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> 
> Has anyone saved the posts from a couple of years ago on gas 
> management for dives on the Doria in order to minimize the 
> number of  tanks to take but not compromising gas mixtures.
>

This the one?

 " Having said that, let's look at a diving situation:

  I want to do the "Doria", but my captain, Janet Beiser, has to limit
my gear baggage since there are others on the boat. I can be a dope and
dive air ( if Janet would let me), or a convolute and put air in my back
tanks and take gas stages, losing redundency in an emergency, and being
forced to air at the worst posible time ( like what happened in the
death of Rob Parker), or sharing air with an out of gas divier, who is
now hammered , scared, and on my long hose, or I can make the exact mix
for one dive and then blow it into oblivion with air, or,

                     I can "Do It Right":

   I can lose the abject fear of helium and low oxygen mixtures, and
make up two sets of doubles with high helium, like 50%, and low oxygen ,
like 14 percent, and take stages of the exact mix for the depth ,
probably something like 18/33, and some stages of 50/50 and my oxygen
bottle.

  I dive the stages and try to save the backgas, but let's say I want to
use the backgas. I can blow it back three times and still be ok on the
oxygen, and probably pretty good on the helium, but what happens to the
deco?

  My first dive for 25 minutes is problably a good hour of deco, my
second with the diluted mix is more like 52, and the third more like 45
( relatively 60,50,40 or padded ratios like that). In other words, for a
few extra minutes in the water, I get to do it safely. I then do the
same with my other set, and/or my stages were dived first, and then I do
a couple of back gas dives . I keep the dives to reasonable bottom
times, and end up making the deco gas last longer, and as the deco gas
gets diluted or lower, the deco using the higher oxygen "reblows" is
getting shorter and shorter anyway, assuming I am giving myself a decent
interval between dives. Most of the deco time is on oxygen anyway.

   POINT here: mixes that are too low in oxygen and too high in helium
are not a bad thing - this is ok. The opposite is not. The former means
a tad more deco, the later means a lot more risk.

  For a shallower dive, lets say 130-160, I can take my doubles with
something like a 16/40 and blow that up a couple of times for back gas
diving in that range with the boat's compressor. The deco pickup over a
higher oxygen mix is not enough to warrant the air, especially at the
more insideous depths, like 150, that have enough impairment to cause an
accident, but not enough to "ring your bell" and make you aware of the
impairment.

  Keep in mind I am talking about trips where you have limited gear
space and want to maximize your gas ."

I
Iain Robertson 		British Telecom
Tel : 020 7843 8723 	Fax: 	020 7843 7399
iain.dk.robertson@bt*.co*
pp2A67, Angel Centre, 403 St John Street, London  EC1V 4PL 

 
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