Diving nitrox on an air computer

[DocVikingo]

And that is that diving nitrox to air tables bestows no demonstrated safety margin or edge.

While the lower N2 loading achieved by diving EAN to air tables may be theoretically appealing as a "safety factor," at present there simply is no corpus of scientific evidence for this. It has not been shown that diving nitrox is "safer" than diving air in the sense of reducing the risk of DCS, the severity of DCS or any other aspect of the sphere of DCI.

In fact, EAN is arguably less "safe" than air no matter what tables are being used. Several posters have raised the OxTox issue, and indeed diving nitrox greatly increases the likelihood of CNS oxygen toxicity if maximum operating depths or oxygen clock limitations are exceeded.

As DrDeco stated, "In practice, the incidence of DCS is very low in recreational diving." It is in fact so low that based on statistical considerations alone it would be a monumental undertaking to show a safety edge for EAN even if it did exist.

What diving EAN to air tables does do is reduce decompression obligations, required surface intervals & dive to fly times. It also reduces air consumption. There is much anecdotal evidence that it reduces fatigue, but this has yet to be proven.

Best regards.

DocVikingo

 

[BRW]

Just an observation about air versus nitrox
diving in the deco sphere -- and what that
might imply in general.

You can deduce nothing about relative
safety of nitrox versus air in the recreational
arena. Nor a lot of related questions.

Stats for recreational diving are in
noise for just about everything -- save large
reverse profiles, frequent and closely spaced
repets, and high altitude ventures -- and
shed little light on important mechanisms.
You need go to deep, deco, long, and frequent
diving scenarios to learn anything really new.
Statistically, you get nothing if your sample
space isn't representative of mechanistics.
And recreational diving isn't a good sample
space.

So little can be inferred about air vs nitrox
diving risk in the recreational arena. Same
said for a number of things. DCS hits rates
in recreational diving are in the 1/10,000 to
1/100,00 range -- virtually zero. Such statistics
are not very useful to modelers, table designers,
and people doing risk analysis on gas mixtures,
shallow stops, deep stops, etc. DAN Project
Dive Safety needs hit profiles to validate and
analysze. Hits are good for risk analysis, but
bad for divers. Catch 22. DAN will tell you
that they need more "hits'.

Oxtox is also of little concern -- do an oxtox
calculation for 3 repets on EAN36 to 100 ft to NDLs
and see. Do one for light deco to same depths.
Enriched nitrox and helitrox MODs at 1.4 atm
have 40 min limits -- not recreational bill fare
on standard 80 ft^3 cylinders.

Oxtox is second order compared to DCS in
recreational diving, and even in most tech diving.
Oxtox limits are far more variable and less
well defined than deco NDLs.

So recreational regimes are poor purveyors
of important data. Our RGBM Data Bank
doesn't request recreational data.

However, in the tech regime, this changes.
Anecdotal and statistical data abound. It's
clear EAN is a better deco, switch, and diving
gas ala tech experience. Air is riskier because of
DCS. Nowadays, helium mixes are standard, with
pure O2 switches in the shallow zone. Stats
are good and helium divers feel better. Whole
other story.

Let me just close with log data from the C&C Dive
Team. This is 15 yrs old -- we don't even dive
nitrogen anymore (as you can gather from above),
but it illustrates my point:

-- in multiple (3) deco dives to 100 fsw range
for 20 -30 minutes, hits on air were
3 times hits on EAN36, and 2 times hits
on EAN32;

-- oxtox was neither a computed nor observed limitation;

-- there were no gas switches here, meaning
travel gas = bottom gas = stage gas;

-- we abandoned air 15 years ago because of
hit rate;

-- today, we dive (only) helium mixtures;

-- deep air has a catastrophic tech history.

Also, you will find more amd more recreational
dive operations, boats, etc leaning towards
the nitrox diver for all of the above. Plus
their own experience.

Finally, gas transfer dynamics for both
free phase and dissolved gas phase buildup
and elimination favor EAN according to
even the crudest medical models. This
is seen clearly in the longer NDLs for EAN
versus air, and proof of pudding is seen
in the scores of nitrox divers following
just such protocols on tables or meters.

What this means to many is that nitrox diving
is far SAFER than air diving. Period.

And it goes even farther -- minimize nitrogen
in breathing mixtures whenever you can for
safety sake. Nitrogen is very soluble, forms
big bubbles, is highly narcotic, and is very
heavy compared to gases like helium. Not a
good diving gas.

Mike stated it nicely, "as N2 dose goes up,
so does DCS risk" -- and that is quantifiable.

Bottom line is simple -- if you have a choice
between diving air or diving nitrox, ALWAYS
dive nitrox.

Cheers, and regards to all

BW

 

[pescador775]

Dr's Vikingo, Deco and Paul T. are correct. The short answer is that a Nitrox diver should use a Nitrox computer. For 30 years I dived air using a Scubapro DCP meter sometimes called a 'bendomatic'. I am one of the few divers on this board who has actually had the bends. It wasn't the fault of the meter. It was a random or freak event in which I failed to look at the meter before surfacing. Even so, I believe that the incident would not have happened if I were hydrated and rested before making that twilight dive. Also, if I had not lifted some heavy stuff right before the hit while onboard the boat. (The X factor of heavy lifting may be doubly important if the diver has an FO which I do not). I have tested the 'Bendomatic' alongside some popular computers and I can assure you that the average electronic computer is far more conservative. There is such a safety factor (but nothing can protect you from random, foolish fate) in these computers that I believe that using an air computer for EAN is unnecessary, and in light of the arguments concerning decompression stops, inadvisable.

 

[scubadobadoo]

How many times have I typed exactly what the "Docs" have mentioned regarding the "safety margin" within recreational diving and been flamed and called an idiot? MANY times.

As for why anyone would even CHOOSE to use nitrox while on air tables, I am at a loss. Don't run if your not being chased.

 

[DandyDon]

How many times have I typed exactly what the "Docs" have mentioned regarding the "safety margin" within recreational diving and been flamed and called an idiot? MANY times.

As for why anyone would even CHOOSE to use nitrox while on air tables, I am at a loss. Don't run if your not being chased.

It's a 5 year old thread. I missed your reason for bringing it forward...?

 

[scubadobadoo]

It's a 5 year old thread. I missed your reason for bringing it forward...?

I didn't revive it. Someone else did.

 

[JeffG]

I didn't revive it. Someone else did.

The post above yours is dated October 13th, 2002, 01:37 PM

Over 5 yrs ago.

 

[DandyDon]

I didn't revive it. Someone else did.

The post above yours is dated October 13th, 2002, 01:37 PM

Over 5 yrs ago.

Perhaps a glitch connected to the whatever caused the outage...?

I'll report this.