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John Heimann (jheimann@sc*.gt*.co*) has two excellent points.
1) Breathing from a Nitrox 50/50 stage bottle is a pretty bad idea for
the exit,
and 2) lowering the partial pressure of O2 in the bottom mix from e.g. 1.4 to
1.05 will increase the decompression obligation.
I would like to point out that I regard this as an entirely theoretical
discussion at this time, and I am very grateful for the feedback that I have
received. My apologies for the length of this missive.
I assume that John's rig is: trimix doubles + nitrox 50/50 stage + O2 stage,
with an extra O2 bottle probably with an octopus hanging on a line.
Relying on any stage bottle during the initial descent and the exit phases
seems to me to be a bad idea too. My assumed rig was:
PMM doubles + nitrox 50/50 30 cu ft PONY + O2 probably stage + O2 regulators
hanging down from the boat (obviously, I have a specific boat in mind).
An alternative where you do not have surface supplied O2 is to hang the
bottle over the side.
I think it is practical to exit on the nitrox 50/50 if it is mounted
as a pony. So at the end of the last O2 to nitrox switch, you exit on
the nitrox - no big deal. I hadn't considered that many divers in fact
mount it stage.
Anyway, you need to deal with this issue anyway below 255 fsw...
2) The extra decompression obligation.
Well - how much is it? It's really hard to talk about this without
publicly available trimix tables, but let's talk generalities.
The way I calculate it, if you are using PMM and set for an Equivalent
Narcosis Depth of 132 fsw (i.e. you are as narced as if you were on air
at 132 fsw), then you have:
pp N2 = pressure at 132 fsw * .79 = 5 ATA * .79 = 3.95 ATA
pp O2 = pressure at 132 fsw * .21 = 5 ATA * .21 = 1.05 ATA
pp He = ambient pressure - pressure at 132 fsw
For example, if the working depth is 231 fsw
pp He = 8 ATA - 5 ATA = 3 ATA
Now if we instead were using a trimix set for pp O2 = 1.4 ATA, then
if we assume that O2 is not narcotic, then we would instead have:
pp N2 = 3.95 ATA (same as PMM)
pp O2 = 1.4 ATA
pp He = ambient pressure - pp N2 - pp O2 = 2.65 ATA
So the increase in O2 leads to a decrease in He, and of the partial
pressure of diluent gases, of .35 ATA.
Now, .35 ATA is the equivalent of 12 fsw - so if we ignore the change
in composition of diluent gases for calculating decompression, then
we are effectively diving about 12 fsw deeper. Not good, but it's
only 12 fsw, which can be adjusted for with longer deco, if we have
appropriate tables.
My understanding is that the Consortium tables are designed to ignore
the relative composition of the trimix, so this should be valid. I am
not sure whether the Consortium tables would require a penalty for the
increased total diluent gases or not, but custom tables designed for
the exact mix certainly would.
If instead we believe that O2 is as narcotic as N2, then instead we
keep pp N2 + pp O2 = 5 ATA = 132 fsw, and get:
pp N2 = 3.60 ATA
pp O2 = 1.4 ATA
pp He = 3 ATA (same as PMM)
So we would decrease the pp N2 by .35 fsw, same 12 fsw, same arguments.
Note that when calculating for PMM, you get limited by the END, unless
you choose a much deeper END than 132 fsw. Calculating END does not
depend on your beliefs on narcosis - if you believe in either flavor,
then you either want pp O2 + pp N2 = 132 fsw, or pp N2 = pp N2 at 132 fsw.
Mathematically you will always get the same number.
SUMMARY
a) For PMM, use your nitrox as a travel mix, and mount it as a pony not
as a stage.
b) For PMM compared with Trimix set with pp O2 = 1.4 ATA, the decompression
disadvantage is approximately equal to diving 12 fsw deeper.
Thank you all.
Safe Computing!
Wrolf
>From uunet!opal.com!owner-techdiver Wed Jan 5 16:41:42 1994
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>Subject: Poor Man's Mix, cont.
>From: <uunet!gte.com!jheimann%scsd.dnet>
>Date: Wed, 5 Jan 94 14:10:48 -0500
>To: "techdiver@opal.com"%BUNNY.dnet@gte.com
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>Status: RO
>
>I should explain why you want trimix to be surface breathable (FO2 >= 0.16).
>There's really no magic here.
>
>As a background note, gases with PO2 < 0.16 ATA are hypoxic; you risk losing
>consciousness if you breathe them. If the FO2 in your mix is < 0.16, as
Wrolf
>points out, you can't breathe it at the surface, but can use your intermediate
>deco mix (EAN50 or whatever) to breath from the surface down to the the minimum
>operating depth of the mix (that at which PO2 >= 0.16 ATA). Sometimes this is
>necessary, as when diving below 255 fsw.
>
>If you don't need your FO2 to be < .16 because of depth concerns, you should
>use more O2 for number of reasons. Probably the best reason is that if you
>run your O2 level as high as possible, you minimize your decompression
>obligation. The extra N2 in PMM not only make you narced, it adds unecessary
>hang time. Just as with nitrox, you want to run your bottom PO2 at 1.4 - 1.6
>ATA, depending on your expected exertion level and bottom time. For a working
>dive to the second class china in the Doria (220 fsw) you want
>something like 18% O2 in your mix.
>
>>From a purely practical point of view, hypoxic bottom mix means that you can't
>Subject: breath from your primary reg at the surface. It's nice to be
> able to put on
>
>
>breath from your primary reg at the surface. It's nice to be able to put on
>your doubles, jump in the water with your primary reg in your mouth, and attach
>the stage bottles in the water. Likewise when getting out of the water - it's
>tough climbing up most ladders with two or more stages clipped on - and as your
>open water instructor taught you, you should keep a reg in your mouth when
>climbing into a boat. Moreover, if (God forbid) you should lose your stage
>bottles in the water, you can't breath what's on your back between the min.
>oper. depth and the surface.
>
>John
>jheimann@sc*.gt*.co*
>
>--
>Send mail for the `techdiver' mailing list to `techdiver@opal.com'.
>Send subscription/archive requests to `techdiver-request@opal.com'.
>
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