Unrealistic Deco Time

[Deep South Divers]

There doesn't appear to me to be anything wrong with your computer. That's the way that algorithm works.

The bottom line is that, according to the computer's math, your body was still ongassing, even at a depth as shallow as 25' or so. The longer you stayed down below that depth, the more "decompression time" your computer added to your dive.

You appear to be surprised at how quickly it added time to your "decompression obligation." Don't be - four long dives in the same day on a high N2 mix (read: air) with too small a surface interval between each dive can and will quickly nail you if you "go into deco."

It appears that, according to the Wisdom's algorithm, not until you hit 25' or less did you pass the "floor" - the point at which your body stopped ongassing and begin to offgas. The reason for the shallow floor was because of a large amount of residual nitrogen - four long dives on air with comparably small surface intervals. Once you went over your NDL, you piled obligation on quickly, and even at comparably shallow depths.

That is to say, that even though you were on ascent around 1:08, it took you four and a half minutes to come up 20' - which was four and a half minutes of continued ongassing... And yes, after four dives, a factor of 8 or 10 can result (because of the residual nitrogen from prior dives), giving you about 45 minutes of decompression obligation at 10'.

So is the problem the computer?

Nope.

Hate to point out the obvious, but regardless of what computer you were using, you'd still have dived the same dive. Kinda like telling the police officer that you were going too fast because of your speedometer.

Would it make sense to consider the purchase of a new speedometer? Perhaps contact the manufacturer of the speedometer and ask what was going on?

I'm not going to "beat you up" either - my point is that if you are considering the purchase of a new computer, you're attempting to apply a solution to the problem that is not a direct solution to the problem. I think you have expressed that you grasp the problem well - what other solutions come to mind?

My suggestion would be to not press on the gas pedal so hard. Perhaps shorter dive times (do you realize that you spent some 7 hours underwater that day?), or longer surface intervals, or shallower dives, or a better gas mix would be a better plan.

For what it's worth, Your 32% idea was a good one - but in-water recomression is a complex issue, best left to those trained to do such a thing. A better idea probably would have been 100% O2 at the surface. If this sounds intriguing, call DAN and ask what their recommendation would have been.

Oh, and tell them that SeaJay said hello.

Edit: Oh, and before I forget... Glad to hear that you're okay. I bet you were TIRED by the end of the night. How long did it take you to feel normal again? Did you ever experience any numbness or tingling? Unexplained rash? Ringing in the ears or nausea? Difficulty focusing mentally or visually?

 

[Thalassamania]

If you're going to dive that aggressively, and yes I consider four dives to about 80 ft in a day aggressive, you really should be diving EAN-36 or even better, EAN-40 if you can get it.

 

[WetLens]

DeepSouth, Thal: Agree, absolutely should have been on 32 (all that was available) and did dive 32 the remainder of the trip. It was my first time diving the area and got wrapped up in shooting photos. Again, understand that I am not making excuses - you can't beat me up more that I already have myself over this. The description of the diving going in was relatively shallow with plenty of slope to ease up towards the end of the dives. All dives were as described - always a slope, always somewhere to rise and still see stuff. It was all that and more. I stuck with the guides (who were on 32%) too long.

On the new speedometer analogy... It is not the same. I would not expect a different outcome no matter what speedometer I used because I would expect standards and tolerance limits on the gage. And I could test it by pacing off measured miles. I have run these dives through numerous simulations - before and since posting - to try and better understand the logic behind the accelerating stop time. I cannot find a model that reproduces the results I got from the Wisdom. That is what I am questioning. I should be able to reproduce the situation and I can't.

Thank you very much for you input and insight and I fully agree - I was hitting the gas pedel pretty hard over the 4 days of diving.

And, no, there were absolutely no symptoms. Not tired, nothing out of what could be considered "normal" for me ;-).

 

[Rick Murchison]

I can find no table or algorithm that'll increase the 10' stop obligation (no, there was no deeper stop obligation indicated on any of the data downloads - look at the "Stop Depth" column) the way your computer did.
The Wisdom algorithm is a trade secret, but based on what you saw, here's what happened:
Because of your previous dives, you were "on the edge" with at least two, likely three or more theoretical "compartments." What you saw was a "cascade" of those compartments from short to long half-times, gaining control - that is, becoming the limiting compartment in the algorithm - on the 10' stop.
You probably couldn't re-create the same cascade in a hundred attempts, and while I'm not going to say the algorithm is "basically flawed" I do think that the writers of the program should consider a mod that'll (in cases like this) either give you an earlier indication that you need to start an ascent or somehow limit the cascade rate.
As for what you should do... if you're going to have that kind of diving day routinely, (and it's the kind of diving day I personally like ), I'd recommend taking the time to plan and run the dives in advance with a computer based planning program like V-planner and using the Wisdom strictly as backup. I'd definitely invest in a Tech Foundations (SSI) or equivalent course as a minimum "knowledge base." Nitrox is nice, too.
Rick

 

[DiveNav]

DiveNav: Data attached in spreadsheet format. I included all dives leading up to the day's events. I ignored prior day's dives in the simulations I built, but thought you might want them. ......

Thank you.
We will run data on our sim and let you know what we see in the compartments

 

[ianr33]

- but in-water recomression is a complex issue, best left to those trained to do such a thing.

Just to be picky, the OP went back down after a "missed" deco stop. In water recompression,as I understand it, is when a diver has DCS symptoms,in this case there were none .
Descending again with symptoms is obviously a much bigger decision to make.

 

[Deep South Divers]

...you can't beat me up more that I already have myself over this.

Oh, I'm sure.

I remember years ago, when I did exactly the same thing... Dove my computer, which did some funky things after a lot of diving in a short time span. Mine was a Suunto Vyper.

It put me into an 18-minute deco on my third dive of the day, and I simply didn't have enough gas to do it all. I cut my deco short because - I like to be able to breathe.

A conversation ensued about "where was my buddy" and "why didn't he go topside to get more gas so I could complete my decompression." The reality was that we were 60 miles offshore, and we'd already exhausted all of our tanks - there simply was no more gas available.

...Which, of course, started the conversation, "Why didn't you guys have available O2 on the boat," which turned into "DAN O2 Provider kit vs. keeping a stage bottle of 100% onboard the boat."

But yeah, I beat myself up pretty good about the whole thing - as I imagine you have, too. Don't kill yourself too badly over it - making mistakes is part of the learning process, and we all know that no matter who you are and how much experience or knowlege you have, there's always more to be learned.

In all, I think you're handling it better than I did when I made the mistake... And striving for a deeper understanding is the right thing to do, IMHO.

My point was that, no matter which gauge you were or are using, no matter which program, no matter whether or not there's a change in Sherwood's algorithm in the future... No matter even if you decide never to wear a computer again at all... The instrument doesn't matter. The core issue is that you were pushing your limits, and you would have continued to push them regardless of whatever gauge you were using - whatever computer you were using, and whatever it was telling you. The fact that you somehow feel slighted when the computer slams you with a long (and apparently unexpected) decompression obligation has nothing to do with the issue.

The solution to the problem is: Don't dive so close to the limits. Wait another hour. Dive a safer gas. Shorten your dive times to maintain a better safety margin. Any of the above, or any combination of the above, would have created very different results.

Consider yourself lucky: Not many people can blow off 30 minutes of decompression obligation and come up asymptomatic. Many people would have ended up going for a chamber ride. Many people do, even when they haven't pushed the limits as far as you did.

I, too, did the same thing - and got away with it. I don't recommend it.

For what it's worth, I suspect that part of the reason why your Wisdom piled on the obligation so quickly is because you've got your conservatism set high on it. If so, there's a variety of ways different algorithms can add conservatism, and reproducing it later using different software may be a lesson in futility. Assuming that your conservatism is set high, know that it's entirely possible that a setting of "zero conservatism" may not have put you into deco at all... Which would explain why you "got away" with missing 30 minutes of decompression.

...However, regardless of what your computer was telling you - regardless of your set level of conservatism - and regardless of your decompression obligation, missed or made - your body and the computer are two totally different things. They will never be totally "in sync" with each other, and never completely agree. In this case, the computer was clearly more conservative than your body, so you got away with it this time. It's a good thing - what if the opposite had been true, and the computer was telling you that you were fine, when you weren't? (And I know of cases where that was true.)

I have found that "diving my computer" can result in unexpected problems, like what you experienced. Ditto for "diving too close to the limits." Doing both can result in disaster. The only real solution is to dive conservative profiles, and diving the plan that you make. Any other solution - including attempting to "understand why your computer put so much obligation onto you in such a short period of time" is only a Band-Aid to the real problem.

I think that Rick's got it right - that the reason for the huge obligation had to do with compartments and a cascading of those compartments... I think it was worsened by some conservatism set in your computer... Let me know if that's wrong. But knowing that, for sure, gets you nowhere - it does not create a solution for the problem.

The only solution I know of is to plan your dive (conservatively) and dive your plan. No electronic gizmo is going to substitute for a good dive plan, executed correctly.

 

[Deep South Divers]

Just to be picky, the OP went back down after a "missed" deco stop. In water recompression,as I understand it, is when a diver has DCS symptoms,in this case there were none .
Descending again with symptoms is obviously a much bigger decision to make.

"Recompression" is just that... Recompressing. Diving a 5th dive in hopes of making up his lost deco time is still recompression, whether he's symptomatic or not.

Medically speaking, the point of being symptomatic is not the point of having DCI... That's the difference between Type I and Type II.

In the chambers at Duke, I learned that there are always bubbles in the bloodstream... After a dive, they're larger than they would be otherwise, as the body offgasses. The point at which they become a problem - that is, the point at which the person becomes symptomatic - is a threshold that varies widely not only from person to person, but within an individual, depending on time of day, hydration level, exertion level, and many other factors. These variables make the concept of "an exact point of being bent" an elusive concept. It's probably more accurate to say that every dive is a decompression dive - and that there is no such thing as an NDL.

Unfortunately, the mentality that we have - the methods for teaching and the computers that we use while diving - give us the impression that there's an exact threshold, and that, if you stay underneath of it, you will be fine.

Reality simply doesn't work that way.

Compare it instead to, "How many beers can you have before you're drunk?" Or, "How much candy can you eat before you become diabetic?" Or, "How much fat can you consume before you become obese or have other health issues?" It's not a real "threshold" kind of thing, but rather, the more you consume (and in this case, it's dissolved nitrogen), the worse off you are for it.

What's your exact limit on that exact day? Exact limit for what? None of the issues come on suddenly.

Just like with alcohol or refined sugar or saturated fats, it's a matter of limiting your intake of nitrogen - and the more you limit it, the better off you'll be.

...Which is why diving at the ragged edge isn't much safer than falling over the edge.

 

[Rick Murchison]

Just to be picky, the OP went back down after a "missed" deco stop. In water recompression, as I understand it, is when a diver has DCS symptoms,in this case there were none .
Descending again with symptoms is obviously a much bigger decision to make.

"Recompression" is just that... Recompressing...

Well, yeah, as a physics term, and in fact, but...
"Recompression" as a term in diving is a treatment: "Returning a diver to the highest pressure experienced (or greater if necessary) for the purpose of minimizing or eliminating the effects of decompression sickness or air embolism; also, for treatment of these; accomplished in a chamber rather than return to depth under water." (NOAA Diving Manual, 4th ed)
"In Water Recompression" is, again, a (not generally recommended) treatment for DCS.
So while technically any descent after an ascent is a "recompression," if we're going to be consistent in the vernacular and within the context of the diving world, "recompression" is a term generally understood to mean the treatment of DCS by repressurization, and in the interest of accurate communication among divers, what the OP did was one of the several (generally, but not by some) accepted ways of handling a missed stop without DCS being present. IOW he was still "decompressing" throughout the maneuver as a procedural description of his activity. ianr33 is correct.
Rick

 

[gcbryan]

The bottom line with the Wisdom (or with any of the Oceanic computers) is that you are diving with a "liberal" computer. So in effect you are pushing the boundaries on every dive. If you actually manage to get into deco (hard with such a computer) it penalizes to a greater degree than some more conservative computers that never would have let you stay out of deco for that many dives.

It may be unreasonable to pile on a 45 minute obligation in that short a period of time but you can't take the liberal algorithm up until that point and then disagree with it after it does put you into deco. It's all part of the same calculations.

I wouldn't conclude that you need a new computer. As others have mentioned I'd conclude that you should modify your profile in the future and if you decide to plan on getting into deco then plan for it before the dive. Otherwise, just continue to follow the same computer that brought you to that point. Either choice is fine.