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From: "Sean M. Cary" <SMCARY@MI*.CO*>
To: "Steve Hogan" <Steve.Hogan@tr*.co*>, <shimell@se*.co*>
Cc: <techdiver@aquanaut.com>
Subject: Re: Re Last Stop Depth
Date: Wed, 9 Jun 1999 18:15:37 -0400
Why do a 10' stop when the PO2  of 100% is lower then 1.6?  I make my last
stop 20fsw.

Sean


-----Original Message-----
From: Steve Hogan <Steve.Hogan@tr*.co*>
To: shimell@se*.co* <shimell@se*.co*>
Cc: techdiver@aquanaut.com <techdiver@aquanaut.com>
Date: Wednesday, June 09, 1999 4:40 PM
Subject: RE: Re Last Stop Depth


>David,
>
>
>>Occasionally, I do a multi-level, bimble-around-the-place dive and dive an
>>air computer but deco on O2.  I'm comfortable to leave the computer on the
>>line or bend it when I've done sufficient deco.  Note: this is based on
>rules
>>of thumb which allow me to do sufficient deco.
>
>I have done many dives using mix (usually 18/40) with 50/50 deco gas
>and an air computer. Like you I am just wandering around and I use the
>air computer as a guide for the deco. I have found that the air only
>schedule
>is longer than a mix schedule with deep stops and deco gas. There are a few
>rules
>though. 1. Do the deep stops.  2. Use the deco gas (50/50) for all depths
>shallower
>than 70 ft. 3. perform the dive like a multilevel dive (no sawtooth stuff)
>This is only for some dives where a set of tables is not really
>necessary (like wnadering around on a reef where the bottom is known but is
>highly variable)
>
>Do not misunderstand me, when on a real mix dive (when I expect that the
>stops will be greater than 10-20 ft) then the dive is planned and executed
>on
>tables (and I bend my poor computer....backup depth gauge/timer. On a dive
>I did Sunday the computer informed me that I needed to stay at 20 ft while
>on my 10 ft (table) stop and will finally clear 60 hrs after getting out of
>the water.... I am glad it is not adaptive...<grin> It still indicated 37
>min
>of deco left when I got out of the water)
>
>Steve
>
>
>
>David Shimell
>Project Manager, Sequent Computer Systems Ltd., Sandton, South Africa.
>Email: shimell@se*.co* <mailto:shimell@se*.co*>
>
>-----Original Message-----
>From: David Reinhard [SMTP:reinhard@oc*.co*.au*]
>Sent: Wednesday, June 09, 1999 5:51 AM
>To: techdiver@aquanaut.com
>Subject: Re Last Stop Depth
>
>Thanks to all who have contributed to my knowledge with your replies.
>Unfortunately there are too many replies for me to respond personally to
>all of you
> A number of you have pointed out that air is a poor choice for
>decompression. I fully agree, and it is not my practice to do so. It was
>remiss of me not to point that out in my original post. Below is my reply
>to David Shimell which I had intended to go to the list but I think may
>have only gone to his personal email address.
>
>At 01:19 07-06-99 -0700, you wrote:
>>David
>>
>>The reason there is a significant difference between a last stop at 6 vs.
3
>>is that you are doing the deco on air which is a poor choice.
>
>In reality I don't do my deco on air. I was just running hypothetical
>profiles to see the effect of different stop depths. While I naturally
>expected a difference it was the magnitude of the difference that surprised
>me. While the pressure difference between 3m and 6m is not insignificant I
>was particularly surprised that 1m could make such a big difference.
> You are quite right, of course. Deco should be done on an
>appropriate
>nitrox mix and/or pure o2. But being very cautious I still deco to the air
>schedule and take the safety benefit rather than do accelerated deco. So by
>choosing 6m rather than 3m I have been spending a lot of extra time in the
>water. However due to the large ocean swells we often encounter where we
>are diving it may at times still be appropriate to go for the 6m stop.
>
>Thanks for your comments,     Dave.
>
> I also received the following comment which I am particularly interested
>in:-
>
>(quote)
>"If you have done sufficient deco, then spending further time in the water
>is
>increasing your risk.  OK, this risk is small but never-the-less a risk and
>it bites some people.  Doing excessive deco is therefore not adding
safety."
>
>Do others concur with this idea? As I said above I deco on nitrox but
>operate on an air schedule. In effect this would constitute "excessive
>deco" for the gas I am using. So is there a problem with this practice?
>Does doing extra deco increase or decrease your risk? If you are going to
>increase your deco at what point should you do it, eg only on the last
>stop? Come to think of it Pyle stops are in effect increased deco beyond
>that required by the tables. One of the thoughts I have had about this is
>what actually constitutes "excess" deco. Given the variability in deco
>tables what constitutes the "right amount" of deco on one table could
>constitute "too much" deco on another table. Given that we don't really
>know what the "right amount" of deco actually is for any particular
>individual on a particular dive profile how can we hope to know what is
>"too much" deco. (The only definition of "right amount" of deco I can think
>of is that amount which leaves you unbent after a dive - but you don't
>really know that until after the dive, since tables are merely a hopeful
>guideline which fortunately are right most of the time).
>
> In reality I would be very surprised if a small variation in stop
>depth
>made a large practical difference to the decompressing diver. If this were
>the case I am sure we would have a lot more bent divers (hmmm... on the
>other hand we do have a number of divers that get bent while apparently
>following the tables... I wonder...).  As has been pointed out the use of
>nitrox/o2 for deco significantly reduces the differences I observed, so it
>is even less of a problem to deco diving than it would initially seem.
>
> It has also been mentioned that Zplan adjusts all stop depths if you
>change the last stop depth. I had noticed this and have found it
>inconvenient compared to other software that keeps the same standard stop
>depths and only adjusts the last stop. I guess I didn't consider the
>possible effect of this change on the total deco time. I noticed another
>post on this list about a new shareware deco program that does not do this,
>so I would be interested in having a look at that program when it is
>available.
>
>
>Thanks to all, regards,  Dave.
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>Ocean Internet - "The Quality ISP"
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