Decom Gas - a debate
From: Tom Mount
Friday, May 15, 1998 8:52 AM
Posted on Techdiver
I would like to take a moment to clarify a couple of issues with you on
IANTDs recommendation of the use of EAN 80 in TRAINING PROGRAMS. I will not
go into detailed history on how we evolved to the use of EAN 80 as I have
already explained that.
When you did your training we were (if I remember correctly) still using
oxygen on deco.
Over time I witnessed as did other instructors some (only a few, but still
important to safety) divers develop oxygen toxicity symptoms on oxygen at
the 20 foot stop. This included one diver who like all of us had been using
oxygen for several years as a deco gas.
Following these occurrences we looked into several alternatives for
screening or avoidance. We considered the use of an oxygen tolerance test,
only to be told by the Navy and others that it really is not a reliable
index except in the event of someone highly reactive to oxygen, plus we had
the additional problem of chamber availability and the cost of doing a
tolerance test. From Navy Days and while at the UM RSMAS I was involved in
oxygen tolerance testing we (UM RSMAS) for several years conducted the Coast
Guard test we never had anyone on the DRY (dry is much different than wet)
testing on a 60 foot one hour exposure have a problem.
This is fairly consistent with dry chamber runs such as treatment etc. In over 100
treatments I only witnessed one oxygen problem and that was pulmonary not
CNS. So for a variety of reasons we decided that oxygen tolerance testing
was not feasible and really would not provide reliable data. Check with
about any physiologist and they will almost always agree with the
reliability issue of tox testing. Combined with the cost and availability of
chambers it is a poor option at best for a screening requirement.
Based on the point that a few people had exhibited in water symptoms we felt
it was not a good policy to REQUIRE the use of oxygen at 20 feet (1.6PO2) So
the big issue for us in training in regard to oxygen decompression is the
20-foot stop exposure.
Then of course there is the additional problem in open sea (caves have more
stability and fresh water a slightly lower PO2) decompression as divers tend
to vary a couple of feet even with good buoyancy control due to changes in
body attitude, waves passing overhead, or when on down lines not being able
to get to the exact 20 foot stop even with the use of John lines in current
(every one does not have the freedom of being able to hang neutral with a
drift line, or ride on a deco station, such as in NE hanging on a line in
the current.) plus in open sea one is never completely at rest as in a cave
type deco environment. Also some divers occasionally vary more than a couple
of feet at some point during a deco schedule. AGAIN another reason for
looking closely at the 20 foot oxygen stop in a training program where the
student is being taught all these procedures
Options then became:
- design in a 15 foot stop for the use of oxygen
2. Look at a EANx mixture for the stops that is significantly higher than
the bottom mix and one that will also allow a faster decompression schedule
to reduce the hypothermia and dehydration problems as well and still provide
good off gassing
After reviewing these options and discussing them with several hyperbaric
physicians and physiologist we elected to go to a high EANx mixture. Thus
EAN 80 was selected. Two sets of tables in a variety of mixtures both Nitrox
and trimix were designed. Based on either EAN 75 or 78 with the intended gas
being EAN 80.
One set begins the EAN 80(75) at 20 feet the other begins at 30 feet
So all current IANTD tables offered feature these mixes in technical
programs. In recreational Advanced EANx we use EAN 50 for safety or required
stops in the process of acquainting divers with the advantage of having more
oxygen in the safety or real stop gas than what is in the bottom mix. In the
intermediate trimix program we have a set of tables with one gas switch. The
switch on that table is to EAN 70 at forty feet.
The intent of these tables is to eliminate a need for a diver to be exposed
to a PO2 of 1.6 and also to stay within the oxygen exposure limits.
As stated these tables were developed for training program use. Even the
trimix tables stay within the confines of two gas switches and do not
involve bottom times such as encountered on exploratory dives.
No one has stated that EAN 80 is a better choice for a deco gas, it is one
we think is more suitable for divers in a training program where they are
involved with learning a lot of new skills and both gas and equipment
Based on Doppler testing I have done it appears that there is very little
difference in bubbles detected on EAN 70 starting at 40 feet, EAN 80
starting at 30 feet and oxygen starting at 20 feet. However with mixtures
less than EAN 70 there is a significant increase in the bubbles detected. It
would be great to actually have some large-scale scientific studies on this;
maybe someday funding will be available to do this.
It is for these reasons that IANTD endorses EAN 80 as a training deco mix.
Interesting some major expeditions have opted for EAN 80 as well due to
logistics and other reasons including risk exposure these include the
Britticana, and the Atlanta both 400 foot dive profiles. These expeditions
were completed with no incidence of DCS
On tables IANTD looked at the following models (all Haldian this will most
likely change as more new concepts are put into practice in validated
1. USN tables we felt that these offered the least amount of safety as the
stops begin to shallow and they act rather like doing a treatment for
bubbles that are formed
2. DCEIM again we felt that the stops begin to shallow and we also felt that
the stop times were excessive
3. Bulhman The stops were deeper than the other tables, it has more diving
history (validation) worldwide than the other tables, at the time we were
advised that it was more in tune with current theories, most dive computers
use this model thus it can be used in conjunction with computers in some
instances, most decompression soft ware is based on this model. We realize
the Bulhman model is not perfect but it seemed to suit our needs better
than all other existing validated models. Recently in our newer tables that
are available we have actually incorporated some deeper stops concurrent
with what we believe will, be the trend in future tables.
(Bill if you remember back to when you were doing your course I
presented material, just as I still do, by Hills and Fife on thermodynamic
models solely because they did endorse the deeper stops. Even when I was
at the UM RSMAS we used to present. This in our classes during the mid 70s)
Now you brought up the boat emergency gas, IANTD does not endorse EAN 80
as an emergency gas for the boat use, we think all boats should have oxygen
available for emergency use. While its is important for both recreational
and technical diving it is especially true for recreational diving where the
risk and occurrence of AGE is greater than in technical diving. I repeat it
is IANTDs opinion that charter boats should have 100% oxygen available as
an emergency gas for diving accidents.
Now on deco bottles they must be labeled per IANTD standards
This is direct from our standards
C. LABELING EANx MIXTURES WITH LESS THAN 41% OXYGEN
1. Cylinders already painted yellow must be labeled with a 4 inches (10
centimeters) green band. The band must wrap around the tank starting at the
flat of the cylinder. The label must state "Enriched Air Nitrox" (or an
applicable acronym). IANTD has EANx decals available for labeling tanks.
2. Cylinders painted with colors other than yellow require a 6 inches (15
centimeters) Enriched Air Nitrox band (decal) with the top 1 inch (2.5
centimeters) and the bottom 1 inch (2.5 centimeters) yellow. The middle of
the band (decal) should be printed with a green background and the message,
"Enriched Air Nitrox" (or an applicable acronym) printed in yellow for easy
3. Note: Scuba tanks without a green and yellow label with suitable contents
description-wording to identify Nitrox, EANx or an equivalent acronym must
not be filled by IANTD Facilities. Any color-coding other than yellow and
green violates community standards, IANTD Standards and International
4. Nitrox (EANx) cylinders must have Cylinder Contents Labels affixed to
them. The Contents label must state the oxygen percentage currently in the
cylinder and the Maximum Operating Depth (MOD). The label must be dated and
signed by the person who purchased (or requested) the mix (verifying he/she
confirmed the O2 percentage).
5. This label may be a separate label, or part of the EANx band. IANTD has
waterproof, reusable Cylinder Contents labels and cylinder content tape
available for order.
6. Nitrox (EANx) cylinders must have a current Visual Inspection (VIP) decal
stated that oxygen-compatible lubricants have been used in the system
(valve, tank neck, and/or cylinder). EANx cylinders used with EAN 41, or
greater, or for direct partial pressure blending use, must have an Oxygen
Service Rated VIP decal. IANTD has VIP decals (which also cover both tank
and valve inspections) available for order.
D. LABELING EANx (GREATER THAN 40%) AND OXYGEN (GREATER THAN 91%) MIXTURES
1. Cylinders containing oxygen levels of 91% or greater must be painted
either green or white, or have a 10 inches (25 centimeters) long by 4 inches
(10 centimeters) wide green adhesive band decal with the word "OXYGEN"
clearly and prominently printed in white on it. IANTD has waterproof Oxygen
Cylinder Decals available for order.
2. Cylinders should not be painted in common sport diving colors such as
black or red as underwater these are hard to read the oxygen label.
3. Regulators used with tanks containing high levels of oxygen must also
have green or white second stage covers and/or hose wraps or other means of
identification as oxygen service regulators, leading to the second stage.
Alternatively, the second stage purge buttons may be green or white.
4. Cylinders containing EAN 40 to 90 must have a color-coded "oxygen green"
and white label decal boldly stating, "Decompression Mix EAN* _____." or a
Breathing Gases other than air decal that is red and white. This label must
be 10 inches (25 centimeters) long and 4 inches (10 centimeters) wide. IANTD
has Cylinder Contents labels available for order. * Or, equivalent acronym.
F. LABELING TRIMIX MIXTURES
1. Cylinders containing Trimix must be labeled with either the word,
"Trimix", or "Breathing gas other than air". A Cylinder Contents Label
reflecting the percentages of Oxygen, Helium and Nitrogen must be affixed to
Again IANTD is a training organization and our standards are based on and
continually updated on those things we feel will provide the maximum safety
to the students in a course.
Bill I have explained some of the process of why we recommend and cut tables
with EAN 80 in training (some of our instructors do use oxygen) and also why
we elected to use Bulhman tables. I really do not care if you approve or
disapprove of EAN 80 in training nor do I care if you think the Bulhman
model provides "weenie decos" In fact the answer is for the benefit of those
who may be interested in the logic behind our deco gas selection and the
tables we use.
These tables were not designed nor intended for use in dives of the
magnitude of the dives performed by the WKPP
We have not and would not recommend our tables for WKPP type projects. Dives
of this magnitude requires custom tables and your liability issue as regards
a validated table are not the same as a training agency.
Even the Hamilton tables you have and the ones Billy and I have differ as
yours were created to your specifications, and ours were simply ones Bill
developed for what he felt would provide us with his standard Decap safety.
From: Bill Mee <email@example.com
To: firstname.lastname@example.org <email@example.com
Cc: firstname.lastname@example.org <email@example.com
Date: Wednesday, May 13, 1998 2:51 PM
Subject: Baker's Dozen Revisited
Once again the subject of 80/20 rears it's ugly head. We are now told
that the use of this peculiar gas mix is somehow or the other possibly
an unwritten defacto IANTD teaching standard. Let me remind you that the
WKPP does not use this gas mixture for anything and if you made the
mistake of showing up with this stuff you would never get out of the
parking lot. For those who may forget the past and possibly repeat it I
reiterate the famous "Baker's Dozen" reasons why we do not use this gas.
One of the reasons for carrying pure oxygen as a deco gas is that it
will be immediately available in an emergency. The administration of
pure oxygen is SOP in the aftermath of almost all forms of diving
related accidents. This may prove to be an issue in subsequent wrongful
death litigation in that having "no pure oxygen" is essentially
indefensible and could be considered negligent.
Using some homebrew dive table program to justify the use of 80/20 is
not reasonable inasmuch as the Buhlman algorithm (upon which almost all
of these programs are based) is a diffusion based compartmental model
which does take into consideration micro-physiological issues.
PLEASE REREAD the Baker's dozen and take this seriously.
A (BAKER'S) DOZEN REASONS WHY WE (WKPP) DO NOT USE 80/20
(By George Irvine)
1) This gas was introduced in an effort to overcome the inability of
unqualified student "tech" divers to control their buoyancy in open
water, and is as such is yet one more concession to doing things in a
convoluted fashion to offset a self- inflicted set of problems brought
on by the "doing it wrong" thinking that pervades diving today.
2) A heavy sea is not a problem for a deco stop if it is not posing a
lung-loading problem. Look at your depth gauge in a heavy sea and "see"
for yourself what the changes are - insignificant, and if they are not,
you should either not have been diving or incurring a decompression
liability of this magnitude in the first place. In the event of a change
in conditions during the dive, see below where the 80/20 becomes a
liability rather than an asset.
3) In the interest of using a standardized set of gases for which you
can permanently mark your bottles , it is a poor concession to inability
to sacrifice the benefits of pure O2 to accommodate a real or perceived
lack of skill - learn to dive before taking up techdiving.
4) In this same interest you will find that when you graduate to real
diving, as in caves, you will not want to accelerate your ppo2 at
lower depths while still being faced with a long decompression at
shallower depths, and making bizarre mixes to do this is a dangerous
mistake (just like the fantasy of holding an accelerated ppo2 on a
rebreather throughout a deco). I am anticipating the thinking that the
80/20 crowd would then go to an additional oxygen in cave without
accounting for total exposure, and subject themselves to the risk of tox
in the final deco steps. Tox you do not get out of - bends you do.
5) The 80/20 mix is in fact totally useless and contraindicated as a
deco gas. At thirty feet it is only a 1.52 ppo2 ( the real 1.6 ppo2
gas would be 84/16) and as such does not either provide the right
oxygen window, nor does it does it work as well as pure oxygen without
an inert gas at any depth. The gas mixing in your lungs has already
lowered the effective ppo2 enough to prevent spiking at 20 feet anyway
with the use of pure oxygen - in other words, we are dealing with a
simplistic misunderstanding here, or "old wives tale" that is typical in
6) If 100% oxygen is a perceived buoyancy control risk at 20 feet, then
why is the same ppo2 ( intended) not a risk at 30 feet? This shows the
total lack of reasonable logic involved in the decision to use this gas,
as well as a lack of understanding of the whole picture ( see the rest
of this discussion).
7) Along those lines, all we hear is howling about "oxygen cleaning"
above 40% mixtures, and dive shop proprietors on here complaining about
scuba tanks with oxygen in them being filled in their shops. With a
pure oxygen system, the tank only ever gets filled with oxygen from
oxygen tanks, not from every dive shop compressor it sees. Again , this
shows the total inconsistency of agency thinking, and reveals that the
true reason for this gas is to pretend to lower liability for teaching
incompetents to dive, which is bull, and to attempt to accrue some
inventive accomplishments to the dive agency pundits who themselves
prove that they do no real diving by making this recommendation
in the first place. This is like the colored regs, the stages on either
side, the quick-release buckle, and the poodle jacket: nonsense of the
most obvious nature developed through one-dimensional thinking by those
whose universe of understanding is not only severely limited, but blinded
by the hubris of not being the "inventor" of the techniques that work.
8) Any perceived decompression benefit of using a higher ppo2 at 30
feet with 80/20 is then given back by the lowered ppo2 at 20 feet, not
to mention the fact that the presence of the inert gas in the breathing
mixture defeats the purpose of using oxygen in the first place ( see
the Physiology and Medicine of Diving) . The ppo2 of 80/20 at 20 feet
is 1.28, not much of an oxygen window, and at 10 feet it is 1.04 -
useless for deco. To make matters worse, you can not get out from your
30 foot stop in an emergency ( not doing the other stops) on the 80/20
mix without really risking a type 2 hit.
9) This is a dangerous method to achieve a greater total volume of gas
for the bad breathers (another obvious reason the gas is in vogue), who
should not be incurring these decos, and even that benefit of having
more gas is lost since it is breathed at 30 feet, and then has to last
for the other stops. The fact is that gas is effectively saved by using
the lower deco gas up to this point, relying on the pressure gradient
to both achieve the deco and provide a break from high the previous
gas's higher PPO2 prior to going to pure oxygen where the spike could
be a problem on an extreme exposure without an adequate low ppo2 break (
again this shows that the 80% user is a neophyte diver with no real
experience or understanding of the true risks of these dives) .
10) The 20-30% longer 30 foot time on the lower ppo2 is not only
overcome on the pure oxygen at the next stops, the breaks do not come
into play until the initial good dose of pure oxygen has been absorbed,
since you are not spiking from a high pervious dose without a break
that is effectively achieved on the previous gas. These things need to
be understood and taught by the agencies, not some superficial
convolution that is designed to obfuscate the problem rather than
openly acknowledge and deal with it in a responsible fashion.
11) In an emergency situation, getting onto the pure O2 for 20 minutes
or so (for long dives something approximating the bottom time or a any
decent interval) would give you a real good shot at getting out of
the water having missed the rest of your deco and living through it
with pain hits only. You have to think these things all the way though,
not go for the transparent superficial thinking of those who merely are
trying to "make their mark" with some "great" idea they can call their
own. The acid test is , as always, is the caliber of the divers who
adopt these practices.
12) If there is some problem with your deco or you otherwise develop
symptoms and need oxygen either on the surface or back in the water, it
is silly to have not had it there all along. 80/20 is a joke for that
purpose, unless you have asthma, in which case any accelerated oxygen
mix would be a nightmare. This is again part of the "thinking it all the
way through" philosophy which is obviously missing from the 80/20
13) Only a card-carrying stroke would do something like this, and
showing up with 80/20 is no different than wearing a sign on your back
saying "I am a stroke, and have the papers to prove it". It announces to
all the world that you have no clue, kind of like wearing clip-on
suspenders or having dog dirt on your shoes.
"Do It Right" (or don't do it at all)
Bill Mee's post:
Thank you for exhaustively laying the reasons why we or anyone else
should not use 80/20. The only thing missing from this discussion is
the Q.E.D. at the end.
Reason #8, reiterated here for discussion purposes is perhaps the
soundest reason, among many very cogent ones, as to why this practice
should be avoided:
" Any perceived decompression benefit of using a higher ppo2 at 30 feet
with 80/20 is then given back by the lowered ppo2 at 20 feet, not to
mention the fact that the presence of the inert gas in the breathing
mixture defeats the purpose of using oxygen in the first place ( see
"The Physiology and Medicine of Diving") . The ppo2 of 80/20 at 20
feet is 1.28, not much of an oxygen window, and at 10 feet it is 1.04
useless for deco. To make matters worse, you can not get out from your
30 foot stop in an emergency ( not doing the other stops) on the 80/20
mix without really risking a type 2 hit. "
The rush to embrace this practice, recommended by technical diving
diving opinion leaders, was widespread and in retrospect, irrational and
poorly thought out, like so many of the "trial balloons" in this field
of endeavor. It seemed to many, at first glance, to be a simple means of
increasing one's supply of deco gas while eliminating its bothersome
volume and mass. In fact, the perceived benefits transform into
liabilities when subjected to a thoughtful analysis. When you view
decompression as a two pronged challenge: to progressively widen the
oxygen window and increase the diffusion gradient to maximize passive
transport of dissolved inert gas, it becomes clear that the 80/20
solution falls short on both requirements at a critical point in the
Section 11 emphasizes a very compelling argument for those who are
concerned with managing dive related crises. When diving in the open
ocean divers and boat operators should always be prepared to "scram" the
deco at any time. This could be for any number of reasons, not the
least of which might be a sudden change in the sea conditions or
unscheduled events such as dive accidents or impending ship collisions.
Just follow the Whitefish Point thread for an excellent example of why a
deco may require being aborted (or never started in the first place).
Most unfortunately the "80/20 problem" bears a strikingly resemblance to
several other ad hoc technical contrivances mentioned in this same
article (section 7) i.e. dual bcs, colored regulators, bilateral stage
bottle positioning, poodle jacketed second stages and harness quick
releases. All of these ideas, while seemingly reasonable, become
tainted when subjected to thoughtful review.