Repetitive dives and dive computers

[G~Diver]

Why does the NDL that the dive computer shows when doing repetitive dives is less accurate the more repetitive dives you do ? what is physical explanation ? is there a problem to model the affect of the repetitive dives?

 

[fppf]

You answered your own questions, its a "Model".
It is a mathematical approximation of real life. The model is always running even after the dive. The more dives you string together the longer the model runs the more the error will increase.

But remember, this also applies to tables as well not just computers.

 

[LeadTurn_SD]

Why does the NDL that the dive computer shows when doing repetitive dives is less accurate the more repetitive dives you do ? what is physical explanation ? is there a problem to model the affect of the repetitive dives?

I'm not positive I understand your question... but if you are asking why it's not a good idea to "push" the NDL limits on repetitve dives over multiple days, here is my understanding: The NDL's in the computer (or printed tables) are based on a model, and any model is only as reliable as the actual testing that has been done to validate that particular model or theory. Although a lot of testing and validation has gone into the current tissue loading models and NDL's, testing cannot possibly take into account all of the variables that can occur over multiple repetitive dives, or the physical variations between divers, so there is a real risk of the model becoming less accurate with repetitive dives as more "unknowns" are thrown at it. So the standard advice is to stay well on the "safe side" of NDL limits, especially for multiple repetitve dives.

Not sure if this is what you were looking for, hope it helps.

Oooops, fppf and I were answering your question at the same time. Same answer, his was more to the point!

Safe Diving!

 

[Gene_Hobbs]

This could be worth a read for you...

Lang, M.A. and R.W. Hamilton (eds.). 1989. Proceedings of the AAUS Dive Computer Workshop. USC Catalina Marine Science Center. 231 p. RRR ID: 4242

and if you like that one, try this...

Lang, M.A. and R.D. Vann (eds.). 1991. Proceedings of the AAUS Repetitive Diving Workshop. Duke University, NC. 339 p. RRR ID: 4243

 

[1_T_Submariner]

I have read the answers and understand but, I'm still not sure what your question is and where you are coming up with the idea that the NDL's are less Accurate?

I would actually think it is more accurate (ok better or worse is not the question accuracy is).Since the computer accurately knows your depth and your times. It also knows when you get out of the water and when you get back in. Other than that its is just math.

Tables and other methods that rely on memory etc... are probably less accurate (ok not debating better or worse etc...) just accuracy.

Edit:
Sorry Dr. Deco, I thought this was the computer forum. I look forward to your response.

 

[fppf]

Replace "Just Math" with "Best Guess"

 

[G~Diver]

there is a real risk of the model becoming less accurate with repetitive dives as more "unknowns" are thrown at it.

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

 

[DandyDon]

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

Computers generally track depths and times as compared to a conservative model, but usually do not consider aspects like water temperature in the calculations, much less your personal hydration, fatigue, physical condition and exertion.

I think Cochran is working with water temps and looking at other aspects, but I don't really know much at all about them. Heard a little...

 

[miketsp]

Computers generally track depths and times as compared to a conservative model, but usually do not consider aspects like water temperature in the calculations, much less your personal hydration, fatigue, physical condition and exertion.

I think Cochran is working with water temps and looking at other aspects, but I don't really know much at all about them. Heard a little...

Just nit-picking but water temperature does not come into the calculation. You'd need a skin temperature monitor inside your suit.
I'm normally much warmer diving my 7mm semi-dry in water at 20C than I am diving my 3mm shorty in water at 30C. The effect is probably even more exacerbated with dry-suit divers.

 

[fisherdvm]

Why does the NDL that the dive computer shows when doing repetitive dives is less accurate the more repetitive dives you do ? what is physical explanation ? is there a problem to model the affect of the repetitive dives?

From what I understand, the single dives are controlled by only one of the 16 to dozens of tissue compartments your computer was programmed to use. As you do repeated dives, different compartments are then appoaching the M-values, thus your dives are greatly shortened and the dives are controlled by many variables.

I would not say that it is less accurate, it is just simply more complicated.

As the first dive doesn't have to give credit for surface intervals, most dive computers will agree with most dive tables and algorhytms.

With subsequent dives, depending on the computer's programming, different models are used, different tissue compartments are chosen for the nitrogen gas off credit, some give credit for each compartment's half life, and some chose one particular compartment for the gas off. I.E., the navy used the 120 minute compartment, and most dive tables use the 60 minute, while some will calculate it for each specific compartment.

Then you will find these computers and tables deviating from one another, not because they are less accurate, they are just simply using different methods for giving credit for surface intervals. The navy's table was intended for decompression dives, and give you alot less credit for surface intervals, whereas most computers are intended for no-decompression dives, and let you gas off faster, and give you more credit for surface intervals.

I think that is the main reason why the tables and computers differs so much more with repetitive dives, and mimics each others quite well on the first dive.