actual NDL calculations

[Thalassamania]

To extend the metaphor further, you know where the cliff edge is, you know it is unstable, so you stay as far back as you feel you need to in order to only expose yourself to an acceptable level of risk.

 

[TSandM]

When I first joined ScubaBoard, it was popular to refer to DIR divers as the Borg, as people accused us of being assimilated and indulging in groupthink. People asking DIR curious questions were warned that they would be assimilated. I was, and in fact, became an outspoken proponent of the system, to the point where I was dubbed the Borg Queen. Thus Assimilated Medical Mod, and my avatar.

 

[peterbj7]

Thanks for the explanation. I have yet to catch the DIR "bug" though who knows? - I'm still diving.

 

[j yaeger]

ya know,ive often wondered about that avatar,tsand m
thanks for clearing that up!!!!!
good diving
have fun
yaeg

 

[scc135]

To the OP: I had the same problem when I started diving. I did mostly shore dives, which involve a long, slow descent and a similar ascent. Plotting these dives as having been spent at the maximum depth put them WAY off the charts for the RDP, but I was using a computer, and the computer said the dives were fine. Of course, the reason for this is that the computer is doing a real-time, iterative calculation of nitrogen loading and unloading. It is not possible to do that mentally, and it would be ridiculously time-intensive to try to do it on paper or on a calculator. That's why we have dive computers!

But it is really important to understand what a computer is actually doing, because what it does really doesn't have much to do with what is going on in your body. Nobody really knows the dynamics of nitrogen in the human body, but what we know is that the models we use result in limits that reduce the incidence of DCS to a very small number. The computer doesn't know if you are cold, or if you had to swim hard against current, or if you are 100 lbs overweight. And the diver needs to know if the output from the computer is making sense; if your computer is giving you numbers that are way out of line, you should recognize that, because machines malfunction, and a malfunction in this one can be very dangerous.

I would very, very highly recommend that anyone who is interested in this stuff, or who intends to dive more than very occasionally, spent the money to acquire Mark Powell's Deco for Divers. This is a book which pulls together a very large amount of information, and presents it in a palatable (and not math-intensive) way.

As Peter said, DIR divers use intense personal monitoring of depth and time, a depth averaging approach (the limits of which have to be understood) and an ascent strategy incorporating mandatory deco for all dives. It takes time to be able to monitor and remember your depth profiles well enough to use the system, and everyone should know that the only validation of doing dive monitoring this way is the number of safe dives that are done using it.

Ah. OK. Guess I'm sticking to my computer for the time being then. I am pretty interested in taking GUE fundies and learning more about DIR, but alas, other worldly endeavors are getting in the way.

 

[Blackwood]

i like your posts and your way of explaining. let me try this

if you do a boat dive to a planned depth that allows 30 min bottom time and you use only 20 min of it and ascend at a rate different than the table used in the calculation. there should not be a dcs problem as you cut your bottom time short. yes there will be some inaccuracies. however they will be moot as you did not push the max bottom time.

In addition, if you go slower (within reason), the NDL is greater for an equivalent set of m-values. So if you go up to 30 minutes in the above example but ascend slower than the table assumes, you're not actually "up against the NDL".

I won't try to quantify that.

 

[Thalassamania]

The real problem is not with that dive (unless you ascend so slowly so as to let the NDL for a shallower depth overtake you), but with the proper calculation of repetitive dives.

 

[caverdave]

I am just telling you how the PADI tables were designed to be used. The proper ascent rate is not a mix and match question but rigidly defined by the table's author.
The table won't account for that ... that is the point. In terms of decompression efficiency or how you fell ... those are separate questions. The issue here is how does one properly account for the "delay" during ascent?

Thanks for clarification and most excellent point; OP was for a particular question which was well answered.

Lots of good follow up as well...

Guess I should have continued lurking, but felt compelled to speak... seemed (?!?) to be getting very complicated....

Appreciate the insights for follow up reading!

Dave