Art, I was kind of thinking that the worse shape you are in , the better
that trick would work on a relative basis: the guy who sits on the
backgas and extends the stops will ongas more stuff than he loses(
although in different places) - his only real chance is to force the gas
out and treat himself with the oxygen.

For the fit diver, helium especially is a great gas even when the ppo2
is low. Bill Me and I deco out on backgas a lot and don't get hit on
relativlely deep dives with short bottom times. I hate to admit this,
but we do a lot of these on single 80's with no deco gas at all. Now
that should get the weenies howling, but see Weinke's stuff - his work
says it is very doable.


Paltz, Art wrote:
> 
> I've been following this thread with good interest.  Are we saying that on
> this particular profile that had we lost the 50/50 mix that we should
> probably just follow the planned 50/50 stops but use back gas?  Then do our
> planned 100% stops as scheduled?  I think this probably works if you could
> just come up to 20 foot anyway.  Knowing that most people are not in the
> same shape as George, the question is for the normal diver in relatively
> good shape who has done similar dives without having to resort to in-water
> contingency planning (i.e. has not lost a gas) and has had no trouble,
> should they just follow the planned schedule substituting back gas for
> 50/50?  Sorry so long but that's just the way it came out.
> 
> I think to summarize and simplify, if in descent shape and experienced in
> these types of dives and you loose the deeper gas should you:
> a). Substitute lost gas with previous gas (possibly back gas) and continue
> with planned schedule.
> b). Do to previous gas (back gas in this instance) and try and fudge a
> little by adding stops (probably dangerous on deep stops)
> c). Panic and surface, signal for correct gas and try and go down and hold
> your nuts hoping someone comes to the rescue.
> d). Something else?
> 
> The above assumes that "previous gas" is probably better than just saying
> bottom mix.  Correct me if I'm wrong but I would assume that if you were
> using say 35% Nitrox, 50/50 and O2 in the schedule you should use the
> remaining 35% for the 50/50 stops if you lost the 50/50 gas.  Maybe on a
> profile where you're necessitating more than 2 deco gases the answer
> changes.
> 
> Thanks,
> Art.
> 
>         -----Original Message-----
>         From:   kirvine@sa*.ne* [SMTP:kirvine@sa*.ne*]
>         Sent:   Monday, January 10, 2000 10:14 AM
>         To:     Shimell, David (shimell)
>         Cc:     Ian PINKSTONE; techdiver@aquanaut.com
>         Subject:        Re: I think I understand
> 
>         Move up = shallower. You can do the 50% as if it were oxygen
> timewise,
>         and if you lose that you can do the backgas as if it were 50% and go
> to
>         the oxygen. What I am saying is do not extend the table because you
> have
>         the wrong gas - the upper deco steps can be screwed with
> unmercifully
>         without any real consiequnces, it is the deep ones that you must so
>         corectly.
> 
>         I'll let you in on a little secret - you could do this dive and come
>         right to 20 in quick steps  , do 20 minues on oxygen and get out
> clean -
>         done it, and done al ot worse and gotten away with it. These are the
>         real measures you take in an emergency.
> 
>         Shimell, David (shimell) wrote:
>         >
>         > George
>         >
>         > Most interesting stuff...
>         >
>         > >In real life, you are btter off moving up and taking the gradient
>         > >offgassing as bubbles while still underwater than you are sucking
> on the
>         > >wrong gas and lengthening the stops,
>         >
>         > Going pack to your suggested deco profile:
>         >
>         > Profile 220' FOR 25
>         > 16/50, 50% and O2
>         >
>         > DEPTH   TIME    GAS
>         > 160-120 1 MINUTE EACH ON BG
>         > 110     1
>         > 100     1
>         > 090     1
>         > 080     1
>         > 070     5       50/50
>         > 060     2
>         > 050     3
>         > 040     5
>         > 030     8
>         > 020     13      OXYGEN
>         > 20-0    8
>         >
>         > I would like to better understand what you mean by "moving up".
>         >
>         > Following a failure of the 50%, are you suggesting that you
> continue the deco
>         > as if 50% had been available i.e. follow the schedule but on BG.
>         >
>         > Or are you saying that 16% is so worthless that you would skip the
> stops and
>         > go straight to O2 at 20', if so what kind of ascent would you do
> from 70' to
>         > 20' to give your system a chance to cope with the bubbles?
>         >
>         > David Shimell
>         > Email: shimell@se*.co* <mailto:shimell@se*.co*>
>         > Project Manager, IBM NUMA-Q, Sequent Computer Systems Limited,
>         > Weybridge Business Park, Addlestone Road, Weybridge, Surrey, KT15
> 2UF, UK
>         > registered in England and Wales under company number: 1999363,
> registered
>         > office as above
>         >
>         > -----Original Message-----
>         > From:   kirvine@sa*.ne* [SMTP:kirvine@sa*.ne*]
>         > Sent:   Thursday, January 06, 2000 4:48 PM
>         > To:     Ian PINKSTONE
>         > Cc:     techdiver@aquanaut.com
>         > Subject:        Re: I think I understand
>         >
>         > Ian, these decos are so severely overstated that a loss of a gas
> is
>         > insignificant. I did a whole Wakulla deco on my 190 bottle right
> through
>         > 20 feet on a 30 minute bt at 300 and never got hit, using a
> schedule for
>         > switching. I kept screwing up the gas switch and not knowing it. I
> only
>         > found out when I got to 20 feet and my O2 was not there, and I
> then saw
>         > that my 120 and 70 bottle were both untouched, and that I was
> about out
>         > of gas on my 190 bottle. So much for deco.
>         >
>         > In real life, you are btter off moving up and taking the gradient
>         > offgassing as bubbles while still underwater than you are sucking
> on the
>         > wrong gas and lengthening the stops, and in real life if you have
>         > anything approaching the deocompression time that you are
> suggesting
>         > here, you not only have support with extra gases available, you
> have
>         > buddies to share with. I just did a dive wher I shared the 70
> bottle
>         > with my dive partner last Sunday, and it worked fine.
>         >
>         > --
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