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From: Jan Werbiñski <janwer@fo*.co*>
To: <>
Subject: Re: What ppO2 for bottom mix?
Date: Mon, 14 Jul 2003 22:01:44 +0200
----- Original Message -----
From: "Steve Schultz" <se2schul@ho*.co*>
To: <janwer@fo*.co*>; <>
Sent: Monday, July 14, 2003 7:28 PM
Subject: Re: What ppO2 for bottom mix?

> Jan,
> Firstly, why do you think that 26/25 is a better mix than 21/35 at 45m ??

Because I think there is the reasons for particular fraction of both gases:
END and safe ppO2 limit.

> I'm guessing that because you are using the maximum ppO2 of 1,4 you are
> gaining by having to do less decompression, right? Also, by using less
> helium, the decompression software gives you shorter decompression, right?


> Well, using 21/35 has benefits over the "custom mix" or "best mix".
> First off, what if you mix your 26/25, get to the dive site and find that
> the wreck isn't in 45m, but rather 47m? What if you get to 47m and see
> something interesting just off the wreck in 49m? You were already at the
> edge of what you considered to be a safe limit, but to see the interesting

The wrecks is usually known. So usually there is no surprizes. And the
reason for whatif when you don't follow the rule "dive your plan" is like
having strict rules because of bad habits. Something wrong there.

If I see something deeper then I don't go there, if I really want to, then
for a 1 minute I see no problem in being closer to ppO2 1,6 for 1 minute
relaxed descent and ascend. Later analysis show that difference in CNS clock
is just a few percent. But anyway MOD should be followed.

> things, you'd have to push the limit even farther.  How lucky are you
> feeling?  If you were diving lower oxygen and higher helium like 21/35,
> you could see those things without risking toxicity or narcosis.
> Also, if you're diving the same mix for a given depth range, you get to
> the deco for that mix.  There are lots of dives that I used to consider
> and complex that I no longer need tables for.

Yes but what about having runtimes and deco computer? Then what's the reason
for memorizing profiles? Tables for planning and computer for controlling.
Maybe I'm too novice I can't get it.

> Now, 25% helium has no advantage over 35% helium.  Helium is a very
> gas, much moreso than nitrogen.  Now, I don't advocate diving to 45m on
> but if you used to do it, do the same dive with the same deco but using
> 21/35 instead of air and you'll feel much better after the dive.  If your
> deco software says that you need less deco with 25% helium, it is lying to
> you.  When I dive to 45m on 21/35, I tell my software that I dive it on
> to get a shorter schedule.  So you get no advantage with less helium.
> helium mixes also lets you blow air on top for a shallower repetitive dive
> and still have enough helium to avoid narcosis.

Why the software is lying? I use GAP and z-plan with similar results.

What is the scientific reason for telling your software you diving air when
you dive trimix? And why you think this practise is another argument for
higher FHe?

The last reason is good - topping old mix with air is to get something like
20-25 FHe sounds like good solution for two day trip.

> Now back to the oxygen. You are going to expose yourself to high oxygen on
> deco, especially if you're going to be diving over many days.  Our lungs
> become inefficient when they get exposed to lots of oxygen, so it is now
> common practice to keep the O2 low when on the bottom (1,1 or even 1,0)
> then go to 1,6 on deco, but take breaks to a low ppO2 mix.

Why it's common?

> So, high helium is good. Low oxygen is just fine too.  Standard mixes let
> you mix, calculate deco and have all the buddies show up to the divesite
> with the same mixes much easier than "best or custom mixes".
> Does that help?

Yes, but... :-)

Jan Werbiński       O0oo....._[:]) bul, bul, bul
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