Reverse Profile?

[nilsdiver]

The bad part of the sawtooth is as always the ascent. Just as a fast ascents can leave you fatigued and perhaps with sub clinical DCS or worse the repeated ascent within a profile can have a similar cumulative effect as you repeatedly on and off gas. A sawtooth profile stresses the algorithm a computer is running and is beyond the scope of table diving. All things considered I doubt that there is a definitive answer to how much is bad. perhaps I'll learn otherwise.

The key to managing your profile is one of rate and magnitude. Any ascents want to be made deliberately and you don't want to on excursions form the surface to the bottom. As an example, on a local shore dive to 40 feet I do not think twice about following a bottom contour that may have ledges rising 10 feet. My profile when plotted will is segments look more like a kiddie roller coaster than a sawtooth though.

Pete

Agree with all that. Still looking for some real numbers to put to what a sawtooth profile is. I'm sure I could drop to 100 fsw and go up and down 5 feet and make my profile look like a saw tooth... but I don't think 5 foot teeth at 100 fsw is gonna be a risk. But I don't know how big a tooth needs to be to be a risk either. Although it would make sense that the deeper you are the bigger the teeth need to be.

 

[boulderjohn]

If I recall correctly, the workshop found that the warning against reverse profiles first came in a 1972 PADI manual that suggested deeper dives first. The reason was not clearly stated there, and PADI was unable to find any clear record as to why that suggestion was made. As the years went on, that suggestion evolved into an edict. I believe the original reason was exactly what Crowley pointed out: when using the PADI tables, you get shorter surface intervals (etc). that way. I once experimented with other systems (like V-Planner) and found that the differences vary.

Notice that this quote from the workshop conclusion limits their conclusion to normal recreational limits:

... no convincing evidence was presented that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of DCS, the workshop participants found no reason to prohibit reverse dive profiles for no-decompression dives less than 130 fsw and depth differentials less than 40 fsw.
UNQUOTE

It is my understanding that Bruce Weinke, a participant in the workshop and creator of the RGBM model of decompression, successfully argued for this limitation for reasons suggested here:

For decompression dives nevertheless, using bubble model as a base for study (Bruce Winke's RGBM, David Yount's VPM, Duke University bubble-volume model and DCIEM bubble evolution based on dopler scores), there was some results that actually pointed out for a higher risk of DCS for decompression models.

I am not aware of those studies, though. I had thought his argument was more theoretical in relation to the effect of deep dives on existing bubbles. (I do not pretend to be speaking authoritatively in this statement.)

 

[Divenomad]

I think it's good that this subject is in basic discussions. While a new diver may not share our understandings or fascination with decompression theory, it does slam home the point that it IS theory, and even experts are confronted with contradictory evidence.

Right now, we don't know, what we don't know, and it's prudent to dive safe and conservatively. While, for personal reasons, it irks me, better safe than sorry. Just a thought.

Nomad