Hi David

    An opinion about your doubts (keep in mind that I am not a deco
expert but only an humble diver...)

    As a general idea, consider that at the beginning of each deco
stop, the body is "stressed" to its maximum tolerance, expressed
exactly by the various M, "a", "b" (and so on) factors. Ideally one
can perform a continuos deco during which the limit is reached in
every instant. In this case, we can get the minimum deco time. To be
clearer, let us put the problem in the Haldane framework and
consider the 9->6 m steps. If M=2 it means that at the beginning of
the 6 m step we have 3.2 bar in some tissue, since 3.2=1.6*M. We
ascend to 3 m when the tissue tension becomes 2.6 bar, for the same
reason just outlined (3.2 is twice [M] the ambient pressure at 3m).
Obviously, some time will be spent to decrease from 3.2 to 2.6 bar.
If we choose to set the step at 5 m, the tension has to decrease
from 3.2 to 3 bar only (check out the calculation!) and less time
will be needed. One can reply that in such cases we have shorter
times but more steps; as intuition suggests, one can see that once
the tissue has reached its limit at the beginning of the step, the
rest is "dead time" (i have used the hyphens to avoid too complex
explanations...); in other words the most efficient deco is
equivalent to "utilizing" tissues tolerance as much as possible.

    Consider that I have used an unrealistic framework. The
shallower steps "activate" less and less tolerant tissues, with
longer halftimes. Since there is no "continuity" in passing from one
to another in terms of their parameters, it is likely that some
combinations of steps, diving profile, and surface pressure (i.e.
diving at altitude without equilibrating the body tissues tension)
lead to great changes as for the shallower stop times even with very
little fluctuations of the diving parameters... This may explain
some strange behaviors one can observe when playing with the various
deco planners.

    In the various cases you quote, the increasing distance between
the shallower stop and surface can meet more or less the
considerations I made above. More and more time is needed when
increasing the distance between deco stops.

Corrado Bonuccelli
corradob@al*.it*
ICQ#25934110


-----Original Message-----
From: David Reinhard <reinhard@oc*.co*.au*>
To: techdiver@aquanaut.com <techdiver@aquanaut.com>
Date: Monday, 07 June, 1999 03:36
Subject: Last Stop Depth


>Hi guys
>
> I have just discovered something of which I found of interest and
wondered
>if I could get some constructive comments. I have been using Zplan
set on a
>"last stop depth" of 6m, basically because of greater ease in
maintaining
>the set depth if there is a bit of a swell etc. What I have only
just
>realised is just how much difference there can be in deco times
when you
>use different last stop depths. When you run the same schedule
using either
>3m or 6m last stop depth the deco times can be dramatically
different. In
>fact even a difference of one metre can make a difference (eg 4m
instead of
>3m). This seems a bit unrealistic given the fact that it would be
>relatively easy to drift up and down by a depth of a metre during
your stop
>(maybe this underlines the importance of a trapeze or a least an
SMB/bag).
> Here are some examples for a 50m, 20min dive (incorporating deep
stops).
>
>SET AT 3 METRES
>
>Waypoint  at  50m for  20:00 (22) on Air,
>Deep Stop at  30m for   2:00 (26) on Air,
>Deep Stop at  21m for   2:00 (29) on Air,
>Deep Stop at  15m for   2:00 (33) on Air,
>Norm Stop at   9m for   2:00 (36) on Air,
>Norm Stop at   6m for  10:00 (46) on Air,
>Norm Stop at   3m for  27:00 (74) on Air,
>
>TOTAL DECO TIME: 45 minutes.
>DIVE RUN TIME: 73 minutes.
>
>SET AT 4 METRES
>
>Waypoint  at  50m for  20:00 (22) on Air,
>Deep Stop at  31m for   2:00 (26) on Air,
>Deep Stop at  22m for   2:00 (29) on Air,
>Deep Stop at  16m for   2:00 (32) on Air,
>Norm Stop at   7m for   7:06 (42) on Air,
>Norm Stop at   4m for  35:00 (78) on Air,
>
>TOTAL DECO TIME: 48 minutes.
>DIVE RUN TIME: 77 minutes.
>
>SET AT 5 METRES
>
>Waypoint  at  50m for  20:00 (22) on Air,
>Deep Stop at  32m for   2:00 (26) on Air,
>Deep Stop at  23m for   2:00 (29) on Air,
>Deep Stop at  20m for   2:00 (32) on Air,
>Norm Stop at   8m for   6:00 (41) on Air,
>Norm Stop at   5m for  39:00 (80) on Air,
>
>TOTAL DECO TIME: 51 minutes.
>DIVE RUN TIME: 80 minutes.
>
>
>SET AT 6 METRES
>
>Waypoint  at  50m for  20:00 (22) on Air,
>Deep Stop at  30m for   2:00 (26) on Air,
>Deep Stop at  21m for   2:00 (29) on Air,
>Deep Stop at  15m for   2:00 (33) on Air,
>Norm Stop at   9m for   2:00 (36) on Air,
>Norm Stop at   6m for  53:00 (90) on Air,
>
>TOTAL DECO TIME: 61 minutes.
>DIVE RUN TIME: 90 minutes.
>
>As you can see from the above examples there is a large difference
between
>the 3m stop (total deco = 45min) and the 6m stop (total deco =
61min). I
>simply had not realised how much the tables would penalize me for
doing my
>last stop at 6m rather than 3m. Notice the 10 min difference
between the 5m
>setting and the 6m setting (however there is only a 3 min
difference
>between the 4m and 5m settings- how can that be?). My question is
how
>realistic is this? Can a 1m difference in depth make that much
difference
>to the efficiency of your deco that there would be a 10 min
difference
>between the two? Hypothetically what if your depth gauge was poorly
>calibrated and was showing that you were at 5m when you were
actually at 6m
>- you could leave the water having done 10 min less deco than the
tables
>actually require. Also, if you are doing your deco in a vertical
position
>there could be a significant difference between the deco
requirements for
>your toes as compared with your head - maybe I should be lying in a
hammock
>for my deco stops?!? (or should I do half my deco head up and the
other
>half upside down to even things up...sorry I'm getting a bit silly
>now...where are my pills...ah, thats better).
>
>As you might expect the differences get larger for longer deco
times. eg
>40m for 40 min (no deep stops) gives 71 min total deco if you do a
3m stop
>but 98 min if you do your last stop at 6m (nearly 30 min
difference!).
>
> Given that the Buhlmann tables on which Zplan is based are
somewhat old
>now how would this fit in with more recent thinking about doing
deco deeper
>(or at least starting you deco deeper as in Pyle stops)?
>
> Any comments greatly appreciated,   Dave.
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