Doing the Deeper Dive Second.

[MikeFerrara]

BW has posted some good stuff here on the board about reverse dive profiles. they recieve thorough treatment in "Technical Diving in Depth" by BW.

The output of the reverse profile workshop have been misstated more than not. They decided that for non-stop dives withing recreational depths and deth differences of no more than 30 or 40 ft that the order didn't seem to matter much. That's often twisted to mean that reverse profiles are ok but that isn't what they said.

If doing the shallow dive first or the shallow part of a dive first was the way to go we would be able to do our decompression before the dive instead of after.

 

[Snowbear]

Here's a few for you to read and decide for yourself. Some of these contain links to more references:

http://www.scubaboard.com/t58713.html
http://scubaboard.com/showthread.php?t=39238
http://scubaboard.com/showthread.php?t=27922
http://scubaboard.com/showthread.php?t=27942
http://www.scubaboard.com/t50033.html

 

[padiscubapro]

I have a dive partner who insists on doing the deeper dive second and a deeper diver if we do it a third time. He feels with longer Surface Interval Times that the depth of the dive and order doesnt matter. I strongly disagree with him and have successfully got my way each time we dive. I know i am cramping his style and our diving outings are becoming less frequent. How important is it so do the deepest dive first.

Whether you do the deep dive first or second really depends on the tables you use.. Most tables severely penalize you for doing the profiles in reverse order.. So doing the deeper dive second will give you shorter allowable bottom times. If you use the PADI wheel to do multi level planning you definately should do the deeper dive first.

one thing that has not been mentioned.. is that from a gas loading perspective a deeper dive can actually be shallower to your body.. fo example say you did an AIR dive to 80 fsw, then the second dive of the day was to 100fsw using 36%.. From a gas loading perspective the second dive is "shallower" the EAD is about 75 fsw, if you were running AIR tables you would choose 80fsw..


Once you start doing decompression dives it becomes less of a concern because the staging is dont with stricter limits and decompression divers typically come out of the water with less gas loading than NDL divers...

Its not uncommon, when I am running trimix classes to run the dives deepr as the day progresses..

My first evaluation dive will be something like 160-180 fsw, the second dive of the day will be 200-220. The second day will start around 220-240, and end up somewhere around 260 fsw for the last dive (deeper for CCR divers)

Once I start running dives beyond 300 fsw I generally limit it to 1 dive a day..

 

[cornfed]

Here's a few for you to read and decide for yourself. Some of these contain links to more references:

http://www.scubaboard.com/t58713.html
http://scubaboard.com/showthread.php?t=39238
http://scubaboard.com/showthread.php?t=27922
http://scubaboard.com/showthread.php?t=27942
http://www.scubaboard.com/t50033.html

Without clicking any of these links I can pretty much guarantee at least one of them contains a link to the Smithsonian reverse profile study. As Mike pointed out, the results of this study have been misquoted and misinterpreted quite a bit.

 

[chrisch]

That's an interesting perspective. How do you measure the gas loading with mix? With EAN its easy as its only the N2 that loads. However, with H do you consider this faster gas to be cleared sooner? Or can you re-load with H and manage the N2 with mix?
So for example after a 20m dive on EAN32 could you then do a 35m dive on 25/30?
3ata amb. @ .68PP N2 = 2.04 ata
4.5ata amb. @ .45PP N2 = 2.025 ata
From a Nitrogen view the second dive is shallower. However, the 'jump' from 20 to 35 meters would be enough for me to say this is unsafe.
This of course leaves the issue of bubble nucleus formation from dive 1.
An oversimplification on my part therefore, but I am curious as to other people's view.

Chris

 

[Curt Bowen]

Since most of our dives are exploration (dont know how deep) and decompression dives we normally do not care what order the depths run. Just do the decompression stops and add a little safety time if you feel it is needed.

As for recreational and unexperienced divers, I would stick with deepest first.

 

[tville23682]

I will read some of the articles you provided........ thank you to everyone for the info. it is apprecitated

 

[Mako Mark]

one thing that has not been mentioned.. is that from a gas loading perspective a deeper dive can actually be shallower to your body.. fo example say you did an AIR dive to 80 fsw, then the second dive of the day was to 100fsw using 36%.. From a gas loading perspective the second dive is "shallower" the EAD is about 75 fsw, if you were running AIR tables you would choose 80fsw..
QUOTE]

This is so if discussing gas tension, but not so if discussing gas phase right? Or am I out of my depth?


My understanding of the historical "deepest dive first" stance was that calculating the logarithms for reverse profiles before the days of electronic computing were so hellishly difficult that the tables designers kinda avoided them.

The reverse profile workshop pretty much seemed to find that it wasnt such a big deal as we previously thought, based on anecdotal and pretty sketchy data.

BW's idea about forcing bubbles completely into solution is an interesting one thought, has a kind of logic to it. I think this is the difference between the older and newer decompression theories, the older did not take into account gas phase (micro bubbles) the newer ones do.

Mike is right though, I think I would prefer to use a newer model, though not so new that it hasnt been tested and established for a little while. Tables test pilot, Im not..

 

[Hank49]

BW has posted some good stuff here on the board about reverse dive profiles. they recieve thorough treatment in "Technical Diving in Depth" by BW.

The output of the reverse profile workshop have been misstated more than not. They decided that for non-stop dives withing recreational depths and deth differences of no more than 30 or 40 ft that the order didn't seem to matter much. That's often twisted to mean that reverse profiles are ok but that isn't what they said.

If doing the shallow dive first or the shallow part of a dive first was the way to go we would be able to do our decompression before the dive instead of after.

I had always been told to do the deep dive of the day first. And I always wondered why, because if you do four dives in a day and have 38 hours reading on your computer until saturation, isn't the first dive the next day, deeper than the last dive the day before in the same category as a deep dive second? Maybe you had a 10 hour surface interval but you're far from saturated. ??

 

[wedivebc]

one thing that has not been mentioned.. is that from a gas loading perspective a deeper dive can actually be shallower to your body.. fo example say you did an AIR dive to 80 fsw, then the second dive of the day was to 100fsw using 36%.. From a gas loading perspective the second dive is "shallower" the EAD is about 75 fsw, if you were running AIR tables you would choose 80fsw..
QUOTE]

This is so if discussing gas tension, but not so if discussing gas phase right? Or am I out of my depth?


My understanding of the historical "deepest dive first" stance was that calculating the logarithms for reverse profiles before the days of electronic computing were so hellishly difficult that the tables designers kinda avoided them.

The reverse profile workshop pretty much seemed to find that it wasnt such a big deal as we previously thought, based on anecdotal and pretty sketchy data.

BW's idea about forcing bubbles completely into solution is an interesting one thought, has a kind of logic to it. I think this is the difference between the older and newer decompression theories, the older did not take into account gas phase (micro bubbles) the newer ones do.

Mike is right though, I think I would prefer to use a newer model, though not so new that it hasnt been tested and established for a little while. Tables test pilot, Im not..

Interesting point Mark, when we do charters in Nanaimo we have to do Dodd Narrows when tide change allows it. It has a max depth of 75ft. If it is the first dive of the day often people will pass on the wreck (Saskatchewan 110') to avoid RP. We use nitrox and EAD profiles to prevent the RP.

cheers,