Reverse dive profiles

[loosenit2]

Recently read a very good post giving a review of diving with a cruise line in multiple locations. One of the recommendations was that one operator should change the Order of their dives to get back to port more quickly.

As I read the dive profiles the effect would be to create a reverse dive profile where your second dive was deeper than your first. My first reaction was no, you can’t do that, the rule is you always do your deeper dive first.

I started to do a little research to see if that rule was still a hard and fast rule and came up with this article. Should Scuba Divers Make Their Deepest Dive First? Not Necessarily

Curious what the general consensus is on this “rule” is in the community.

 

[TMHeimer]

I'm certainly no expert on this often debated topic. All of what the article says seems logical to me. It seems logical that aside from other factors (like ascent rate, hangover, etc.), the amount of nitrogen in your tissues would be the same whether you did the shallow or deep dive first--thus the risk of DSC would be the same. You can't use tables for true multi-level diving anyway or you will often be over the NDL regardless of which dive is first.

 

[tbone1004]

@loosenit2 I'm sure it's in that post you linked, but the "rule" came from the old US Navy tables where you were penalized heavily for repetitive diving. You wanted to make the deeper dive first so you maximized bottom time. With the PADI RDP that came out and now with modern computers there is no real reason to do the deeper dive first.

 

[Damselfish]

I've long been under the impression the deeper dive first rule originated from the dive tables - that if you work the dives on the tables putting the deeper first allows you a little more NDL so was inferred to be more conservative. (I seem to recall actually doing this exercise in my OW class in 1990.) But when most people are diving computers, and the tables are rarely an accurate representation of what you're doing anyway, that's not especially relevant.

I have seen people say well if you don't have to do the deeper dive first, a reverse profile on one dive is no problem, which was a clear misunderstanding of the whole thing.

 

[shurite7]

There are still plenty of people out there that will state a reverse profile cannot be done. Whenever I've asked some of these people why that is the case they can never seem to give an answer. The most common theme is people learned not to do it in their BOW course and never questioned what they learned. They tend to stick with what they heard in the course rather than continuing to advance their knowledge and experience.

I used to do them all the time when living in the tropics with customers; 100 ft dive in the morning followed with a 50 minute SI and then a dive to 50-60 ft, head back to the dock, eat lunch, swap tanks, then repeat the two profiles.

 

[dmaziuk]

Bubble models have the concept of "bubble crushing radius" that is affected by this, however, bubble voodoo is largely educated speculation with little data behind it. Even so, if you look at VPM closely, it says deeper first dive results in smaller bubbles -> good, but it also likely results in higher overall gas loading -> bad. So a reverse profile could go either way.

Anyway, I think reverse profiles were "bad" before bubble models, so bubbles are not the origin of it.

 

[CathyE]

The theory was mentioned in our OW class ( about 10 years ago), not as a 'rule', but as a discussion point leading to stressing that one plans their dives and sticks to the plan.
I have done reverse profile diving on several occasions since and never taken a hit. I have however never done a third dive on those days. In addition the second deeper dive was rarely deeper than 80 feet(ish).

 

[rabe]

There are absolutely no reasons to do the the first one deep and the second shallow.
It's well explained in Deco for divers, by Mark Powell.

In the book, he also explained that this notion was introduced in one of the early PADI manual but with no reason or explanation at all, and it has been copied and repeated since then.

 

[Lorenzoid]

There have been a number of good discussions on this. Some I found include:

Reverse Profile?

Reverse Dive Profiles

As I understand it, the only proven reason is that the PADI RDP would advantageously indicate a shorter surface interval to do a second (shallower) dive after a first (deeper) dive than the surface interval it would indicate if you were to do that same shallower dive first and the same deeper dive second. If you're following a computer, then it doesn't matter whether the second dive is shallower. Physiology-based rationales that have been floated with the idea of supporting the common admonishment against so-called reverse profiles don't seem to be persuasive to the expert consensus. Apparently, it isn't clear how this admonishment got started, but it seems to live on as a result of repetition by well-meaning instructors, as others have pointed out.

 

[tursiops]

This is a dead horse. Stop beating it.

The issue was put to bed in 1999.
https://medschool.ucsd.edu/som/emer...Dive Profiles Proceedings 2000.compressed.pdf

It is STILL a good idea, however, for your most efficient nitrogen management and off-gassing. Take any two different-depth dive profiles with the same SI and look at your nitrogen status depending on the order in which the dives are done. You can do this with tables, computers, it doesn't matter. You end up with less nitrogen in the body and possible a longer NDL on the second dive, if the deepest is done first. But it is not a "rule," it is just a best practice for nitrogen management.