Repetitive dives and dive computers

[1_T_Submariner]

Replace "Just Math" with "Best Guess"

The algorythm that is used is a "Best Guess" the math to get the answer is not. You could still end up with an Accurate incorrect answer (and a chamber ride).

I guess this is a play on words I just was trying to explain what I didn't understand about the OP's question.

 

[bleeb]

So what are the main "unknown" parameters that the repetitive dives model is missing ? Is it all about microbubbles ?

Most of them.

Sorry.

Here's one way to look at the problem: As another poster explained, most (all?) mathematical models used by dive computers represent the human body with a few (<12) 'compartments'. Each one has a fixed size (amount of storable nitrogen) and the rate at which it exchanges nitrogen with other compartments is fixed. Obviously, people are made up of a whole lot more lumps of tissue, their sizes vary by a lot (both from person to person and also within a single person) and how well they're connected varies. The exact conditions they're subjected to also vary from one dive to another. The quantities of nitrogen that cause problems are also not well known.

Techniques like taking into account possible microbubbles are based on things which are known to happen, but their effects aren't actually well understood, so to some extent, they're added into the "model" as additional fudge factors. When looked at statistically (with a large sample of divers and a large number of different dives) they seem to help, but there are so many other variables, that I don't think anyone has ever said for certain that a particular calculation would definitely help or not for a particular diver at a particular point in the dive on a particular day.

 

[Dr Deco]

Hello G-diver:

NDLs

I am not certain of the meaning of the original question. I believe that others have indicated that the NDLs will change as you load dissolved nitrogen. It is not until this is completely removed that you will return to the original [base set] of NDLs.

Models – What is Missing?

The first item to understand is that “model” is a very loose term when applied to decompression. JS Haldane actually had a concept in mind with his “limited, stable supersaturation” and “tissue compartments,” but it is understood today that these ideas are incorrect.

The real term is “algorithm.” This means that it is a calculation tool and may or may not be grounded in reality. It actually is not physically real, and my Decompression Physiology class uses two days to explain why.

Some items that are [probably] incorrect are:

- totally separate, unconnected “compartments;”
- the invariance of the compartment halftimes depending on workload;
- the inability to adjust for “microbubble” levels in the tissues related to DCS; and
- the inability to account for individual “bubble generation tendency” of a diver.

Dr Deco :doctor:

 

[NiclasG]

All simulation models used in dive computers are simple
aproximations to partial diffrential equaitions. You can not implement
the equations in a dive computer because of calculation power & battery
constraints.

The good news is that you can reduce the dimensions and still
end up with something that behaves in a similar way and with
a little bit of fiddling with parameters you are done.

Many divers seems to be pushing the NDL to it's limit. The NDL is
not a black/white measure. Whenever you are getting close to zero
you should take care and ascend and behave like you are in
decompression.

Why are NDL:s more uncertain? Simply because the simulation
and reality are diffrent.

Niclas

 

[DandyDon]

Many divers seems to be pushing the NDL to it's limit. The NDL is not a black/white measure. Whenever you are getting close to zero you should take care and ascend and behave like you are in decompression.

I generally don't push NDL - too much of an air hog, but even on my "liberal" Oceanic PDCs, I don't worry if I get into the yellow zone approaching it. I'm not sure about "behave like you are in decompression," but I will adjust my remaining dive plan to allow for deep and longer safety stops with a goal of surfacing in the green. Extending a safety stop and using half my contingent reserve down to 250 psi would be a prudent action in my mind with that goal in mind if there were no other challenging factors in play - but then I have tested my reg and spg below 250 psi, I do carry a pony, and I have not panicked when I had to orarlly inflate my BC on the surface, etc. Riding to 250 psi does have its risks even at 15 ft. :11:

 

[Zieg]

Riding to 250 psi does have its risks even at 15 ft. :11:

What risks?

 

[DandyDon]

What risks?

That probly should be discussed on another thread, maybe in Basic Scuba?

 

[Zieg]

That probly should be discussed on another thread, maybe in Basic Scuba?

But you made that statement in this thread.

 

[DandyDon]

But you made that statement in this thread.

Well, yes - but to discuss that possible subject would be a significant hijack. The standard 500 psi contingency margin is taught in OW courses, and any discussions of possibly using part of that contingency would probly be best discussed in its own thread. It might be a New Divers subject, but I'd not want to see it in that forum; I'd prefer Basic Scuba.

 

[Zieg]

Well, yes - but to discuss that possible subject would be a significant hijack. The standard 500 psi contingency margin is taught in OW courses, and any discussions of possibly using part of that contingency would probly be best discussed in its own thread. It might be a New Divers subject, but I'd not want to see it in that forum; I'd prefer Basic Scuba.

You introduced this subject into this thread about computers and NDL's. You said riding to 250 psi had risks. I'm asking what are the risks. Is there a risk concerning NDL's? Support your statement.