reverse profiles??

how important is it to do the deepest dive first??

today i had 2 dives,,first was 14 metres and the 2nd 18 mentres,,is it a bad idea to do this my guide says its fine

There as a major workshop on this topic about a decade ago, with experts from around the world participating. No one knew the origin of the prohibition of reverser profiles. The earliest reference to it was a suggestion in a 1972 PADI manual, but PADI (who participated in the workshop) could not identify who put that suggestion into the manual or why. Over time the suggestion somehow became a rule.

The workshop concluded that there was no danger to reverse profiles for recreational divers, but it maintained the recommendation against them for decompression divers at the insistence of Bruce Weinke, who argued that such dives bring additional problems that I don't need to go into here.

Common sense will tell you that the most likely reason for the original suggestion is related to the way tables work. If you do the deepest dive first, you can do the shallower dive after a shorter surface interval than if you do the shallower dive first.

If you have had a sufficient surface interval to do the second (deeper) dive safely, according to either a table or a computer, then there should not be any additional worries.

"Typically" you'd want first dive to be deeper to be on the safe side and to get better NDL. As I understand it, it is a recommendation.

Your first dive was at 46ft and the second at 59ft ---- so 50ft & 60ft, both dive were not deep dives. Also, a 10 or 14ft difference will not really affect your NDL that much.
If you have a nice surface interval, let's say 1hour.
You can pretty much make 2x 30min dives and still be far from NDL. No deco limit.
Would it be better if you did the 18m one first, maybe.
Will you have DCS, I can think of no reason that it should.
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It is also recommended (if I remember correctly) to start a dive at the deepest you want to get on that dive and then slowly go up.
I guess it helps decompression by slowly going up while diving.
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Here are some threads about it:
Deepest Dive First? Not Anymore
Always Do Your Deep Dive First? - The Cyber Diver

If you use the tables, like folks did when the no reverse profile rule originated, you'll screw yourself out of BT on the second dive if you attempt a reverse profile:

:40 @50 puts you out of the water at a F group and would allow for :01 BT on a subsequent dive to 90'.

:25 @90 puts you out of the water at a G group, but would allow for :57BT on a subsequent dive to 40'

It wasn't about safety, it was about BT.

There have been several studies on the topic that showed either no effect or a dramatic effect.

I think one good reason to do the shallower dive first is if you haven't been diving for a while you get a bit of a workup dive before getting serious.

boulderjohn:

The workshop concluded that there was no danger to reverse profiles for recreational divers...

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To be precise, they concluded that “We find no reason for the diving communities to prohibit reverse dive profiles for no-decompression dives less than 130 feet / 40.6 meters and depth differentials less than 40 feet / 12.5 meters”

http://rubicon-foundation.org/dspace...MS_V32N2_9.pdf

Since then (2000) the findings of the study have been questioned by some parties, although without any concrete proof that they were wrong

Personally I am happy to do reverse profile under the conditions above, assuming it fits the dive plan for SI & BT

aquaregia:

There have been several studies on the topic that showed either no effect or a dramatic effect.

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What study showed a dramatic effect?

If you don't violate the deco limits with either dives, then what's the big deal?

Most of my dives are reverse profiles (build-up dives) under 50m (160ft). I had some reading material on the subject, will see if I can find it and share it here.

As a guide, it is far from uncommon for work to put me in a situation where I teach in the morning (Max 60 ft), and guide in the afternoon (Max 130 ft). Tropical instructors dive morning intros and do boat dives in the afternoon as a matter of course, or guide a morning trip (deep/shallow) and then an afternoon trip (again deep/shallow).

As a guide doing these profiles, the recent widespread adoption of dive computers, and their increasing ways of enforcing slow ascents (deep stops, audible alarms for ascent violations) has been a godsend. I can do nothing about having to dive reverse profiles, but fast ascents make me sad. Deep stops, and safety stops are really nothing more than ways of drastically reducing ascent rate, and the fact that computers encourage and enforce them makes happy.

boulderjohn:

There as a major workshop on this topic about a decade ago, with experts from around the world participating. No one knew the origin of the prohibition of reverser profiles. The earliest reference to it was a suggestion in a 1972 PADI manual, but PADI (who participated in the workshop) could not identify who put that suggestion into the manual or why. Over time the suggestion somehow became a rule.

Click to expand...

boulderjohn's comment about the fact that no one really knows where the recommendation came from is a good thing to remember. Scuba Diving in general has many things like this. (Turning the valve all the way open then back, Blasting off a dust cap with air, etc.) In general, it is better to assume their is wisdom in accumulated experience, and do wht others do until you know better by some means, but it is also good to remember that people used to think a lot of things, many of which look hilarious from our perspective.