What's the current thinking on a reverse profile series for technical dives?

[Akimbo]

For me, my first question is “how far will I be from a chamber” when considering profiles that push the limits of statistical proof. I think you have to put reverse profile dives into context.

Reverse profiles came up briefly in my saturation training in 1971, which also covered advanced/experimental deep surface supplied. It was taught by two ex-EDU hyperbaric docs who had done some of the human testing. Remember this was the time when virtually all diving was done on tables based on square profiles. These guys were involved in the early consideration of calculating on-site dive-specific profiles. They had this $80,000 computer/calculator made by the now-defunct Wang.

Understandably, they were very hesitant about calculating reverse profile dives because there wasn’t much data available and Doppler bubble studies were in their infancy. The Navy’s concern was, and still is, almost exclusively for square profiles. They had plenty of bodies they could swap out to split those rare reverse profile dives into two. That mind-set, and the logical preference for front-loaded profiles, is probable cause for a lot of the continuing reservations.

However, when reverse profiles were applied to fixed US Navy tables, these Docs were in general agreement that it was probably better than the full square wave due to less gas absorption. Fast forward to today where computers give us credit for lower absorption rates at the front end of the profile. There are mathematical models, but not a lot of statistical data back the tissue theories up for reverse profile.

This might be an interesting experiment. Assuming you are using desktop decompression modeling software, how do your decompression times compare when you mirror the same profile? Take those same two profiles and apply them to a different decompression model just to get a sense of how close they are. That might give you some insight on how you might increase safety margin, perhaps by end-loading more decompression time than indicated.

You would still let the desktop modeling software calculate the decompression, but nothing stops you from defining slower staged ascents as part of you dive profile.

 

[Bubbletrubble]

@Akimbo: I always thought that the term "reverse profile" referred to a set of repetitive dives in which later dives are deeper than earlier ones.
It sounds like you're using the term "reverse profile" to refer to the depth profile within one particular dive. Am I misunderstanding something here?

 

[Akimbo]

I have always understood “reverse profile” to be a single dive where the diver slowly reaches the max depth, maybe spending part of the dive at the max depth or a bounce, and then starting the ascent/decompression phase. My exposure has treated it as part of the family of dive profiles including:

• Table oriented “square profiles”
• Recreational “meandering profiles”
• “Front-loaded” profiles being rapid descents to max depth and slow ascents as part of the dive, often used on wall or pinnacle dives.

I have never heard very much concern over multiple dive reverse profiles since they have been allowed longer than I have been around. Of course there is a greater decompression penalty so they tend to be avoided by dive charter operators as well as military and commercial diving supervisors. In turn, that results in lower statistical samples.

Edit: I have been following the discussion and lost track of Tortuga68’s original post. Commercially we dive for weeks at a time, usually square profiles (on each dive) and one jump/day, to these depths with no noticeable increase in decompression hits. Logically, doing some shallower dives the day(s) before “should” be to your favor.

The differences may or may not matter but I offer them for your consideration. Gases were air or HeO2 rather than Trimix. They also included pure O2 in-water and surface decompression, which produced lower hit rates than all-water stops. The general consensus was higher hit rates resulted from the fact that water stops were colder and physically taxing compared to chamber stops. However it may also be that there was less residual gas after Sur-D-O2 because there was little incentive to rush chamber stops when tables were developed.