2nd Dive Deeper -- Is this the new recommendation?

[Richeod]

It was a DIR diver telling me that PADI and DAN had it all wrong. I've found what I think is his basis for his statements and quoted it above.

What makes him a DIR diver. Does he wear a Halcyon rig? In my GUE Tech class we were NOT taught this. We were taught to double the shallow stops for a repetitive dive.
GUE, read DIR, does not really seem to care if you do reverse profiles or not but they certainly don't say you have to.

Everyone reads things George has wrote and interperates them the wrong way. If you do a 300 ft dive for eight hours and intentionally bubble to decompress faster as George does you WILL have bubbles in the venous system. If you then do a shallow dive those bubbles can cross into the arterial side and cause problems. Hense second dive deeper to recompress bubbles.
If you do a second 300 ft dive for another eight hours you can run the same profile because you are essentially saturated.
This is what GI is refering to, not for an OW diver to ignore the repetitive dive groups.

If you are going to break the established norm you better make damn sure the person giving you advise knows what he is talking about.

If you don't believe me try doing a 100 ft for 20 minutes then a 120 for 12 and a 130 for 10 with a 30 minute SI between them. I would guess that the results would answer your question.

 

[Epinephelus]

Richeod once bubbled...
If you don't believe me try doing a 100 ft for 20 minutes then a 120 for 12 and a 130 for 10 with a 30 minute SI between them. I would guess that the results would answer your question.

Trying to apply GI3's article to recreational no-stop diving is BAD advice. Trying Richeod's test profile above is REAL BAD advice.
I know he isn't actually proposing you try those dives, but just in case that escaped anyone - and some of the posts latching on to "the DIR article" as gospel indicate that that's a possibility - let me reitterate DO NOT TRY THE THREE DIVES PROPOSED ABOVE!
E. itajara

 

[Dr Deco]

Dear Readers:

Reverse Dives

This thread has seen an explosion of replies in the two days it has been on SCUBA BOARD. I do wish to reiterate that the dives covered by the Smithsonian Conference were 130 feet or shallower and the reverse depths were not to exceed a 40 foot differential (as pointed out by “reefraff”).

As most experienced divers know (at least those who read this FORUM), the dive tables are derived from the no-decompression limits (NDLs). The NDLs were the only quantity taken from the US Navy tables to construct recreational diver tables. As surprising as it might seem, the US Navy did not determine all of the NDLs; many simply were derived from the original studies wherein decompression tables were tested. Hardhat divers seldom bother with NDLs; that is partially the reason for having a surface supply. The US Navy tables were tested for long, decompression dives.

Multiple Dives

It is not common for military divers to perform more than one decompression dive in a short period of time. Generally, the dive teams will switch and a new, “fresh” diver will go down. Thus the repetitive schedules were an afterthought for special conditions where this simply was not possible. When the tests were performed, the dive depths were equal of the second dive was shallower. They could have tested a dive in which the second depth was much deeper and created dive tables for these situations. This test plan was not done. It should therefore be remembered that it is for want of data and not because it is beyond possibility that we do not have these types of tables.

Dive Tables Today

There are but few tables that have been tested. In North America, these are basically the US Navy, DSAT (PADI), and DCIEM and those derived from them (e.g., Oceanic from DSAT).

Deco meters could have reverse profiles - with large depth differentials - if the manufacturers would pay for a test program. I do not see this coming under the auspices of private industry.

Deep Tables

As far as I can tell, the deep dives and decompression associated therewith are performed in the water. The situation is the same as decompression of astronauts in null gravity. When all of the gravity forces are removed from the legs in altitude decompression, the DCS incidence is reduced by 3 to 10 fold (not ten percent). This is quite possibly the case in diving also, and I suspect that is what is occurring in these situations.

Virtually all commercial and military decompression is performed in some type of a bell and deck chamber. It began in the early 1920s with the diver entering the bell at the first Haldane stop. When this occurs, the diver is subjected to considerable musculoskeletal stress, and all decompression tables are referenced to long chamber decompressions. Recreational divers have only recently began to perform long decompressions and these are all buoyant, in-water deco (without the physical and mechanical stresses). If the WKPP dives were performed in a chamber with the divers encouraged to get up and walk around, I suspect that the DCS incidence would really go up. If the divers were given gas switches and told to stay seated, then the ascent could be fast in the fashion of the dives tested in Europe by the German DFVLR.

Deep Repets

Can you perform two 300 fsw dives and not count the first one. If all deco has been by a buoyant diver, then it might be possible. If they say they have done it, then I believe them – providing they follow that procedure.

Dives to 100 feet and a surface interval where you walk around during decompression (yes, surface interval is decompression), will result in a hit for a large fraction of the divers. One would never do this in anything but a chamber in a laboratory with Doppler equipment at hand. I am not exactly sure why you would wish to do this, however. Possibly to demonstrate that it will not work. Some times you might get lucky. I have seen large laboratory animals do it.

And sometimes they died. :wacko:

I can see that more material must be added to my decompression physiology class.

Dr Deco :doctor:

 

[barb]

One of the Dive Operators in Cozumel are now doing all their dives this way. I questioned it and was told that PADI has found that it's no longer necessary to do the deeper dive first.
At the time I just thought: oh yeah, here is a way to accomplish 2 objectives of the "large" Operator. 1 - be able to check out diving skills of new divers in shallow water before taking them deeper. 2 - give shorter dives so you can get back to the shop faster. This particular Operator added to #2 objective by almost eliminating a surface interval. The S.I. consisted of the time it took to get to the next dive site. Add to that the fact that the 2nd dive was deeper (especially for some of the divers I watched)
What I found very alarming is the number of new divers following along with this routine and they did'nt have a computer and I did'nt see them checking any tables during the S.I. so they just bindly followed the DM (who BTW was often well above the divers during the dive) It's very difficult for one DM to keep 10 excited divers together let alone keep them within a safe depth.
I had two instances where I had to make a choice of whether I was going to go into deco or cut my dive very short and follow along near the surface.
This last trip to Cozumel was a big eye opener for me and I now, unfortunately, have a greater understanding of how divers can and do die.
I am currently completing my rescue diver course where one of the most significant things I've learned is the idea of self-rescue first.

 

[BRW]

Folks,

Got an email suggesting a post or two on RPs. So,
consider:

1) at the Smithsonian RP Wkshp, we reported a 16% DCS
hit rate on RPs deeper than 100 fsw, and with deltas
greater than 40 fsw. See Proceedings. These were
typically repetitive deco dives with 2 - 3 hr SIs

2) present RGBM meters (Suunto, Hydrospace, Plexus,
Mares, ABYSS, GAP, others coming out) ALL address RPs
consistent with above for tec and rec diving.

3) these dives are part of the RGBM Data Bank, plus others.
Check RGBM diving.com

Regards to all.

BW

 

[QKRTHNU]

One of the Dive Operators in Cozumel are now doing all their dives this way. I questioned it and was told that PADI has found that it's no longer necessary to do the deeper dive first.

:wacko:
I hope they don't kill anyone.

I can see how this deeper 2nd dive stuff could be applicable to Decompression dives (Since they're already planning on going through in-water decompression), but Non-Deco dives forget it! Makes no sense what-so-ever. You ALWAYS have to take in consideration any residual nitrogen when calculation NDL.

 

[BIGJC]

Some would call this "reinventing the wheel".... It just sounds dangerous to me, plus what advantage do divers get out of it??

But hey if they're doing it in mexico who am I to questions it???

:flusher:

 

[Albion]

With respect to the DAN angle

last year there was an article in the DAN SEAP magazine which stated that they could find no definite evidence that reverse diving was more prone to causing DCI than conventional deepest first profile. In so far that the dives are not too far different (ie like first to 10m second to 40M). Thier information, as i remember was based on reported DCI cases and recreational non decompression diving.

 

[barb]

the problem of course, at least what I observed in Mexico, is if divers are not watching their depth and descend significantly deeper on the second dive.
I would guess that any vacation location would have issues around this.
It seems that many divers on vacation rely on the DM to look out for them - forgetting to take care of their own profile/plan.
Also, (as I've already said) the issue with this practice is the short S.I. that seems to happen between dives (in order to facilitate getting back to the dock faster) could prove "deadly" if divers end up doing 2 deep dives one after another. I say this because I have also experienced divers relying on the DM's computer for executing their dive plans - very worrisome!!!