Jim

I hear what you are saying. Fortunately most of the dives I do here in
Sydney are in the 50m to 60m range and are quite achievable on 50% only. 

Do you have any ideas as to how one might minimise the risk of oxtox while
still getting the benefit of the oxygen window at an appropriate gradient?
Full face masks for deco appear to raise as many if not more questions than
they solve. 80/20 is a bullshit solution which does not achieve the optimum
balanced PPO2 of 1.6, even if taken at 9m/30ft - 1.52. 

Cheers
Steve Bliim



-----Original Message-----
From: Jim Cobb [mailto:cobber@ci*.co*]
Sent: Tuesday, 5 September 2000 0:13
To: Steven Bliim
Cc: Tech Diver
Subject: Re: 80/20 deco


Some practicable information from what I've picked up over the years...

While bubble formation has it's hazards in deco diving, it's generally
manageable outside of clusterfucks. What does concern me is the
unpredictability of oxygen toxicity. As anyone who works in a hospital can
tell you, oxygen will fuck up your lungs. They are very careful about how
long they put patients on 100% because O2 can be toxic and will cause
permanent tissue damage.

With diving we compound the issue by breathing high partial pressures of 02
which increases the potential for damage. This is just a fact of life for
technical divers, just like getting deco hits, barotrama, and all the other
stuff. Exertion makes the situation worse, it is good idea to keep the PP02
reasonably low during the working part of your dive. Personally I like the
1.1 to 1.2 range.

And more importantly, beyond the tissue damage, we divers also have to worry
about toxing out on 02. Going into convulsions makes it impossible to breath
the regulator in your mouth. To be frank, oxtox scares me more than any
other aspect of tech diving. The list of divers killed by oxtox is long and
has some prestigious names.

At what point an oxtox occurs while on high PP's of O2 is an inexact
science. It behooves the diver to be very cautious when doing high PP02
deco, this includes having support divers keep an eye on you when you are at
the 20' stop.

It is also important for you to recognize the onset of a oxtox, and be ready
to break to your back gas quickly. If you have to do some sudden exertion
while at an high PP02 level, for example dealing with a Jon line in a high
current condition, don't hesitate to switch to your back gas until the
exertion is over. Just as there are "undeserved" deco hits there are also
"undeserved" oxtox hit's which take place outside of all the theory. You are
a very lucky sonofabitch if you survive one.

That said, I know that the roots of 80/20 were to "idiot-proof" the 20 foot
stop, Tom Mount said so. But 80/20 has become institutionalized to the point
that in order to justify it outside of allowing idiot divers a nice
brainless deco, they now say to do your 20' stop at 30'. This is so stupid
as to boggle the mind. And in going to 30' foot to justify their stupidity
they did not think it through and think that 80/20 still has it's magical
ability to save you from an oxtox and thus no air breaks. This is really
stupid, dangerous bullshit.

Air breaks are good. Outside of your "official" 5 min breaks, switch to your
back gas any time that you feel funny no matter how short/long you've been
at a high PP02. You don't have to stay on for long and you don't have to
increase your deco because of it. An oxtox can kill you, and there is no
sure gauge of when one will strike, so act accordingly.

Just some practical thoughts that you no doubt learned in training, but good
to go over again to keep it at the forefront of your "things to think
about."

   Jim
 -------------------------------------------------------------------
 Learn About Trimix at http://www.cisatlantic.com/trimix/

> From: Steven Bliim <Steven.Bliim@Mc*.co*.au*>
> Date: Mon, 4 Sep 2000 17:16:32 +1000
> To: 'Jim Cobb' <cobber@ci*.co*>, Guy Morin <xnet@vi*.ca*>
> Cc: Tech Diver <techdiver@aquanaut.com>
> Subject: RE: 80/20 deco
> 
> Jim has hit one of the nails on the head here. Anyone doing their deco on
> 80/20 from 30 feet as opposed to 100% from 20 feet is still going to be
> facing a similar PPO2 for similar times. They are going to need backgas
> breaks in the same way that those using 100% will need backgas breaks -
that
> is if their deco goes for long enough to need backgas breaks. So much for
> that argument!
>  
> Have I got it wrong, can anyone tell me why the 80/20 crowd will not need
> backgas breaks?
>  
> Regards
> Steve Bliim
> Still waiting for my trip to the NE!
> 
> -----Original Message-----
> From: Jim Cobb [mailto:cobber@ci*.co*]
> Sent: Saturday, 2 September 2000 1:33
> To: Guy Morin
> Cc: Tech Diver
> Subject: Re: 80/20 deco
> 
> 
> So what you are telling me that you will use whatever it gets you out of
the
> water quicker, damage to your body be damned, and whatever I say or
anybody
> else says will not make you change your mind.
> 
> I don't know what it takes to get you guys to realize that the idea with
> deco is to remove nitrogen from you body and you don't do this by
breathing
> more nitrogen. Nitrogen is what causes the damage.
> 
> I also, IMHO, your thinking is flawed when you presume that using 80/20
> obviates the need for air breaks. Seems to me if you are doing *any* mix
> where you are spending extensive time at 1.5 PP02 or above you would want
to
> do air breaks to avoid long term damage to your lungs.
> 
> Oh, yeah, the only thing that matters is getting out of the water fast, I
> forgot.
> 
>   Jim
> -------------------------------------------------------------------
> Learn About Trimix at http://www.cisatlantic.com/trimix/
> 
> 
> 
> From: Guy Morin <xnet@vi*.ca*>
> Date: Fri, 01 Sep 2000 09:24:54 -0400
> To: Jim Cobb <cobber@ci*.co*>
> Cc: Tech Diver <techdiver@aquanaut.com>
> Subject: Re: 80/20 deco
> 
> 
> 
> 
> Hi Jim, 
> 
> Thank you Jim for confirming what was elucidated
> in the post. For myself, you are quite right that I will
> be using the theoretical model since it has a good
> track record, and consistently estimates what I am
> actually diving. Doesn't the WKPP have something
> in regard to diving to what is planned? I would have
> to guess not, since they never dive what they plan.
> 
> Only a fool would dive following the WKPP method
> without knowing all the details of what they practice,
> and that certainly isn't available on the net, on this list,
> or through word of mouth.
> 
> As far as your practice versus theory, well that part
> is going to be ignored the WKPP obviously hasn't
> put in the same effort in 36/80 deco as it has in 50/100.
> 
> On another note, remember that it is someone who was
> promulgating the WKPP method as being superior that
> pointed out the tissue loading from the theoretical model,
> not me. Let's keep that little detail in mind, shall we? He
> who lives by the sword...
> 
> Finally, I will point out that from a logistical and consumables
> perspective, 50/100 deco does make better use of resources.
> If I was wanting to get hundreds of divers through the water
> to stage a big push, that would be my choice too. The major
> costs for a large operation is gas consumables. Using 36/80
> deco requires a greater investment in blending, and gas
> matching, and wastes a lot of gas. The reasons are obvious,
> getting off back gas at around 100' prevents the back gas
> from getting drained as would be the case in 50/100 deco.
> Using back gas in deco allows it to be drained following more
> liberal rules than thirds. In addition, since shallow deco allows
> switching to back gas, that makes further use of back gas,
> and economizes the O2. Using O2 is simple to blend, and
> is insensitive to error. Since a lot of time is spent at the
> shallow stops, not having to blend huge volumes of gas,
> and the subsequent analysis required, and schedule tweaking,
> it makes a lot of sense, especially on that scale.
> 
> So, from an operational standpoint, 50/100 is by far the best
> choice. For the diver who doesn't have to worry about consuming
> millions of cubic feet per year, and who wants accurate deco
> estimation in a field environment that often does not afford
> the luxury of a decompression chamber, something more "consistent"
> makes a lot of sense.
> -- 
> Guy  
> 
> 
> 
> 


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