Repetitive dives and dive computers

[Web Monkey]

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?

It's not necessarily less accurate, although there's a good chance it is.

The computer contains a mathematical model of a generic "you".

Given the information it does have (breathing mixture, depth, time, ascent rate and tank data if AI), it maintains an estimate of the amount of nitrogen in various tissue types in your body.

If the model doesn't fit you perfectly, the differences between the computer's model and your body (errors) will tend to increase over time. This could be good or bad, depending on which way the errors are accumulating. If the computer consistently estimates that you contain more nitrogen than you do, and you follow it's recommendations, you'll have less chance of DCS. It also works the other way, meaning that if the computer thinks you contain less nitrogen than you do, and you follow it's recommendations, there is an increased chance of DCS.

This is one of the reasons that many people recommend taking a "day off" in the middle of a week-long series of repetitive dives.

Note that this isn't specifically a "computer problem" and the exact same problem can occur when diving tables. If you off-gas slower than the model the tables are based on, you could easily end up getting bent.

It's actually a problem that occurs when applying any mathematical model to a physical process, without feedback from physical measurements.

Terry

 

[DandyDon]

Is there a risk concerning NDL's? Support your statement.

Not in my mind, which is why I won't expand on the idea in this thread - and I've been here long enough that I should set good examples. My apologies if I seemed to imply that....

 

[Zieg]

Not in my mind, which is why I won't expand on the idea in this thread - and I've been here long enough that I should set good examples. My apologies if I seemed to imply that....

.... you don't have an answer? Is it that your statement had nothing to do with the subject of the thread in the first place and should have been omitted?

 

[Papa_Bear]

It's the Lawyers! The Logarithms are written to get more conservative the closer you come to saturation end! Its not science because Physiology is involved otherwise it would be hard and fast, but we are all different! So getting DSC is not a Red line it is a best guess! So the more guess the program has to deal with membranes and Logarithms it is written to keep the Lawyers from the door! The Military does many things that you wouldn't because it's harder for the Lawyers to get on base!:11:

 

[dschonbrun]

Zieg, have some respect for Don... he has more than enough knowledge and experience to answer your inquisition, but he's choosing not to do it here.

 

[Zieg]

Zieg, have some respect for Don... he has more than enough knowledge and experience to answer your inquisition, but he's choosing not to do it here.

Please tell me how I am disrespecting anyone here. The amount of knowledge or experience is not in question. Don made a statement and I simply asked what he meant. It appears that he feels that his statement is not related to the subject of the original post (off topic), therefore his hesitation to answer.

If Don wishes for the related posts to be moved to another , that is his option.

 

[DandyDon]

.... you don't have an answer? Is it that your statement had nothing to do with the subject of the thread in the first place and should have been omitted?

Perhaps I could have handled this better, but - I try my best. Yes, my statement about using some of the 500 psi contingency reserve if need to get back into the green zone on my computer was addressing NDLs, but the "other risks" of using part of that reserve seemed like off-topic to me. I thot the risks were kinda obvious, but maybe not. Perhaps it would be good to discuss this for others to comment and/or learn.

I discussed this with a Mod who agreed that I should answer, with which I have no problem, but left it up to me whether I do it here or in another thread linked here. I chose to open this http://www.scubaboard.com/forums/ba...need-use-some-500-psi-contigency-reserve.html in Basic Scuba.

thanks