Reverse profile dangerous?

[lefrogster]

Just finished the section on reverse profile diving in "Deco for Divers" and was surprised to learn that there is no scientific basis for discouraging reverse profile dives, and that they are already being done in commercial, scientific and military diving. Yet this is against everything I've been taught by the agencies.

What's the deal?

 

[GrimSleeper]

DAN compiled, if memory serves, over 200,000 dive profiles and couldn't find a single instance of reverse profiling leading to DCS. The latest consensus seems to be that, within recreational depths (ie 40m or less), with no formal decompression involved, a second dive up to 12m deeper than the first dive presents no increased risk of DCS. IIRC the original recommendation was going to be more relaxed, but Bruce Wienke had concerns that RGBM wouldn't work with bigger depth differences (it doesn't work properly under any other circumstances, I guess, so why would it&#8230 so they settled on 12m.

As far as deeper/decompression recreational diving is concerned, nobody's done any research and there just aren't enough of those dives being done to come up with numbers, so there aren't any recommendations. I do recall one of GI3's tracts, though, in which he said that the WKPP's preferred approach was to do shallower dives before deeper dives, and I guess that worked for them, given their safety record.

I've heard that PADI tried to work out when and why 'deepest dive first' became dogma, and it turned out to be something that one of the senior members had said years ago and it just stuck...

 

[MB]

The most practical reason "deeper dive first" involved maximizing dive time when using tables. If you run several table exercises, particularly with dives between 50-70 feet, you'll soon see that shallow followed by deep results in very short dives, and the need for much longer SITs. Its a practical reason that particularly affects newer divers doing training dives, as well as occasional divers who rely on tables. Like so many traditions, it took on a life of its own and now engenders some pretty wild reactions. When you add computers and nitrox and long safety stops to the dive plan, the profiles can open up quite a bit.

 

[packrat12]

Everyone is saying it without saying it...

On deeper dives, you are more likely to be NDL limited than air supply limited (or close on both). In shallower dives you are more likely to be air limited with lots of excess NDL time. If I add residual nitrogen to a deep dive from diving shallow, I will be NDL limited, e.g. less dive time having decreased my NDL time. If I add residual nitrogen to a shallower dive, I will still be more likely air limited.

This also works well in early diving where computers and tables may not have been used at all. I know many divers foolishly to today standards, did not think they could bend on a single tank. Following the deep --> shallow principle helped.

 

[mathauck0814]

No. Reverse profiles generally make more sense to me than the other way around. The only problem is that unless you're chartering the boat or driving it yourself you may be bound to a schedule that would restrict the length of your second dive. The reverse profile rule was born out of economic needs, not physiological needs.

 

[drbill]

I've never particularly worried about reverse profiles. My dives are determined by the subjects I hope to film and the sequence in which I get to the sites where they are. I generally do my deepest dive first, but on many occasions have done much deeper second or third dives. I'm not particularly worried about going into deco though (assuming I have adequate gas supply and redundancy).

 

[WetLens]

We always taught "deepest dives first" because those profiles maximize allowable bottom time based on diving air and Navy tables - all that was available to us at the time. I bought an Edge shortly after it was introduced and was amazed at the difference in allowed bottom time, even above what we were doing with multi-level table techniques. Spent a lot of time listening to Huggins at various seminars, read everything available at the time, and did not find anything that led me to believe "deepest dive first" was anything but a method to maximize bottom.

Then came a trip to the Great Barrier Reef in 1990 - a 10-day liveaboard. We were forced to abide by what we called the Marquis of Queensberry rules of diving - some arbitrary depth limit, always deeper dives first, and no more than 4 dives in a day. We found ourselves lying to the DMs regarding depth. Our first dives were always to the maximum allowed depth - even if we only went 60'. That allowed flexibility on subsequent dives. Utterly stupid rule which we followed only in our reported depth, not our actual depth. We were lucky that computers were relatively new at the time and the DMs never checked them.

In practice, where and when ever possible, I still conform to deepest dive first and especially deepest part of an individual dive first. It just makes sense from a time-management standpoint. It shouldn't be a "rule," rather a best practice.

 

[packrat12]

With the use of computers, I really also do not care reverse or normal profile either. When the 'rule' was made, all we had were tables. I did a quick profile that may have not been uncommon. 120' Dive followed shortly by a 50' dive.

120 for 13min --> K group --> 30 Surface Interval --> F Group --> 50' dive, Residual: 25 min with a total NDL time of 56 min. So I could have done 120/13 and 50/56 with a 30 SI.

Reverse it.
50 for 56 min --> R Group --> 30 min SI --> K Group --> 120', Residual: 13 min with a total NDL time of 0 min. I cannot complete this dive on tables alone.

A computer may also give similar results if the profile is square.