Deep stop question

[tursiops]

What is the correct way of defining the "time" under any given depth in the PADI RDP - I was of the understanding it was total bottom time but I've also heard it said as total time under the water....???

It is the time from initial descent until leaving the bottom to head for the surface.
Unfortunately, dive computers don't know how to do this, so "dive time" includes the ascent. Worse, most dive comnputers don't even turn on until you get to 5 ft or so, and turn off when you get back to that sort of depth.
The good news is the RDP "BT" is always less than the "dive time" from a dive computer (or bottom timer), so using the latter builds in a bit of a margin.
In BoulderJohn's case, his BT was a few minutes, his dive time was 80 mins. And THAT is one of the problems of using tables for real diving.

 

[dmaziuk]

But you did violate the NDL according to the planner, 100' max depth with a run time of 80 minutes is way over the 20 minutes max allowed - correct?

No: 5 minutes at max depth. It's a typical reef dive: you follow the wall up and have the total runtime of an hour on an Al80.

 

[Storker]

When you said "those tables" to contradict my comment about what I learned about the research on the PADI tables, I assumed you were talking about the PADI tables.it?

When I said "those tables", I was referring to the tables I had previously mentioned. It obviously wasn't as evident as I believed it was.

So did they publish anything along those lines? Is there any official warning about it?

Frankly, I don't know. I only referred what I was taught from my instructor and had confirmed from the maker of the table.

 

[boulderjohn]

But you did violate the NDL according to the planner, 100' max depth with a run time of 80 minutes is way over the 20 minutes max allowed - correct?

The problem I was trying to point out is that tables (All of those kinds of tables) do not address a minimum ascent rate--they assume you will ascend at close to a given rate, but they don't talk about ascending too slowly. At some point, you can ascend so slowly that you are in violation of the NDLs as would be determined by a computer. Obviously, the dive I described was more of a multi-level dive, but how do you differentiate between a multi-level dive and a lingering, slow ascent, and how do you differentiate between a multi-level dive and a deep stop?

What is the correct way of defining the "time" under any given depth in the PADI RDP - I was of the understanding it was total bottom time but I've also heard it said as total time under the water....???

As Tursiops said, when using the tables, bottom time is the time from descent until you make your direct ascent to the surface. So you could say that I had only 5 minutes of bottom time, or you could describe some pauses along the way as multi-level stops, meaning I it was quite some time before I finished my bottom time and began the true ascent. When using a computer, you normally count total run time for purposes of logging dives.

I get what you are saying about the no set too slow ascent rate but in that sense, defining time as the total time under water sort of covers that....

Total time under water really only works if you are using a computer, which recalculates your dive continually. As I hope I showed in the comments above, if you are using tables and doing the kind of slow, multi-level ascent I am describing, you really don't know what is going on. Bottom time ends with a DIRECT ascent to the surface. It can be slower than the ascent rate associated with the table, but if it is a lot slower, you are doing a multi-level dive, and the tables don't cover those. If you go slowly enough, at some point you will go into deco, but you won't know when that is.

I think you can see why I decided not to write my article on NDL ascent rates. After all my research, I have less to say that is helpful than I did before, when I thought I knew what I was talking about.

 

[KenGordon]

In my research on deep stops in NDL diving, I learned that this study is no longer highly regarded.

Having read all the research on NDL ascents I could find last year, I urge you to get your contact to publish that information, since no one I encountered knows about it, including apparently DAN and PADI leaders.

Fair’s fair, tell us specifically what is wrong with the 2004 paper, rather than the article that quotes it, or maybe details of how it is no longer highly regarded.

The problem I was trying to point out is that tables (All of those kinds of tables) do not address a minimum ascent rate--they assume you will ascend at close to a given rate, but they don't talk about ascending too slowly.

BSAC88 tables give numbers which are the time to the first stop, so 6m for a no stop dive. This means that it does not matter what the ascent rate is (obviously not too fast), the time that matters is the arrival time at the stop, just like in a deco dive.

 

[tursiops]

BSAC88 tables give numbers which are the time to the first stop, so 6m for a no stop dive. This means that it does not matter what the ascent rate is (obviously not too fast), the time that matters is the arrival time at the stop, just like in a deco dive.

Doesn't that mean an ascent rate is implicit? If you must go from depth to 6m in a certain amount of time, what is the ascent rate used for that calculation?

 

[boulderjohn]

Fair’s fair, tell us specifically what is wrong with the 2004 paper, rather than the article that quotes it, or maybe details of how it is no longer highly regarded.

As I understand it, the results of that study have not been substantiated by later study. (I believe I am accurately repeating what Simon Mitchell told me about it.) I am not the one who wrote the warning on the article stating that it did not reflect current thinking. DAN gave it that warning.

 

[rjack321]

Fair’s fair, tell us specifically what is wrong with the 2004 paper, rather than the article that quotes it, or maybe details of how it is no longer highly regarded.

The problem with Maroni's methodology is they didn't compare equal length dives. They added time at intermediate stops. So there is no way of knowing if the differences were a product of the slower ascent or if the deep stop was the cause or the longer ascent was the cause. The flaw in the methods is hinted at by Gutvik here:
"If the deep stops are only added on top of the conventional procedure, we cannot conclude if it is the extension of decompression or the deep stop itself that is beneficial. It is possible that the extra decompression time added is more wisely spent at a shallower depth."
Alert Diver | Deep Stops

 

[NAM001]

and I think again it makes a difference whether you start your ascent at the NDL point or midway. NDL would offer the greatest off gassing rate 100 foot dive 100 ft tissues offloading while you go to 30 ft vs that same ascent scheme where 100 ft in say 5 inutes your tissues are at 75 to start vs 99 ft. the faster the off gas rate the less any on gassing in other If you were at 100 ft for 3 minutes at the start of the dive would be an all together different consideration that if that 3 minutes were later in the dive. tissues would matter. This aspect whether valid or not has always been a discussion point I have with others when the talk about 3 min vs add 2 if greater than 60 or 10% from ndl comes up. depending on the order of things would drive the answer. with a 2.5 min tissue a 5 min dive to 100 would have some tissues at 82 +/- ft. after that 5 min segment going to 40 ft say for 20 min would be almost all off gassing and tissues would be perhaps 40-45 ft.... compare then going from say 40 to 100 being saturated at 40 would be on gassing from a 2.2 ata to 4 ata and for 5 min they should ongas to about 85 ft. It seems that no matter where you stop between the loading of the slowest tissue and some where deep you will continue to on gas some where but that only looks as the on gassing side of things and not the off gassing side. is that amount of on gassing have a tangable consequence given the value of the off gassing that has occured. This thought has come up many times since some one said it dies not matter if you do the deep dive first and then do shallow last vs the opposite. perhaps that was only meant to apply for deep dive surface int then shallow dive as opposed to deep/shallow segments of a multilevel dive. I see them as no different one dive multilevel or multiple dives with out a SI.

 

[NAM001]

That's one of the worms in the no-stop can: your surfacing supersaturation is computed from something that hasn't happened yet: the ascent. So there is some predefined ascent rate in there, most likely the recommended 30 fpm, but you won't really know unless the vendor spells it out for you or lets you read the code. If you deviate from it, theoretically you could invalidate the computed NDL.

It shouldn't ever happen in practice, but mathematically it's possible.

However once you actually stop, that may change things. E.g. if your fastest TC is 2.5 minutes, a one-minute stop may be long enough to on-gas it -- if it wasn't saturated at that depth (this is the part where they started calling it "profile-dependent"). If your fastest TC is 5 minutes, one minute is probably negligible even if it is on-gassing, but 2 minuets may not be.

When you predict an outcome and you can not control what has not happened then you have to have an an assumed ascent rate. that rate will be the worst condition to calculate with. if you ascend at 25 rather than 30 it is a benefit to do so and at 35 then the predicted data goes out the window. I want to say that IN my aeris computer it used 60 fpm to 60 ft and then 30 fpm to the surface for its worse case protocol. Im not sure about my shearwater but I think it is similar. but as you deviate form the assumptions in the algorythm it will update and give new info based on compliance to assumptions from time of calculation. The reason I say I think my shearwater is the same is that there was a change to the alarm tripping on the computer to trip at 31 ft so that you can do the max 30 fpm and that it also did that for the 60 fpm rate tripping at 61 fpm. I could be thinking about another computer though.