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George,
In response to your post, a couple of questions and
obsevations. Please respond.
> From: "G. Irvine" <gmirvine@sa*.ne*>
> Subject: Gas Diving Made Easier
> Common Gas Diving Topics
>
> We need to look at some topics in gas diving that are being misapplied
> due to misunderstanding, and then used to justify deep air diving.
>
> #1 - "air tables" are wrong. We all learned some simple things first,
> like PADI tables, and then Navy tables. PADI tables are Navy
> no-decompression tables. Navy tables are nonsense - they bend you by not
> doing the deep stops and then treat you by extending the shallow stops.
I agree.
> #2 - Gas tables are more correct: unlike the more arcane Haldanean
> models in the air tables, gas tables were mostly developed using
> Bulhmann's theories which started deco deeper, but not deep enough.
I agree.
> #3 - In real life, stops start deep, helium is your friend, it makes
> you fell better after the dive and keeps you from narcosis, it is EASIER
> to decompress from.
You say," it (Helium) is easier to decompress from." I don't feel
helium is harder to decompress from than air or Nitrox. What makes it
easier? Are you implying shorter hang times?
> The result is that people were first taught to believe that gas needs
> deeper deco than air, when in fact air needs deeper deco than is in the
> "air" tables.
I agree.
> The term "decompress from gas on an air table" is an oxymoron - there
> is no "correct" "air" table as we know it, the gas tables are what is
> needed for air.
If you are referring to PC generated custom dive tables using 12 to
16 tissues and real values for inert gases as well as deco gases, I
would agree.
> 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?
I disagree with you here for a couple reasons and offer an
alternative:
A) Any time you switch bottom mixes that have different END's you
are adding unnecessary complications. Although what you suggest is
obviously better than switching from Trimix to air.
B) If you have two bottom mixes with different Helium and Oxygen
percentages, this would require you to use tables for the
strongest mixes for safety sake. Also, after every air top you
would be cutting tables for what ever your new mix is. Personally
I feel this is unreasonable. For four dives to the same depth
in two days, I feel that I should be allowed to have the same
bottom mix and END for all four dives (my choice 15/40/45).
My suggestion would be the following.: If you would normally
wear say 240 cuft. of gas on your back in a set of doubles
and plan to use 160 cuft. (rule of thirds), why not carry a
stage with 120 cuft. with the same bottom mix. Use your stage
first then switch to your doubles. On each dive on average
you would use about 40 cuft. out of your doubles. After 4
dives there should still be 80 cuft. of mix left in your
doubles. Doing it this way would allow you to use the same
bottom mix all the time and eliminate the need for wet boat
fills. A set of doubles, 4 stages of bottom mix, and 2 to 4
stages of deco gas shouldn't be an unreasonable request when
you consider the price of a Doria trip!
Ted Green
> 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 .
>
> Technical diving is fun, but it is geting a bad name due to the
> accidents.AAAThe accidnets are due to impairment from nacosis. The
> accidents need to stop. I repeatedly do dives that were not even thought
> possible by my original dive partner , Bill Gavin, and I do them safely,
> and I do them all the time, and they are fun to do. If I can do 300 for
> three hours and then go out to dinner with my pals, then you can dive
> the Doria or anything else without the self-imposed risk. I know what I
> am talking about - learn this stuff right and stop the nonsense.
>
> If you need information to keep yourselves alive, ask me or any of the
> WKPP guys, and examine your own misconceptions - there is no such thing
> as an "air table", and the real risk is death. - G
> --
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>
>
Ted Green (owner)
Tidewater Aquatics (Dive Store)
Salisbury Maryland USA
TDI IT #029
SSI MI #178
The world contains but three types of people:
1. Those who make things happen.
2. Those who watch things happen.
3. Those who wonder what happened.
--
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