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Date: Mon, 07 Jun 1999 13:09:01 -0700
To: techdiver@aquanaut.com
From: David Reinhard <reinhard@oc*.co*.au*>
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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http://www.ocean.com.au/info.html

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