Deep dive first! Why??

[Jeroenelout]

Hi Dr. Decompression,

I always tell my student to make the deepest dive first, but every now and than they ask me how it makes sence.

If I make my first dive 30 min 40 feet, than the second dive should be more shallow. If a friend dives the same moment 30 min to 60 feet, than according to the RDP, he can still plan a dive to 55 feet. That while he has absorbed a lot more nitrogen than I.

If I would pretend to have dived the same deeper profile as he has, en look up my pressure group for that deeper dive, can I than also make a dive to 55 feet with him?

Hope to hear from you soon. Jeroen

 

[coberry7]

I'm not Dr. Decompression, nor do I try to play him online...Having said that I can say that the dive the deepest dive first and then a shallower dive afterwards has to to with diving DEEP, and then RELATIVELY shallow. i.e. Dive to 100' then follow it with a dive to 45', versus going to 45' and then going to 100'.

Diving so shallow (40') on your first dive, when you have access to diving on a deeper site, that you plan on diving THAT DAY, makes no sense. Hit the deepest dive first. If you plan on doing repetitive dives to 30', you're OK. Just stay within your computer or table limits.

In theory, what you said does raise an interesting question...in theory.

The intent of the suggestion to dive deeper first and then dive shallower is intended for greater differences in depth on the two dives than the example that you proposed. i.e the example I provided above.

I would tell my students to use the common sense rule here, not using the 'Dive deep first' suggestion as an absolute.

I apologize, as I do not have my RDP handy to fully answer your question right now. I'll look it up to answer your last question later.

Colin Berry

 

[Walter]

When I teach the tables, I give an example of why. Then I tell them they'll get more bottom time if they do make their deepest dive first.

Using YMCA tables

My example is with a 58 ft dive for 47 minutes Repetitive Group H followed by a surface interval of 1 hour 30 minutes and a second dive to 48 feet for the max time allowed by the tables. You enter the second dive as an F diver with 47 min RNT. You have a max of 33 minutes and end as a J diver.

Do the 48 ft dive for 33 minutes first and you'll surface as an F diver and go back in after the 1 hour 30 minute surface interval as a D diver with an RNT of 24 minutes. Your second dive to 57 feet gives you a maximum of 26 minutes bottom time.

By making the deepest dive first, you picked up 21 additional minutes of bottom time.

Using NAUI tables, the results are the same except you are allowed a maximum of 31 minutes instead of 26 on the dive to 57 feet, causing you to lose 16 minutes bottom time instead of 21.

I don't have the RDP handy, but if memory serves, you lose 12 minutes bottom time and your dive to 48 feet is 30 minutes longer since the RDP is such a liberal table.

 

[SDAnderson]

The Reverse Dive Profile Workshop, held in Washington, D.C. in October of '99, was attended by the who's who of the dive industry.

The Workshop was organized by Michael Lang, Diving Officer and head of the Smithsonian Institution's Scientific Diving Program, and by Charles Lehner, Biotron Laboratory, University of Wisconsin at Madison.

Sponsors included the Smithsonian, the American Academy of Underwater Sciences, the Divers Alert Network, the Dive Equipment Manufacturer's Association and others.

Findings and Conclusions​

Neither the U.S. Navy nor the commercial sector has prohibited reverse dive profiles, and they are performed in recreational, scientific, commercial, and military diving. Since the prohibition of reverse dive profiles cannot be traced to any definite diving experience that shows an increased risk of DCS and no convincing evidence was presented that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of DCS, the workshop participants found no reason to prohibit reverse dive profiles for no-decompression dives less than 130 fsw and depth differentials less than 40 fsw.​

Read more about it in an article published in the May 2000 issue of Undercurrent: Always Do Your Deep Dive First?

 

[DepartureDiver]

If I would pretend to have dived the same deeper profile as he has, en look up my pressure group for that deeper dive, can I than also make a dive to 55 feet with him?

Hi Jeroen

Okay, to answer your question, yes you are thinking right and that is a fine way to plan a reverse profile. Now to add to your knowledge which others have started here, while I believe it is always to your benefit to do your deepest dive first and work shallower, reverse profiles are no longer viewed as a risk they were once considered. Here are some other articles on reverse profiles for your enjoyment.

EUBS Newsletter

Technical Reverse Profile Paper by Yount, Maiken and Baker

 

[Spectre]

There are quite a few threads on the subject here. Search for "Reverse profiles" and see what you find.

 

[Dr Deco]

Thanks to our Scuba Board readers for the replies. A deeper dive first has more o do with table calculations than physiology (within limits). It is a matter of bottom time of the second dive as to which dive you wish to perform first.

Dr Deco