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

[Charlie99]

WreckWriter once bubbled...
I think a more reasonable statement might be that the thinking is turning towards the old "first dive deeper" thing may not be as critical as we once thought.

WW

These recommendation go even further, by saying that the 2nd dive SHOULD be deeper; and also that residual nitrogen levels from the 1st dive should be ignored when planning the 2nd dive.

I have a hard time reconciling the WKPP/DIR recommendations with what I learned in OW class.

 

[Scubaroo]

This really sounds like it needs a bunch of dry divers, a buttload of chamber time and proper study to settle.

Maybe DAN could turn their attentions to it, similar to the flying after diving studies they've recently done. It would be a major shift in diving industry doctrine to dive deeper repetitively.

 

[WreckWriter]

Charlie99 once bubbled...
I have a hard time reconciling the WKPP/DIR recommendations with what I learned in OW class.

WKPP recommedations are not generally meant for someone just out of OW Charlie

 

[buff]

George gets it partially right. Aside from his rambling, almost incoherent writing style I would agree with his points but he also has a lot of "ifs" and that's the kicker.

"If you do you decompression the way I have described, including the way I ascend to the surface, you will greatly reduce the heavy bubble-form offgasing that generally occurs post-dive. If you are basically clean, you can dive again without penalty. If you are using the correct gas, the "residual" effect is greatly reduced."

A healthy margin of safety is demanded in diving-especially technical diving. George assumes that everything is perfect-equipment, health of the diver, temp, etc.(hypothetical world to be sure) and therefore nothing will go wrong if all the steps are followed. That doesn't happen in life and it doesn't happen in diving.

I don't know where he gets the idea of doing a second dive deeper to "diminish micro-bubble formation from the first dive". This is true but what about micro bubble formation from the second dive? Does this mean I have to do a third dive? And therefore a forth? I love to dive but this is a life time project!

George has been good about pushing a VERY slow, continuous acsent rate and I really agree with him on this.

 

[Rick Murchison]

buff once bubbled...
George has been good about pushing a VERY slow, continuous acsent rate and I really agree with him on this.

Which ascent (slow continuous) has just recently been shown to create far more bubbles than a staged ascent (more rapid ascent interrupted with stops covering the same time as the continuous one from bottom to surface).
Which just goes to show that there's still an awful lot we don't understand about the physiology of bubble formation in the human body.
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As for the "deepest dive first" debate, the Doppler modified Navy tables greatly favor that profile to remain within NDL (take dives to 80' for 20 min and 60' for 30 minutes, for example - if you do the 80' dive first, you need a 1hr 58m SI, but if you do the 60' dive first a 3hr 58m SI is required). The "deepest first" favoritism is less in most dive computers but the ones I've been able to check out still have it built into their algorithms. Running the 80/20 - 60/30 dives in V-planner, for example, allows the 80-60 order with a 10 minute surface interval stop free but requires 20 minutes SI for a stop free 60-80 order.
So, while there may not be any need for the "deepest dive first" dictum, in the real world of recreational no-stop diving, you'll be penalized with longer surface intervals - and less diving - if you don't adhere to that general guideline.
Rick

 

[Charlie99]

Rick Murchison once bubbled...

Running the 80/20 - 60/30 dives in V-planner, for example, allows the 80-60 order with a 10 minute surface interval stop free but requires 20 minutes SI for a stop free 60-80 order.
So, while there may not be any need for the "deepest dive first" dictum, in the real world of recreational no-stop diving, you'll be penalized with longer surface intervals - and less diving - if you don't adhere to that general guideline.
Rick

The the DIR article says that loading from prior dives doesn't matter, so your program obviously is incorrect if it generates different profiles or NDLs depending upon the surface interval.

Read the article!

 

[Rick Murchison]

Charlie99 once bubbled...
The the DIR article says that loading from prior dives doesn't matter, so your program obviously is incorrect if it generates different profiles or NDLs depending upon the surface interval.
Read the article!

I have read the article.
And what, pray tell, gives the "DIR article" authority over all the other tables, computers, algorithms and other papers on the subject?
Hmmmm?
Rick

 

[SDAnderson]

In 1999 at a scientific symposium called "Reverse Dive Profiles Workshop" produced some results that many in the dive community still are having a hard time getting comfortable with. For a short summary of the conference, you can read a Rodale's article, but here is the official word from the participants, published at the end of the conference:

Key Findings of the "Reverse Dive Profiles Workshop" held at the Smithsonian in 1999:

• Historically neither the U.S. Navy nor the commercial sector have prohibited reverse dive profiles.
  
• Reverse dive profiles are being performed in recreational, scientific, commercial and military diving.
  
• The prohibition of reverse dive profiles by recreational training organizations cannot be traced to any specific diving experience that indicates an increased risk of DCS.
  
• No convincing evidence was presented in the workshop that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of DCS.
  
• The attendees found no reason for diving communities to prohibit reverse dive profiles for no-decompression dives less than 40 meters (130 feet) and depth differentials less than 12 meters (40 feet).

Other reading:

Undercurrent article about reverse profiles

USC Chamber Reverse/Forward Profile Study

Implications of the Varying Permeability Model for Reverse Dive Profiles

Steven

 

[Rick Murchison]

reefraff once bubbled...
Key Findings of the "Reverse Dive Profiles Workshop" held at the Smithsonian in 1999:

• Historically neither the U.S. Navy nor the commercial sector have prohibited reverse dive profiles.
• Reverse dive profiles are being performed in recreational, scientific, commercial and military diving.
• The prohibition of reverse dive profiles by recreational training organizations cannot be traced to any specific diving experience that indicates an increased risk of DCS.
• No convincing evidence was presented in the workshop that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of DCS.
• The attendees found no reason for diving communities to prohibit reverse dive profiles for no-decompression dives less than 40 meters (130 feet) and depth differentials less than 12 meters (40 feet).

This is quite correct - but what they don't say is that there is a penalty built into the tables/computer algorithm for doing the reverse profile. So, on the one hand we can say "there's nothing to prohibit reverse profiles" - but just realize that you're going to be faced with longer surface intervals than if you do the deepest dives first - and that's still a practical reason to do the deeper dive first when you have the choice. Unless, of course, you ascribe to the theory that the shallow dives don't matter and ignore their contribution to your nitrogen "bucket"... (which is NOT what the Reverse Dive Profile Workshop concluded at all - they require that you stick to your tables/computer algorithm)
Rick

 

[ERP]

Now I have no Idea if diving deepest first or last is better, but as I understand it, the majority of the empirical data used to build the models used to construvt the tables was gathered with deepest first profiles, so it makes some sense to be cautious when using a different assumption than the tables were built on.