divers that you 

might botch a yellow smb deployment and intend to indicate 

any emergency situation by rhythmic pulling of the smb line. 

A dancing smb on the surface will be the cue that something 

is not well below the waves. If in doubt, check it out, quickly! 

Spare gas shouldn’t be deployed from the surface on long 

lines below 21 metres as it might simply get dragged 

insufficiently deep by any prevailing currents. 

Ensure that all ‘next regulators’ are working properly by 

testing them while still BEING ABLE to breathe something 

acceptable BEFORE SWITCHING to a potentially broken reg! 

Switching regs underwater is no problem if you are relaxed 

and practised. 

Traditional downstream 2 nd stages allow the breathing of 

tanks to far lower supply pressures than upstream/servo 

assisted 2 nd stages which typically free flow at low supply 

pressure especially at high ambient (surrounding) water 

pressures – you’ve been warned! 

Regarding plans that include longer bottom times or deeper 

dive depths, it is recommended that you simply find out how 

deep the dive site is likely to be (accurately) and follow the 

original dive-plan based around this maximum depth. As a 

rule of thumb its recommended that if you stray a metre or so 

deeper than planned for 75% of the bottom time then 

subtract 1 minute of bottom time from the plan for every 

metre you went deeper. Momentary lapses in depth control 

at the bottom can be ignored. If you leave the bottom early 

the runtime can be merged (caught up with) by waiting at the 

first stop depth until actual time coincides with run-time. 

Once times are merged continue with the original schedule. 

62

Making pencil notes on your dive plan can help a stressed 

mind! 

Overstayed bottom times cannot be easily predicted as the 

causes are not predictable for example will it be 1 minute or 

12minutes or more! Follow advice from your wrist computer 

from 40metres and upwards. If the computer has a lookahead 

feature compare this against your slated plan. If you 

feel as though you spent longer at depth or ascended too 

slowly then add time to your shallow water deco times (from 

21metres and upwards)…in this scenario an extra hour of 

preventative deco is less painful than a ton of chamber rides. 

Following purely wrist computer predicted plans is fine as 

long as you have 2 of them and they read very similar timesto-the-surface. 

REALISTIC DIVE PLANNING IS CRUCIAL – NOT ALL 

CHAMBERS ARE SWANKY LUXURY! 

PROPER 

PLANNING 

POSSIBLY 

PREVENTS 

PISS 

POOR 

PERFORMANCE 

63

DIVE COMPUTERS 

Dive computers are 

available that can reliably 

handle complicated 

deco plans, even those 

using combinations of 

open circuit and closed 

circuit equipment with 

multiple Enriched air or 

Trimix mixtures. Dive 

computers need to be understood and used properly 

otherwise they end up being ‘locked out’ and even being 

useless. 

If your software generated dive plan approximates the plan 

generated by your dive computer, then it would be 

acceptable to use the computer as the primary or secondary 

dive plan, or vice versa. You should ensure that the ascent 

rates used by the dive plan and the computer are similar and 

ideally 10 metres per minute. Many computers and dive 

timers like the one pictured above use variable ascent rates, 

that are not suited to technical dives especially those using 

Helium in any of the breathing mixtures. 

1 Mix Nitrox as 

back-up gauge 

3 

7 Mix Nitrox/Trimix 10 Mix Nitrox/Trimix 

OC or CCR 

Well heeled divers may even use multiple multi-mix trimix 

computers without any slated or laminated back-ups. This 

does not negate the need for pre-dive planning of course. 

Virtually any dive computer can be used as a back-up gauge 

for advanced and technical dives. Before purchasing, make 

sure that the device uses a fixed 10 metres per minute ascent 

rate and reads to the depth required. Some models will give 

64

depth and time even after they have entered a ‘Lock up’ 

mode, which is useful for redundancy and repetitive dives. 

Happy Crazy Deep Diving :D 

65

Chapter 6. What Comes Next 

Candidates should enter training completely ready and able 

to dive to the maximum depth reached during their previous 

technical level course. 

Right, hopefully we’ve read and understood everything. It’s 

now time to get wet…in case we haven’t studied hard then all 

material will be covered on Day 1 

Day 2 will cover equipment setup and a practise dive to 

40metres. During this dive a series of skill evaluations will 

take place. 

Adequate trimix dive planning skills 

Switching from Primary to Back-up mask and back 

Manifold Shutdown drills without struggle within 30secs 

SMB deployment 

Buddy awareness 

Gas sharing with long hose and buddy breathing 

Runtime follow +/- 2 minutes 

Gas switching through 2 decompression gases 

Exhibit advanced buoyancy control skills 

Remove and replace 2 decompression cylinders 

Remove and replace decompression regulator underwater. 

Subsequent levels of training will follow these depth 

progressions over at least 4 days. Further demonstration of 

the above skills may be requested at anytime on any of the 

following dives. 

MOD 1 training 

Day 3 Plan and execute a dive to 55m lasting 50-70mins. 









66





Day 6 Plan and execute a dive to 120-130m lasting 100- 

120mins. 

Maximum depth may not exceed 130metres to stay within 

current maximum depth guidelines stated in DAN diver 

recompression insurance documentation. 

SAFE DIVING! 

67

Greenforce-Dive lights 

Illuminating the 

darkness, wherever 

you’re going… 

68

1 Author: Mark Ellyatt – Technical Instructor Trainer - Inspired Training

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