Sawtooth profiles and conservatism?

[northernone]

I'm having a gap in my knowledge, might someone point me in a more informed direction of thinking?

Background thinking:
Sawtooth profiles increase dcs risk, add conservatism to compensate, computer algorithms attempt to penalize for them...

Question: What might that look like like on my dive profile?

Deco background:
I'm basically a tables diver, computers backup ZHL16/GF 40/75 and vbm-b +3 for sanity checks. Generally add 20% to the last stop.

Sample dive profile:
Cruising somewhat deep looking for sea caves, up and down the wall as I spot them. About 30 minutes bottom time, sawtooth from around 260-220ft, did that 15 times. 60fpm descent, 40fpm ascent. Total runtime around 160 minutes but with cruising at 90-70ft as well.

No symptoms, no weakness after. Just wish I knew how conservative I was being or a guess at how foolhardy it is to sawtooth at those depths. I'd mix in shallow o2 rebreather dives or stop the sawtooth profiles if it's likely unwise and a largely incalculable risk. They are not necessary dives, unlike the unavoidable sawtooth of cave diving.

Haven't come across source material research, just the standard training manuals and dig up a few threads from the turn of last century.

Anything welcome.
Cameron

 

[KevinNM]

Last I knew it was not really established that they were higher risk. It was more that it seemed reasonable to think they might be.

 

[TONY CHANEY]

Am I the only one that has not thought about sawtooth and reverse profiles in years? I thought that the sawtooth profile was frowned upon because everything was based on a square profile at least until The Wheel came out. Since I started cave diving and just using computers I really give it no thought. Since I am new to rebreathers, I will use tables to back up my computer on a really technical rebreather dive.

 

[northernone]

Am I the only one that has not thought about sawtooth and reverse profiles in years? I thought that the sawtooth profile was frowned upon because everything was based on a square profile at least until The Wheel came out. Since I started cave diving and just using computers I really give it no thought. Since I am new to rebreathers, I will use tables to back up my computer on a really technical rebreather dive.

Given the resounding silence I this thread, I think you have a point.

I'm unable to find a good answer or apply my thinking about deco theory in a way that's reassuring while I yoyo. I can hypothetically plan a computer dive with repeated 100m to 20m ascends and descents and it will give me a deco profile.

Where I'm concerned is the safety of believing it. Particularly in my case where I could be doing this over multiple days. (On a single dive the two different algorithms penalty adding one 'tooth' with 80% additional runtime and the other only 20%.)

It's nice to imagine I have an educated guess where the safety margins run thin. Confidence in my deco decisions for peace of mind... As well as avoiding the chamber as a patient.

At worst, anyone have a reassuring antidote of significantly more aggressive profiles involving sawtooth which didn't end in agony?

Thanks,
Cameron

 

[KevinNM]

I think John Adsit had a history of where some of these sorts of statements came from.

 

[KenGordon]

I can hypothetically plan a computer dive with repeated 100m to 20m ascends and descents and it will give me a deco profile.

Where I'm concerned is the safety of believing it. Particularly in my case where I could be doing this over multiple days. (On a single dive the two different algorithms penalty adding one 'tooth' with 80% additional runtime and the other only 20%.)

ZHL16/GF does nothing for the sawtooth, it is all area under the curve. In the past I have plugged shallow sawtooth dives essentially as long as you like into V-Planner and not got stops. I am not sure how a deeper dive would change - which algorithms gave you 20and 80% extra? How big was the tooth?

Personally I think sawtooth profiles are risky. I think then you are ‘off the tables’ as what you are doing is quite unlike the dives that were tested to design the algorithms. Maybe the time shallow makes up for the risk of bubbles forming, being crushed , growing some more crushed and then being supper big the next time round. Maybe that doesn’t happen at all. It Isn’t tested though.

I think we are reasonably sure repeat dives bring extra risk, so maybe that mechanism applies?

Consider the difference being warm at depth/cold on stops or working at depth makes to risk. Another example of being outside the testing which led to algorithms.

The example dive you give above is not especially scary though (in terms of sawtooth) Your repeated change in pressure is a small fraction of ambient. The scary stuff, for me, is a dive that gets significantly shallow, deep again etc - like rescue training - the ambient might drop by 50% repeatedly. Another, similar maybe, bad practice is free diving after scuba. I am not sure what evidence there is that it is dangerous and I assume there’s have been no proper trials, but that doesn’t mean I will be trying it out for myself.

For your particular dive, which sounds a bit like a survey, I might think about doing a deep pass and a return shallower pass. It is a pretty big dive and you will be surfacing with a significant nitrogen loading so you don’t want to be pushing any more. If you did get bent and then posted the profile you can be sure there will be people pointing at the sawtooth and saying “obviously!”.

 

[Michael Guerrero]

I think I've read a study (UHMS maybe) that showed that reverse profiles were no more dangerous that standard profiles--at least in terms of bubble scores and diagnosed DCS during the study.

For sawtooth, I'd be more concerned about doing it shallow than deep. The gas volume changes going from 7 to 8 atm is only going from 1/8 to 1/9 (less than a 2% change), whereas bouncing around in the 2 to 3 atm range is a gas volume change of nearly 10%. And doing this at the end of the dive, where your tissues are loaded, may make that more risky.

Also, I've read a few accident reports where instructors got DCS from chasing their students up and down and up and down. The one that stands out most clearly was in a DAN article where the instructor ended up on the surface five or six times chasing the student and ended up getting slightly bent. However, these weren't deep dives.

I have done sawtoothing below 150ft (shipwrecks) with no noticeable difference in planned deco or negative aftereffects.

EDIT: lol, basically, what Ken said.

 

[TONY CHANEY]

Well thought out reply Ken. I think I get your point. Small depth changes like +-10 feet not so bad but +-50 foot changes could get you a ride in the chamber. Has anyone checked V-planner, as Ken did, but with the depths northernone posted. I might play with it later tonight.

 

[RyanT]

As mentioned previously, the depth matters. Doing a sawtooth between 260 and 220 is a change of just over 1 atm and proportionally, the pressure change is relatively small. I generally try to avoid sawtooth profiles, but I'd worry more about doing them between 0-70' ish range.

 

[rossh]

From the model math perspective, in ZHL the profile shape (slow ascent, zig-zag, reverse or square), does not add or subtract anything extra beyond the normal on/off gas tracking values that are present always, and which form the basic deco ascent limits. i.e. if the current gas loading exceeds the limits, then a stop is produced.

In VPM-B, the zig-zag profile does have some extra carry over effect, but its not significant to the overall plan.

Yes, some research says that reverse profile dives are the same risk and the usual sequential shallower approach.


But practical experience shows that big zig-zag profiles are not a good thing. Perhaps the most common one is the dive master, who when finished guiding the dive, will then race to the bottom and quickly unhook the anchor, and return to the boat. The gas tracking and deco calculations says its OK, but DCS's have occurred this way.

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