Nitrox...with AIR computer

[MechDiver]

sharpenu once bubbled...

The response by mech diver that said you wont get near the O2 limit is incorrect, as is his statement that nitrox does little to increase bottom time.

Here are the profiles:

Mix Depth Time Surface Interval
37 67 51 na
37 62 61 46
36 62 32 57
36 64 41 16 hours
36 65 60 70
36 65 51 62

I suggest you go back and reread my posts. I did not say it does little to increase bottom time. The poster was also talking about depths in the 70 - 120 ft range. Take the posts in context please.

Also, there are different levels of "O2 limits". I stand by my comments.

MD

 

[sharpenu]

Sorry.....no offense

 

[MechDiver]

sharpenu once bubbled...
Sorry.....no offense

No problem to disagree with something. If you're reading the posts differently than how I intended them to read, its fine to question. Thats one of the problems with ng postings.

MD

 

[The Iceni]

jonnythan once bubbled...
Breathing response is controlled completely by CO2 partial pressure, not oxygen. At the surface, you breathe more than enough oxygen.. and at only 33 feet you're inhaling twice theo oxygen... but producing the same CO2, which is why your breathing doesn't slow down to half.

Breathing is reflexly controlled almost completely by CO2 partial pressure, but oxygen receptors do exist. However, I will accept what you say, jonnythan - as general rule.

However minute volume (i.e gas consumption or SAC) is also affected by conscious influence which will overide reflex respiratory depth and rate (unless the CO2 threshold is exceeded). Thus for a given level of exercise a relaxed, comfortable, warm diver will tend to have a lower minute volume than a tense, stressed diver in cold water.

There are many anecdodal stories of Nitrox divers having lower SAC than air divers. I am sure this must be multifactorial.

As for the original question I am not sure a diver needs such an added safety margin provided by using air "tables" with Nitrox since the incidence of unexpected DCI in recreational diving is so very low. However, I have experienced the downside.

We used my buddies air computer (at her request) on a 30 metre dive during which we were both using Nitrox. Towards the end of the planned (table) bottom time we were momentarily separated so the ascent was slightly delayed, but not beyond the planned run time. My buddy's computer subsequently gave us a total of 15 mins of quite unecessary deco since the table (and my own computer, set to EAN32) had given us both a no-stop dive.

My buddy was very perturbed by the deco obligations and subsequently balled me out during the debrief. I quietly pointed out that the extra fifteen minutes spent hanging about in the water was quite unecessary. We would both have been better off out of the water after a simple safety stop.

I am not really sure if she understood the point I was making.

 

[Wreck Wench]

Hi Doc....you hit the nail on the head...at least as far as most divers are concerned. If you are diving with another nitrox diver who is diving a nitrox computer then your air computer will give you tons of obligated deco that in reality is not accurate. Assuming of course that the diver with the nitrox computer stayed within no decompression limits.

If a diver gets annoyed at the 'bogus deco obligation' and bails like your friend did....then the computer will be locked out and you won't be able to dive until the next day....and in some cases even more if you own a very conservative air computer and the person diving the nitrox computer was more aggressive....

If you are diving nitrox on an air computer and end your dive within no decompression limits then yes you are safer assuming you do not exceed the maximum operating depth for the gas mix.

 

[Dr Deco]

Dear SCUBA SOURCE Readers:

Nitrox and Safety

The tables, and meters, are quite conservative with respect to safety, and you actually gain little in safety by gas mix alone. A DCS incidence of 0.0001 reduced from 0.00015 is not much.

It is difficult in chamber tests to see small differences in gas loads with respect to DCS. In fact, incidences must be based on filed data since tables will (generally) not have any DCS by the time they are released for use.

What is not done, however, is to test dive tables with exercise during the off gassing (i.e., decompression or surface) phase. Yes, all tables will be tested with working divers at the bottom, but not working on the surface. DSAT (PADI) tables did have field trials with divers ascending the ladders with their gear and the differences in Doppler bubble scores between them and the laboratory dives were small.

Nonetheless, when you go to large gas loads (as in altitude decompression for aviators or astronauts), the differences of exercise versus no exercise are readily apparent even if you have only ten test subjects. In fact, the differences in DCS incidence are several fold (not several percent). Thus my take home message would be, it is more important to watch the physical activity when climbing onboard and afterwards than considering nitrox versus air.

Gas Usage

As others have indicated, this is largely a matter of self-control at low levels of exertion. When you are very active, the carbon dioxide in the blood will govern the ventilation (= breathing) rate. This is similar to heart rate. When you are very active, heart rate is determined by the metabolic demands of the body. When you are not very active, heart rates vary to a large extend based on relaxation or anxiety.

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[BRW]

Folks,

In perusing thru this post, some seem to regard nitrox as
some kind of voodoo gas, one frought with problems.

In actuality, the exact opposite is true. It's really higher
concentrations of N2 that are frought with problems, and air
is the worst. Statistics show that DCS risk decreases with
decreasing proportions of N2, as seen at the DAN Nitrox Wkshp
a few years ago (Proceedings available thru DAN).

And most tech divers know this as an operational principle for
staging. Lower and/or eliminate N2 in your breathing mix, and
backfill with He once your O2 content is fixed by ppO2 concerns.
We have a sign in the C&C locker room that says "God gave
us helium to dive, the devil invented nitrogen". All this sums
for us by noting that we never dive with more that 25% - 35%
N2 bottom mixes. No matter what the application -- shallow,
deep, deco, bounce, repet, altitude, etc. Since going to this
"minimal N2" mode, we have not had a single problem for the
past 10 yrs. Period -- from trimix at 350 fsw up to enriched
heliair (triox or helitrox) in the 50 fsw zone, deco, repets, altitude,
bounce, etc.

For recreational diving, nitrox is a far better gas to dive than
air, gives you longer bottom times, safety, and can scarcely
broach the oxtox limits on a single 80 ft^3 cylinder. Tables
and meters abound for nitrox, are tried and tested, and should
be employed just as air tables and meters now. It now makes
no sense to dive EAN32 and EAN36 on air tables or air meters,
because you are wasting mix utility for little else.

From a Training Agency point of view, the day is coming when
introductory courses will be nitrox courses, with air a "subcase"
of interest because of mix availability. But not the diver "choice"
when some standard EANs are available instead. It's happening
now on dive boats, resorts, and in basic courses. And this is
a good thing. Might I also point out that enriched helium
mixes (heliair) are replacing EAN mixes where He is available.
And this is even a better thing -- especially for deeper exposures
and repets.

Best regards to all,

Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Leader

 

[The Iceni]

Wreck Wench once bubbled...
. . If a diver gets annoyed at the 'bogus deco obligation' and "balls you out" like your friend did....then the computer will be locked out and you won't be able to dive until the next day....

Perhaps that's why a few still reckon this is a sensible practice?

If a diver has invested in an air computer he is stuck with it until he has the readies to upgrade to a Nitrox computer, and what sensible diver would want to buy such an inflexible, and to my mind now obsolete, item of kit from him?

If you are a novice seeking advice on what computer to buy my advice would always be to buy a (programmable) Nitrox computer. Even if you do not intend to use Nitrox at the time of purchase, after a few more dives I guarantee you will change your mind. And of course a Nitrox computer can always be set to air.

This is not to say that I do not consider tables to be the best option for genuine technical diving but we are talking here about the way most people dive and as BRW confirms Nitrogen is the devil's gas - not Oxygen.

Keeping the ppN2 as low as possible is the way to go.

 

[BRW]

Folks,

Amen to using nitrox computers. And full up dual phase
computers like the EXPLORER for any mixed gas and tec/rec
diving (yes, it's RGBM).

Expect others also using the RGBM before long -- see
RGBMdiving.com for instance.

Regards,

Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Leader

 

[The Iceni]

BRW once bubbled...
Amen to using nitrox computers. And full up dual phase
computers like the EXPLORER for any mixed gas and tec/rec
diving (yes, it's RGBM).

Expect others also using the RGBM before long -- see
RGBMdiving.com for instance.

Regards,

Bruce Wienke

Yes Bruce, I like the Hydrospace Explorer. I was intending to invest in one and played with the downloadable simulator for many, many hours. Whether I would have used it for technical diving during a dive, as oposed to using it as a dive planner, I am not so sure.

Indeed RGBM is most probably the way to go. However, like Dr Deco I now believe we must sort out how to allow for varying levels of (isometric) exercise during a dive, as I gather that this is now the single most important variable in the generation of all those nasty unwanted micronuclei.