reverse profiles

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Paul Foley - 28 Oct 2007 20:03 GMT

There's a letter to the "Ask The Instructor" column of the Nov. issue of
Scuba Diving Magazine that puzzled me.

"On a recent dive trip, my divemaster scheduled a 1 pm dive on a 90-foot
wreck. I also wanted to do the 9 am dive, so he insisted I go deeper
than 90 feet in the morning if I wanted to do the afternoon dive."

Doing as he's told, the guy goes briefly to 92 feet in the a.m. dive,
completing the rest of it at 40-60 ft. Then he does the p.m. dive.

The expert advice he gets is basically that the deepest dive first rule
is largely an artifact of the dive tables, going on to cite examples of
the penalties incurred by diving a reverse profile. He concludes:

"Is it safe to make a reverse profile dive? I found very little research
on the topic, and like most dive leaders [I] tend to err on the side of
caution, relying on the standard that has kept divers safe for decades."

This answer sounds distincly bogus to me. I did a little research (very
little) and came up with this link:

http://www.soc.soton.ac.uk/OTHERS/SDSC/dn1_00a.html

which concludes that there is no evidence supporting the rule and
tracing the whole sad business back to PADI <g>

I'm a guy who believes in following the rules. (I'm also the kind of
guy who wears a belt and suspenders.) But I can't for the life of me
think of a physiological reason behind the deepest dive first rule. And
it seems downright looney to deliberately dive deeper than you want to
in the a.m. just so you can dive to depth in the p.m.

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Matthias Voss - 28 Oct 2007 21:50 GMT

You may want to look up reverse profile conference, Seattle,
UHMS.

Conclusion:
No evidence of "reverse" profiles better more risky when 2nd
dive's depth does not surpass first one's for more than 12 m.
For greater difference, "no sufficient data".
Remember this does not have anything to do with the profile
within a single dive. There it's "max depth first".

Matthias

> There's a letter to the "Ask The Instructor" column of the Nov. issue of
> Scuba Diving Magazine that puzzled me.
[quoted text clipped - 27 lines]
> it seems downright looney to deliberately dive deeper than you want to
> in the a.m. just so you can dive to depth in the p.m.

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Dan Bracuk - 28 Oct 2007 23:15 GMT

Paul Foley <paulfxfoley@earthlink.net> pounded away at his keyboard
resulting in:

:"Is it safe to make a reverse profile dive?

Yes it's safe. That whole deepest dive first thing was a way of
maximizing bottom time in the dive tables days.

Dan Bracuk
Never use a big word when a diminutive one will do.

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Ron - 28 Oct 2007 23:54 GMT

>There's a letter to the "Ask The Instructor" column of the Nov. issue of
>Scuba Diving Magazine that puzzled me.
>
>"On a recent dive trip, my divemaster scheduled a 1 pm dive on a 90-foot
>wreck. I also wanted to do the 9 am dive, so he insisted I go deeper
>than 90 feet in the morning if I wanted to do the afternoon dive."

--snip--

I read that article, and they gave dangerously out of date
information. For a while, the "reverse profile problem" was
considered an old wives' tale.
More modern research showed that there is a real effect in
reverse profile diving. Research by McInnes, Edmonds, and
Bennett was written up in the May 2006 issue of Undercurrent.
They subjected guinea pigs to simulated dives. The two groups
made the same dives with the same rate of ascent. The only
difference was that one group did the deep dives first, the other
group reversed the order.
The deep-dive-first group showed no signs of DCS. The other
group had six DCS hits. Two of the six died before they could
try to treat. The other four died during the oxygen
recompression treatment. They got similar results in further
testing.
Conclusion: "We advise against advocating reverse profiles,
until the limitations of this format are determined more
factually and the decompression requirements are redefined."

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Ron
(user ron
in domain spamblocked.com)

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Adam Helberg - 29 Oct 2007 01:44 GMT

>>There's a letter to the "Ask The Instructor" column of the Nov. issue of
>>Scuba Diving Magazine that puzzled me.
[quoted text clipped - 23 lines]
> until the limitations of this format are determined more
> factually and the decompression requirements are redefined."

That the reverse-profile pigs got bent in this experiment does not mean that reverse
profile diving is unsafe. It's possible to chose borderline dives that exceed normal
safety limits and in this case reversing the profiles will push the dive time farther
beyond the no deco times.

The way I understand it, if you want to maximize the dive time the traditional
profile gives longer no deco dive times. The corollary to this is that if you dive
two dives with fixed profile, choosing the deeper one first gives greater margin of
safety with regards to staying away from deco times. But it's perfectly safe to dive
a deeper dive after a shallow dive if you stay within the no deco times of your
computer. And it makes no sense to deliberately deepen the first dive to avoid a
reverse profile.

Adam

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Sheldon - 29 Oct 2007 06:36 GMT

Your post is interesting. It just seems to me that even if you do a reverse
profile, if you follow the tables, or your computer, you're still sucking up
the same amount of nitrogen either way. Maybe it has something to do with
the ascent at the end of your day. A "normal" dive plan would have your
ascents getting shorter and shorter as the day goes on. A reverse profile
has you taking all your residual nitrogen down to a deeper depth with a
longer ascent.

Just a thought.

Sheldon

Sheldon

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Matthias Voss - 29 Oct 2007 11:42 GMT

> More modern research showed that there is a real effect in
> reverse profile diving. Research by McInnes, Edmonds, and
> Bennett was written up in the May 2006 issue of Undercurrent.

From what we read below, this is a flase statement. I
cannot see efforts to realign the guinea pig a) into a model
working for guinea pigs b) into a scaling down to fit human
physiology.

> They subjected guinea pigs to simulated dives. The two groups
> made the same dives with the same rate of ascent. The only
[quoted text clipped - 8 lines]
> until the limitations of this format are determined more
> factually and the decompression requirements are redefined."

We have done hundreds of 2nd dive deeper w/o problem.

Matthias

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-hh - 29 Oct 2007 12:14 GMT

> [Ron wrote]:
> > More modern research showed that there is a real effect in
[quoted text clipped - 5 lines]
> working for guinea pigs b) into a scaling down to fit human
> physiology.

The statement also raises more questions than answers for me as well:
the supposed explanation for the "A before B" sequence was because of
"time efficicency" for conducting dives A & B.

As such, the implication is that the Surface Interval (SI) between A &
B is smaller when the dive sequence is A-->B than it is for B-->A.
Insofar as to why, one can argue that the deeper (A) profile loads
faster compartments more so than B, so therefore, it also requires
less SI to reduce them down to some suitable M-value for the next
dive.

With the sequence reversal, this accomodation is no longer present,
and since we "don't care" what the M-values look like 6-12 hours
later, all we see are the disadvantages.

-hh

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Joerg Hahn - 29 Oct 2007 15:54 GMT

Hello Paul,

I don`t think the guinea pig issue is relevant.

It is more relevant to have a look on how you dive.

First, if you stick to RDP, ascent rates, no deko, short SI
, I would not do/plan invers dives.

Second, if you plan and dive with deko stops, deep stops,
proper ascent rates, proper dive profiles, SI longer than
3...hours, even very deep air dives, within broad safety
margins..

I do inverse dives.

Imagine (with the above said) that the first profile is
completely covered by the second profile, the length of SI
determines how much the slow tissues still contain N2 and
will prolong the shallow stops for a relativly short amount.
Which by the way at least Buehlmann-DC can handle.

Joerg

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Don Gingrich - 29 Oct 2007 22:37 GMT

> There's a letter to the "Ask The Instructor" column of the Nov.
> issue of Scuba Diving Magazine that puzzled me.
[quoted text clipped - 4 lines]
> deeper than 90 feet in the morning if I wanted to do the afternoon
> dive."

[snip]

> which concludes that there is no evidence supporting the rule and
> tracing the whole sad business back to PADI <g>

Interesting that you should mention that.... The time that
I got bent was on a PADI course where we were strictly following
the PADI rules. (OK, for the record, I was a *little* bit dehydrated
and I was silly and did a couple of somersaults at the surface in
twin 100's to entertain a seal -- I've been told that exercise right
after a dive is not smart.) But the real kicker was that since the
second dive was going to be a wreck penetration at 27-28 metres and
the first dive was a wreck at 24 metres, we swam about 100 metres out
from the wreck against a current to get to 27 metres -- then, as the
tide turned, we swam back 100 metres to the wreck against a current.
Hard work at depth -- a great way to increase nitrogen loading. And it
probably affected me more than most of the others since I had twin
cylinders and they mostly had singles.

I woke up at 3:00 am the next morning with an ache in an elbow and
wrist that gradually worsened until I got to the chamber.

It could have been a lot worse. But, I cannot help but think that
it might not have happened at all if we had not been going to such
great lengths to avoid the dreaded "reverse profile". Particularly
since we're only talking about 4 metres here.

> I'm a guy who believes in following the rules. (I'm also the kind
> of
[quoted text clipped - 3 lines]
> And it seems downright looney to deliberately dive deeper than you
> want to in the a.m. just so you can dive to depth in the p.m.

I fully agree, if you are going to extremes to avoid this, I believe
that you are more likely to do harm than good.

-Don

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Adam Helberg - 30 Oct 2007 06:17 GMT

>> There's a letter to the "Ask The Instructor" column of the Nov.
>> issue of Scuba Diving Magazine that puzzled me.
[quoted text clipped - 26 lines]
> I woke up at 3:00 am the next morning with an ache in an elbow and
> wrist that gradually worsened until I got to the chamber.

Can I ask you if you were using a dive computer, and if yes, which one. Did you
violate the computer? Make a safety stop?

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Don Gingrich - 30 Oct 2007 07:40 GMT

>>> There's a letter to the "Ask The Instructor" column of the Nov.
>>> issue of Scuba Diving Magazine that puzzled me.
[quoted text clipped - 30 lines]
> Can I ask you if you were using a dive computer, and if yes, which
> one. Did you violate the computer? Make a safety stop?

A Suunto Viper and yes we made safety stops and stayed within
the computer profile. (However, it should be noted that the second
dive was very much "straight down to about 25 metres, then on to
28 for most of the dive while we were doing linework inside the
submarine and then fairly directly (with a safety stop) back to the
surface. If you've read "Shadow Divers Exposed" or at least have
seen the cover, you've seen the submarine wreck we were diving on
for the second dive. Possibly a bit close to the limit on the
second dive, but most were diving singles which rather effectively
limits bottom time at 28 metres.

ABT for first dive was about 24 minutes, second dive 20 minutes.

Most of the first dive at 18 metres most of second dive at about
28 metres. Surface interval of about 120 minutes.

-Don

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Matthias Voss - 30 Oct 2007 11:37 GMT

> A Suunto Viper and yes we made safety stops and stayed within
> the computer profile. (However, it should be noted that the second
[quoted text clipped - 6 lines]
> second dive, but most were diving singles which rather effectively
> limits bottom time at 28 metres.

Reminds me of the event where I got a little hit lately.
(slight bends in elbow and knee).
First dive to 38m, 17m BT, divetime 37', uneventful.
4 hours interval, 2snd dive to 27m, 27 minutes divetime.
Current 1.2 knots, viz 2.5 m, laying line.

Both dives within safe limits with some margin.
I attribute the nuisance mainly to the line laying ( some
work in the current), and a possible dehydration because of
drinking cidre. I had wanted to do some extra minutes at the
3m stop, but there was a one armed guy with us whom I
supposed to need help, so we surfaced as a group.

Matthias

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