David & Manos,

You hit the nail right on the head with your 11 step evolution of a tech 
diver. However, I would think that 28% O2 is a bit high for a 42 mtr 
dive.  The PPO2 at depth is 1.46ata.  I don't think that is a good choice 
at all.  I would recommend rethinking not only the profile but also the 
bottom mix.

For about the last 4 years now I have been using 21/25 (O2/He) as bottom 
mix for dives from 130' (40 mtrs) down to 170' -180' (52 -55 mtrs).  The 
Idea of using such a high O2 is a through back to the old school of nitrox 
where we were taught a 1.5ata PPO2 for a bottom mix.  I think (I hope)by 
now we know that is a bad idea even in the best of conditions.  Most of the 
people I know now are recommending 1.4ata or less, depending on who you 
talk to.  I am in the "or less" camp!!!  PPO2s of 1.3ata are high enough 
for a bottom mix and less is better still (within reason).

Trimix 21/25 is a good choice for midrange diving because it keeps the PPO2 
in the 1.0 - 1.3 ata range, for dives between 130'~180', and also manages 
the "END" to >130' (40 mtrs).  It is easy to mix and the 25% He is high 
enough to cause vocal changes so the likelihood of mistaking it for air, if 
not properly marked, is greatly reduced.  The Deco times are also still 
quite manageable for these short duration dives.

The profile I would run for this dive is:

Bottom mix: 21/25 (O2/He)
Deco gas:    O2

42 Mtrs for 25 Min

27    1 Min
21    1 Min
18    2 Min
15    2 Min
12    2 Min
9     3 Min
6    10 Min

This is only 4 additional minutes of deco over David's profile (which I 
have no quarrel with the numbers just Manos' gas selection) and reduces 
both the PPO2 and the END!!!  Sound like a good trade-off to you??



At 06:33 AM 21-07-00 , Shimell, David (shimell) wrote:
>Manos
>
>Firstly, they are only numbers popped out of an algorithm.  Abyss allows a
>multitude of settings and algorithms, and you have not indicated which you
>selected.
>
>The stages of progression in a diver deco-wise are:
>
>1.      Taught to use tables and believe them implicitly.
>2.      Decide that a dive computer is a must-have.
>3.      Be forced back to tables due to mixed gasses, and hate it.
>4.      Calculate and execute your deco to the second.
>5.      Have aspirations to buy a wrist weight such as the VR3.
>6.      Realise that the deco is horrendous on such computers.
>7.      Start to question Buhlmann's algorithms and computer implementations.
>8.      Decide that the training you have received on deco is way off the
>mark.
>9.      Start to reduce the deco by changing the shape of the profile.
>10.     Find that one feels better and more vital after this so-called
>aggressive deco.
>11.     Enjoy the diving more due to the reduced requirements to carry 20
>litre stages :-)
>
>The trick is to miss out as many of the earlier steps as possible!
>
>FWIW, this is what I'd do:
>
>24      1
>21      1
>18      1
>15      1
>12      2
>9       3
>6       8 (O2), followed by a slow ascent over 3 minutes.
>
>Others may use different gasses, and a build on the benefits of the O2 window
>at the gas switch.
>
>David Shimell
>shimell@se*.co* <mailto:shimell@se*.co*>
>DDI: 01932 814096 * Mobile: 07770 282 202 * Fax: 01932 814343
>Project Manager, IBM UK Web Server Group, Sequent Computer Systems Ltd,
>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:   Manos Manoli [SMTP:manos@ma*.co*.cy*]
>Sent:   Friday, July 21, 2000 7:02 AM
>To:     Tech Diver
>Subject:        Table Vs Abyss
>
>Iam doing a simple Wreck dive on 42meters this weekend on Trimix.
>I run the plan on Abyss and then i thought to compare with the iantd
>table ?
>So at the end of the day one of them or maybe both are bulshiting me.
>( i know some will run and say try Decoplaner ) But thats not the point
>!!
>Why is there so much deference between a simple dive like this ?
>Which one you would dive ?
>
>So the results on 25minutes on 42 meters with 19%he 28%o2  was:
>
>Abyss:
>12 Minutes on O2 at 6 meters.
>
>Iantd tables:
>1   min on 12 meters
>3   min on 9   meters
>12 min on 6   meters on O2
>15 min on 3   meters on O2
>?????????????????????????????
>
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Bill (aquadart) Bott

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