Dive Tables vs. Computer Algos - repetitive rec dives

[tbone1004]

well that's a big discussion...
So the NAUI/Navy tables are optimized for one big dive. NAUI's tables are one stop short of the Navy tables, and the Navy set theirs up for one dive per day. First dive, pretty good, repetitive dives suck. You get one good long dive, then are shafted for any repetitive dives
PADI decided that divers don't dive like that, so they created the RDP to optimize repetitive dives at the cost of bottom time on the first.

PADI roughly correlates to 45/95 in GF's per Shearwater. The GF-Lo doesn't really matter that much because PADI's tables are no-deco, but the 45 may cause a mandatory "safety stop" on deeper dives which in this case is a deco stop that @Diver0001 was talking about. "Mandatory Safety Stop" is a stupid term IMO but because so much emphasis is put on staying out of deco, it can cause people to freak out a bit which is quite annoying. Seen it happen, usually comes from people thinking that if they violate the tables or their computer by a minute they are automatically bent.

The computers obviously track real time, and unfortunately the vast majority of computers have proprietary algorithms that can only be correlated by actually diving the things because you have no idea how to model them. Really stupid, but thankfully a bunch of people have taken them into pots and figured out what they're going to do.

IMO the PADI tables, if used in truly square profiles, are very aggressive and I won't use them. Thankfully very few dive truly square profiles, so it's usually a non-issue.

 

[Jay]

At the 15 min. point of the 1st dive, starting ascent, the NDL was 4 min. That dive was 20 minutes on the Cobalt, probably because the safety stop generated was 2 minutes.
At the start of ascent on the 2nd dive, the NDL was 3 minutes, total dive time was 30 minutes.
Dive 3 showed 13 min NDL at the start of ascent, 26 min. overall dive time.
These dives would be within the "folded" RGBM algorithm- basically a Buhlmann model with bubble factors added. At 45 m we would be switching over to a fully iterative RGBM calculation.

I'm not sure what this would prove, however. ... Ron

Thanks for that. Much appreciated. I guess if all the results had lower NDLs we could say that the tables are more aggressive?

The mathematical algorithm underpinning the tables, however, would be considered fairly aggressive if it were just put 1:1 into a computer that sampled your depth every few seconds and eliminated those rounding errors.

I think there is an article you can find on the internet from ... Baker called "Understanding M Values" that has a table in it showing the exact 1/2 times and M values used to calculate the tables. Most computers these days are tweaked to be considerably more conservative than that.

R..

Thanks too for that. It's a neat way of answering the theo question without such an abstract dive profile. I'll have a study of it over the day. I'm wondering then, given 1:1 comparison then a DSAT computer (unmodified) should match the DSAT dive tables?

 

[tursiops]

On apples/other fruits etc:

I've selected times and depths and times that don't involve (table) rounding, neat square dive profiles (as per tables), SS as required by tables, and adjusted for Bottom / Dive Time differences. I don't see where or why there's other comparison errors inherent or added to the question in the way I put it. If so, please do specifically say so.

Edit: obviously I've rounded the ascent times (minor error), and I guess that the NDL times generated by the computer or algo might be rounded?

Take a look at the Scuba lab DC comparisons in Scuba Diving mag over the years. They compare various computers in a pressure pot on "real" 'repetitive dives. It shold tell you what you want to know.

 

[boulderjohn]

My view on the tables was that they are relatively aggressive, they've been scaled back over the decades, and any computer on the most aggressive setting wouldn't be more aggressive than the tables (using their square table profile).

I am curious as to how the tables have "been scaled back over the decades." Are you talking about changes in a specific set of tables, or are you talking about changes from one kind of table to another?

 

[Jay]

Take a look at the Scuba lab DC comparisons in Scuba Diving mag over the years. They compare various computers in a pressure pot on "real" 'repetitive dives. It shold tell you what you want to know.

Yes, I've studied this comparison in detail and did a comparison chart in this thread: Place of dive tables in modern diving however that is a computer vs. computer comparison and unrelated to PADI Dive Tables (exc the possible DSAT computer, DSAT table link). Also those dives are very non square (very real).

 

[Jay]

I am curious as to how the tables have "been scaled back over the decades." Are you talking about changes in a specific set of tables, or are you talking about changes from one kind of table to another?

I was meaning to refer to PADI Dive tables, and how the NDLs have changed. e.g. 30m/100ft used to be 25min.

 

[boulderjohn]

I was meaning to refer to PADI Dive tables, and how the NDLs have changed. e.g. 30m/100ft used to be 25min.

No. The PADI tables have not changed since they were first created. The 25 minutes at 100 feet was (and still is) part of the Navy tables and those that are derived from them.

If you look at post #11 from @tbone1004, you will see the problem that those tables created for diving when they were pretty much the only ones in use. Divers and dive operators wanted to do 2-tank dives, but the long required surface interval before the second dive made that a real problem. That long surface interval was based on the fact that when the Navy created them, they added a new, longer compartment, the 120 minute compartment, to the original set and then made that the basis for surface intervals. Since it takes 6 iterations of the controlling half time to clear a table, you will see that in the fact that the Navy tables (and those derived from them) clear (wash out) in 12 hours. That was fine for them because they were really only doing one dive a day, but it was not fine for typical recreational divers.

PADI spent a lot of money on research to determine what compartment should govern the kinds of dives that are typically done by recreational divers. They used Doppler bubble imaging and checked the actual dives of a lot of divers. They determined that for the vast majority of those dives, the 40 minute compartment could safely govern the surface intervals, which would make a significant difference in the timing of repetitive dives. In making their final version of tables, they did the following:

1. They made the 60 minute compartment the controlling compartment, both to be more conservative for most dives and to include dives for which the 40 minute compartment was not appropriate. You can see that in the fact that the PADI tables wash out at 6 hours rather than 12.
2. The shortened the first dive NDLs in comparison to the Navy tables. For example, their NDL of 100 feet became 20 minutes instead of the Navy's 25.
3. They made more pressure groups in order to limit rounding. The Navy tables have roughly half the number of pressure groups as the PADI tables, so you have to round off much more. Rounding is always in the direction of conservatism, so having fewer pressure groups unnecessarily adds to the required surface intervals.

As a result of starting with shorter NDLs, increasing the number of pressure groups, and using the 60 minute compartment, required surface intervals were dramatically shortened, which is what really made the modern 2-tank dive possible. Pretty much all recreational dive computers have algorithms with surface intervals more consistent with the PADI tables than the Navy tables.

 

[boulderjohn]

I want to add a thought to my last post (#17) related to several threads that have appeared on ScubaBoard in recent months.

Using the Navy tables (and those derived from them), it is possible to have some easy memory jogs that allow you to remember the NDLs for first dives without having to look at the actual tables. Some divers have been saying that is how they plan their dives--they just use those memory jogs for their dives. I get the distinct impression from their posts that they do that for all dives, even the repetitive dives.

Although they don't say this, I get the distinct impression that they do the first dive to the Navy limits, sit out the typical surface intervals required by boat crews, and then get back in the water using the same limits as the first dive. There are no memory jogs for repetitive dives, and they never mention consulting their tables for those second dives. In several of those threads, I have mentioned this and asked about it, but they never responded.

Since the typical boat operator goes by surface intervals consistent with the PADI tables, if they do indeed go into the water after doing a dive to Navy limits without using the Navy tables to calculate the repetitive dive limits, then they are not actually diving the Navy tables. Their second dives may well seriously violate both the PADI tables and the Navy tables (and those derived from them).

 

[tbone1004]

@boulderjohn

I use rule of 120 for first dive, then assuming a 2 hour SiT, then I use rule of 80 for dive 2 if for whatever reason I don't have computers or tables with me.

 

[dmaziuk]

On what it is I'm trying to achieve:

I'm trying to understand how conservative or aggressive PADI (DSAT) dive tables are versus various computer algorithms on repetitive rec dives. We generally know how various algos stack up versus each other on single / rep (rec) dives, but we (or at least I) don't know if my thought that tables are relatively aggressive (as are DSAT computers, rel to other computers / algos) is an accurate thought or not. Yes, this is quite a theoretical question.

Well theoretically authors of DSAT recommended that you don't dive every third day and more recently e.g. @Dr Simon Mitchell stated the effects of repetitive diving over multiple days are pretty much not studied. So theoretically if you dive for more than 2 days straight doing more than one 80-100 cf tank/day it's a crap shoot regardless of the algorithm.

Theoretically the slowest "tissue compartment" in DSAT in 60 minutes. Which means that by DSAT, after a SI of 3 hours and 1 second you're clear of any residual gas loading. A computer with slower tissue compartment(s) should theoretically not declare you "clear as a newborn baby" after 3:00:01 and therefore it will be "more conservative". And it's not gonna matter until you accumulate enough theoretical gas in the slow theoretical compartments to actually make a difference. I have not run the numbers myself, e.g. the author of popular deco planner software recently said you never get that much gas in there. Theoretically.