Decom Gas - a debate

From: Tom Mount

Friday, May 15, 1998 8:52 AM

Posted on Techdiver

Bill (Mee),

I would like to take a moment to clarify a couple of issues with you on

IANTD’s 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:

    1. 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

management techniques

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

decompression models)

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 70’s)

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 IANTD’s 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


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.


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.


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

the tank.


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.

Tom Mount


-----Original Message-----

From: Bill Mee <

To: <

Cc: <

Date: Wednesday, May 13, 1998 2:51 PM

Subject: Baker's Dozen Revisited


Fellow Divers,

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.


Bill Mee



(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.


George Irvine

Director, WKPP

"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

decompression profile.

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.


Bill Mee