Deepest dive first, or not?

[ScubaBB]

Hi,

I've always learnt that when you make two dives a day, you always should make the deepest dive first. And I've always done it like that.

Lately I hear sometimes that this is oldfashioned and that there are good reasons to do it the other way around. But until now nobody has been able to clarify this good reasons to me.

Maybe somebody on this board.

 

[Tortuga68]

In 2000 DAN's "Reverse Dive Profiles" workshop on NDL dives concluded that:

“…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” and “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”

http://rubicon-foundation.org/dspace...MS_V32N2_9.pdf

There are times when it's not convenient to do your deepest dive first. I have done reverse profiles within the above guidelines and not had any problems. However, there's no harm in sticking with conventional thinking on the topic (ie don't do reverse profiles)

YMMV so make your own mind up (I voted 'other')

 

[ScubaBB]

What were the reasons for "deepest first" anyway?

 

[AfterDark]

What were the reasons for "deepest first" anyway?

More bottom time on the 2nd dive.

 

[ScubaBB]

Ok, but the other way you've more bottum time on the first dive.

 

[koozemani]

Ok, but the other way you've more bottum time on the first dive.

Not necessarily. As far as ndl limits, your deep dive is more restrictive. Doing you deep dive first allows for more time at depth, whereas, a deep second dive would be shorter than if you had done it first because your first dive would negatively impact your ndls for subsequent dives.

Just guessing, but that is how I understand it.

 

[ScubaBB]

I think I understand you

 

[AfterDark]

Not necessarily. As far as ndl limits, your deep dive is more restrictive. Doing you deep dive first allows for more time at depth, whereas, a deep second dive would be shorter than if you had done it first because your first dive would negatively impact your ndls for subsequent dives.

Just guessing, but that is how I understand it.

What he said... Bottom line if you don't violate NDL with your total bottom time for the day you should be fine, put you never know. Of course the more SI you have the more BT you'll have when you dive.

 

[thani]

I always follow the deep dive first approach to be on the safe side like others said. This way in case you run into a situation and had to stay a bit longer for your deep dive, hence it is your first, you will not have to violate NDL. If it was your second dive you already have a penalty from your first dive.

Actually the standard practice works well for me in this example:
I always dive a 110 wreck - NDL dive- and immediately I dive a shallow 40 feet reef dive for about an hour.

 

[TSandM]

You've already had the Smithsonian paper cited.

There is also an essay floating around the web entitled, "Why we don't bounce dive in the WKPP," by George Irvine. In that essay, he postulates that any bubbles formed after the first dive may, if a shallow second dive is done, be compressed enough to pass the pulmonary filter and end up in the arterial circulations. He suggests that doing a deeper second dive can crush the bubbles, and if the deco done for the second dive is adequate, the net result will be better, not worse.

If this is, in fact, the case, it is probably only pertinent to doing very deep dives, where the pressure is enough to crush microbubbles.

At recreational depths and times, it is probably better to use forward profiles, or at least if you are doing dives with very dissimilar maximum depths, to do the deep one first. We have a lot of experience with that, and far less with doing the reverse.