Deepest dive first?

[VooDooGasMan]

I can answer with my own experience and I have dove for 30 mins shallower and ran down deep, had to get anchor line unhooked that was deeper than dive. Just ascend slowly and I have had no problems with even doing 4 dives in deep water a day. Of course I also get my air down there from GPO's when they breathe out, they have no co in there air, its air from water.(apparently co is absorbed in the ocean so I am not sure)

 

[DivemasterDennis]

While there is no hard and fast rule, there is a practical reason for doing deeper planned dives first- more bottom time. As you do multiple dives in a day, you accumulate residual nitrogen and your no decompression limits ( by rdp or computer) are shortened. That's my reason for doing deeper dives first, much more than any safety issue. A no decompression dive is a no decompression dive.
DivemaasterDennis

 

[VooDooGasMan]

What is a no decompression dive I have never done one?

racer and i am sure Devon has explained in a pm back at ya, and in waters in your neck of the woods.

 

[fdog]

I have my own opinions about the Reverse Profile myth...however, I encourage independent analysis by all my students, and would encourage the OP to do the same.

A very targeted study about Reverse Profiles

Multiple studies on the effects of Reverse Profiles and their safety at the Rubicon Foundation


All the best, James

 

[DevonDiver]

That makes sense to me,

It does to me also...and it did to the agencies. Lacking definitive research, they set (liability driven) policies based on such assumptions.

However, more recent research is proving that it isn't the case, and that there is no discernable trend in that respect.

Thus, "The Gospel" now says you can dive shallow(er) first.



AAUS Proceedings of Reverse Dive Profiles Workshop.

The above paper, and similar works, have been adopted by PADI and other agencies when making recommendations on repetitive dives. Specifically;

1. Historically neither the U.S. Navy nor the commercial sector have prohibited reverse dive profiles.

2. Reverse dive profiles are being performed in recreational, scientific, commercial, and military diving.

3. The prohibition of reverse dive profiles by recreational training organizations cannot be traced to any definite diving experience that indicates and increased risk of DCS.

4. No convincing evidence was presented that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of DCS.

CONCLUSION: We find no reason for the diving communities to prohibit reverse dive profiles for no-decompression dives less than 40 msw (130 fsw) and depth differentials less than 12 msw (40 fsw).




DAN 2001: No Basis for Deep Diving First

> “…no convincing evidence was presented that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of decompression sickness;”

> “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.”

 

[sam1]

My understanding of the original basis for the deeper-dive-first "rule" was, as was stated previously, the very short bottom times allowed on the second dive if the first dive was the shallower one. This was to a large extent due to the use of the U.S. Navy tables, which assumed the 2-hour compartment controlled repetitive dives. One of the innovations of the PADI tables was the use of the (faster) one-hour compartment as the controlling compartment on repetitive dives.

 

[beanojones]

Has anybody run a algorithm in Divepal? It would be interesting to see if there's a difference in +N loading, and if it does indeed, cut into your 2nd dive bottom time.

(Of course it cuts into bottom time. All repetitive diving does.)

But more importantly overall is the fact that getting a computer program to say anything about dive safety is missing some important facts about what it means to be safe. We are not safe because a PC program says we are or because a table says we are.

The only 'safe' profiles are those that are completely tested in both wet and chamber dives, and frankly no one is going to be doing that sort of organized testing anymore. No one did it before PADI, and PADI has turned to computer diving. The people who could do the most research (military and commercial diving) have no particular need to test recreational profiles, because everything they do is either going to be under old rules forever (Navy still has to dive in J-Valves, and OSHA still requires them for OC commercial diving), or going to be done surface supplied.

So the only option now is to look coincidentally collected data, and see it there is anything that can be seen. (But that can never result in a recommendation to do things differently.)

The only actually tested tables were tested with the presupposition of no reverse profiles both, in a dive and over repetitive dives. As it turns out there was not much reason for that presupposition, but that is not important, except to say that reverse profiles were not tested. If they had been tested, something could be said. Since they were not tested, no amount of looking at incidentally collected data can ever justify discarding the presupposition.

People get bent all the time in undeserved hits, so it is clear that tables (or PC programs) just miss some important facts that no one understands. Since there are unclear factors at work, hammering away with the old theory details is just hammering the old theory details. It is not any closer to actually making testable predictions.

But more importantly, no one is going to be doing that testing anyway.

(Thats said, anyone working as an instructor (not doing it on a weekend, but actually doing it for a living) has done reverse profile dives of all kinds. Then again, most people who get bent dive for a living.)

 

[racerx_]

of course i also get my air down there from gpo's when they breathe out, they have no co in there air, its air from water.(apparently co is absorbed in the ocean so i am not sure)

lol!!!!