actual NDL calculations

[wve]

If your ascent is other than a direct ascent to the surface at 60 fpm, the additional time used in the ascent must be added to you bottom time.

I don't disagree. But the instructions for the RDP do not exactly say that, and instructors do not always make it clear, either. Moreover, most teachers I know nowadays are encouraging a 30 ft/min ascent, especially above 60 ft.

The calculations of a square profile can be challenging enough to new divers, much less getting into multilevel dives, compartments, half-times, the pros and cons of deep stops, and all that.

Which gets back to the question that the OP asked. If I understood his question, he was wondering why they don't teach new divers how to calculate NDLs from actual depths and times rather than using tables which end up taking away bottom time by all the rounding and stuff. He then went on to say that he had just done a dive which he could not make work using the RDP.

Now, I assume that he used a computer to make the dive. (I sure hope he didn't make a dive that he already thought wouldn't work!)

So I tried to give him two reasons why he was able to make a dive that didn't work on the RDP.

(1) Computers and tables figure Bottom Time differently. Tables assume a direct ascent at a fixed rate whereas computers are making real-time estimates.

(2) The RDP and PADI Nitrox tables are for square dive profiles. He did a multi-level dive. Had he had a Wheel or an eRDPml, he could have come closer to planning the dive that he had made -- or, more correctly, closer to retro-calculating the dive -- but even then you have to know how Bottom Time is defined by the tables compared to a computer.

I really just wanted to make the point that definition of terms is important, and they can change depending upon the type of dive, tables and equipment you use.

Maybe I'm overly sensitive to this because early in my career I had the same problem as the OP. I dove with a computer, but then when I logged the dives and tried to retrofit the dive to the RDP (just for fun and for practice), I found that it didn't always work. When I investigated, I learned that part of the problem was in how each defined Bottom Time.

 

[Thalassamania]

I don't disagree. But the instructions for the RDP do not exactly say that, and instructors do not always make it clear, either. Moreover, most teachers I know nowadays are encouraging a 30 ft/min ascent, especially above 60 ft.

While this is no biggie, it happens to be wrong. It just goes to point out how little our current crop of diving instructors are taught concerning decompression models, theory and practice.

The calculations of a square profile can be challenging enough to new divers, much less getting into multilevel dives, compartments, half-times, the pros and cons of deep stops, and all that.

IMHO, without those details no diver can be considered to have provided informed consent concerning the activity that they are about to undertake. When coupled with the general, "DIVING IS SAFE" advertising of the industry this creates, to my mind, an untenable liability situation.

Which gets back to the question that the OP asked. If I understood his question, he was wondering why they don't teach new divers how to calculate NDLs from actual depths and times rather than using tables which end up taking away bottom time by all the rounding and stuff. He then went on to say that he had just done a dive which he could not make work using the RDP.

Now, I assume that he used a computer to make the dive. (I sure hope he didn't make a dive that he already thought wouldn't work!)

So I tried to give him two reasons why he was able to make a dive that didn't work on the RDP.

(1) Computers and tables figure Bottom Time differently. Tables assume a direct ascent at a fixed rate whereas computers are making real-time estimates.

(2) The RDP and PADI Nitrox tables are for square dive profiles. He did a multi-level dive. Had he had a Wheel or an eRDPml, he could have come closer to planning the dive that he had made -- or, more correctly, closer to retro-calculating the dive -- but even then you have to know how Bottom Time is defined by the tables compared to a computer.

I really just wanted to make the point that definition of terms is important, and they can change depending upon the type of dive, tables and equipment you use.

Maybe I'm overly sensitive to this because early in my career I had the same problem as the OP. I dove with a computer, but then when I logged the dives and tried to retrofit the dive to the RDP (just for fun and for practice), I found that it didn't always work. When I investigated, I learned that part of the problem was in how each defined Bottom Time.

While your answers were correct from a strictly operational viewpoint (e.g., it is unlikely that they would cause someone to suffer DCS, they were not completely correct from a decompression model, theory and absolute practice standpoint.

 

[peterbj7]

It just goes to point out how little our current crop of diving instructors are taught concerning decompression models, theory and practice

It goes further than that. It shows that many instructors have been taught the PADI way, to LEARN rather than UNDERSTAND, and as most instructors are teaching at the maximum level they have reached in their own diving, in general they haven't a clue. In any activity I look for an instructor who has gone WAY beyond the level he is intending to teach me.

 

[Mike Boswell]

To the OP -

I am just finishing reading the last section of the PADI Encyclopedia of Recreational Diving, and I'm in the middle of a very good discussion of decompression theory where they explain the work of Haldane, Tissue Compartments, Navy dive tables, the development of the RDP, and many other related issues.

I think you would get a much better understanding of Deco theory if you read this section of the book. It's pretty expensive to buy, but I understand that DMs or instructors might have it, so maybe you know someone who would loan you a copy.

 

[Thalassamania]

Here's a piece that I rather like: Understanding M-values

 

[wve]

While your answers were correct from a strictly operational viewpoint (e.g., it is unlikely that they would cause someone to suffer DCS, they were not completely correct from a decompression model, theory and absolute practice standpoint.

Maybe I misunderstand the question...

 

[Thalassamania]

The question is what to do with any extra time you spend during an ascent that is over and above the time that you should have spent had you ascended at the specified rate. When using the U.S. Navy tables you add any delay in ascent to bottom time and recalculate your status. The PADI tables, regardless of what information the hack writers at PADI HQ decide to put in or to neglect to put in because the adjudge it too complicated for you to understand, Ray Rodgers (the table designer) was quite clear in the many conversations that we had that delayed ascents should be handled exactly the same way as the U.S. Navy Tables.

 

[scc135]

Wve, you understood my question perfectly. And yes, I did use a comp and tried to retro-fit the dive with the RDP. I'm starting to not like how PADI just gives you a method but there is no reasoning or rather understanding taught to the student. I really want to know the how and why.

So really, my choice seems to only be the erdpml? So if a DIR diver plans a shallow multi-level no deco dive (if memory serves me right, they don't use computers, right?), it would be with an erdpml or similar?

 

[Thalassamania]

No, the true DIR would do an on the fly Ratio Decompression calculation.

 

[disbelief]

Here's a piece that I rather like: Understanding M-values

Classic read.

IMHO, the confusion arises due to the fact that pure Haldanian models do not feature ascent rate directly. They concern themselves only with maintaining inert gas tension below a critical value. By definition, within NDLs, the gas tension cannot ever become critical. (That is not to say that ascent rate is not important when following recreational tables, just that it emerges from either empirical data fitting or a free-gas modification to the pure Haldanian basis).

The 60ft/min comes not from recreational, "no-deco" diving but from a compromise made in 1957 between CDR Doug Fane (West-coast Underwater Demolition Team) who wanted something like 100ft/min for his frogmen vs the hard-hat guys who's winches could not practically sustain that rate. 60ft/min was selected as the compromise.

If you're interested in ascent rate theory, I'd recommend the 1989 Proceedings of Biomechanics of Safe Ascents Workshop as a good primer. Some good stuff on phase dynamics from Wienke and Yount, and interesting experimental work from Van Liew, Lehner and Pilmanis.