Ascent from Minimum Deco Dive

[VTernovski]

Well first off the confusion is the lack of consistancy, with different instructors giving different statements. Yes, that is simple... sort of. I mean, even this itself is confusing. i.e. you talk of 30 second stops, 30 second moves, and then you talked about 1 minute stops at 30, 20 and 10.

Well, technically a stop is the time you arrive at the stop to the time you arrive at the next stop. So a '1 minute stop' vs. '30 second stop, 30 second move' really -is- the same thing, with two different terms being used.

So the confusion still gets propagated with some people interpreting that as 1 minute stops every 10 feet from 50 to the surface, while others may interpret that statement as 1 minute stops every 10 feet from 50 to 30, then 1:30 stops from 30 to the surface [1 minute stops, 30 second moves].

Now then it gets even -more- confusing when you compare that with the DIR-F lecture that discusses 1 minute from 100 to 70 with a pause at 70.

So as you see... there is different things being said, and while yes... they are all conservative enough to be perfectly fine; they definately confuse people as not every one with the same training is on the same page.



That is definately the most disturbing thing I've heard in quite a while. ATA to Depth and Depth to ATA is a skill that is taught in OW class. In a Fundamentals class, which is, as I understand it meant to teach students the fundamental skills that they -should- have learned in their OW and AOW programs, I can't believe that you can just brush off the anability to make such calculations; let alone -require- it to be done.

When is it that they are taught that skill? In tech 1? When they have quite a bit more task loading that they to focus on?

But ok... let me hypothetically assume that I accept that disability as acceptable. So how about tell them to pause at 75% of their depth, and start their stops at 50% of their depth, or 30 feet; whichever is deeper. That hits them pretty close in recreational depths, gets them familiar with making those calculations, and considering they damn well better be able to learn how to divide their tank pressure by 3, I don't think it's much to ask to expect them to be able to divide their depth by 4.



I think if you look around, you will find that 30fpm ascents are what is being taught. There are some threads on other boards that asks specifically 'what ascent rate were you taught, and what methods were you taught to gauge that'. There were quite a few that were very surprised to learn that my OW book [(c) 1999] still stated 60fpm. I would be quite surprised if you found many DIR-F students that weren't already running 30fpm ascents, if not aware to some level of 'slower ascents' and 'deep stops'.



See... that's exactly the point I'm trying to make. You say you like to give students more credit, but in the same post you don't expect them to able to convert depth to ATA and knock off 20%, or even to be able to take 75% of their depth.

You speak of building blocks; but you don't seem to think that teaching them to be aware of the depths where later you will expect them to be able to not only calculate but calculate ascent profile changes at. Don't you think a better building block to learning to be aware of a curve shape is to start getting them used to the points on the curve, rather than giving them a linear ascent for 50 feet, and then another linear ascent for the rest?

Jeff, why are you publicly attacking MHK on this forum on that subject? We all know that you know this stuff, hence it just seems that you are playing devils advocate and if I would be a mod here I would have closed this thread and sent you a PM asking to take such discussions directly with MHK and to stop this trolling.

 

[MonkSeal]

I can't tell you how many times I ask a student to compute how many ATA's at 100', and all I get is blank stares.

Maybe it's because of imperial units - in metric system it's much easier. Also, using 70% of max depth (rounded to deeper) is quite easy in both systems.

 

[rossh]

...<snipped - see above>...
Michael H. Kane

Hello Michael, Good to see this issue in the public area for discussion.

I would like to comment on the 80% ATA rule (70% max depth) first stop depth and particularly for these shallow dives. The 80% rule forces a diver to stop deeper with reduced depth. i.e the 80% is linked to a proportion of max depth, when in fact is I think it should be linked to distance from max depth alone. The 80% creates increasing obligations, for dives wich have a reduced gas load? That would appear to be applied in a reversed order, and introduces errors into the shallower dives.

In current deep stop bubble models (vpm,rgbm,others), the first stop depth is primarily a function of supersaturation, gas choices and inert gas loadings, ascent rate and limits set to prevent onset of bubbling. This typically resolves to a distance of about 2 ATA off the bottom. Also a check of many NDL tables, will show that the 2 ATA value has significant impact on tables, as they begin to limit NDL times around this pressure offset.

For a quick and easy "rule of thumb" first stop placement, I would use a distance of 50 or 60 ft off the bottom value applied to all dives including deco dives. You can see this also fits neatly with the WKPP 300 ft dives (first stop typical at 240). This would give a much different first stop for many dives from 0 to 200ft bottom depth than your currently using, and eliminate all the extras stops that your organizations procedures add on to existing model and plans. WKPP experience has found that 2 ATA works at 300 ft. NDL testing and data shows the same 2 ATA at the shallow end. Why would we want to employ different value for dives between these two extremes of depth?

On any dive, when a diver is slow to leave the bottom, he experiences additional gas loads. His achieved profile will look very much like a multilevel dive which will require additional deco obligations, or reduced NDL's. If we take the slow ascent procedures, coupled with the overly deep 80% first stops on a shallow dive plan, then it's probable a divers body will register this as a multilevel dive, or an extended bottom portion. These two items coupled (slow ascent, too deep stops), will subject the diver to an increased gas load and deco obligation - more so than had he used the conventional ascent profile and safety stops. The too deeper stop when used in this area, is likely adding obligations unnecessarily to a basic dive.

Not too many years ago, a deep stop was anything deeper than a basic Bulhman table. Then came Pyle stops and WKPP deep stops, and then bubble models computed the deep stops for an accurate first stop value. Now GUE standards is promoting the deepest stops of all.

I think the GUE standards has overshot the mark with the 80% ATA / 70% depth idea when used on dives in the 0 to 150ft range. As pointed out in a post above, the linear ascent method has been well tested and shown to be unsatisfactory. I think GUE's procedures are getting too close to this situation. Our bodies will not distinguish the subtle difference from a linear type ascent in the deep portion versus the slow ascent and too deep first stops. The only saving item here on these shallow dives, is that they have a small gas load, and can afford a large error factor.

Regards
Ross Hemingway

http://www.v-planner.com

 

[MHK]

Jeff, why are you publicly attacking MHK on this forum on that subject? We all know that you know this stuff, hence it just seems that you are playing devils advocate and if I would be a mod here I would have closed this thread and sent you a PM asking to take such discussions directly with MHK and to stop this trolling.

Vadim,

I think part of the reason that many GUE instructors are reluctant to engage in scuba forums is evidenced in this thread. We try as hard as we can to communicate the notion that decompression theory isn't an exact science, and the methodolgy and tools we provide to help divers understand and shape their ascent rate curve is the end goal. George, and by extension the WKPP, have been using 80% of ATA's for some time with great success. As GUE moved into offering a DIR-F class many students only had access to information procured from George's postings and the WKPP site. The 80% of ATA rule is very applicable to the diving done in Wakulla, as a way to introduce the same methodology to recreational students many had already become accustomed to the 80% of ATA rule. Frankly, 50' of max. depth is more applicable in the recreational environment, but anyone that is fixating on the distinction is severly missing the larger point, which is that either way it's simply a tool to use as a starting point for a more complete ascent rate strategy. Similiarly, by way of example, we teach a 120 rule, as a methodology to compute NDL's. I say all the time if you want to use 115 instead of 120 I could care less, the point is that we offer the methodology, not necessarily a myopic fixation on 120, or in the instant case 80% of ATA. As you note, it makes little sense to engage in discussions of this subjective nature with professional contrarians on a scuba forum.

I note someone else in this thread pointed out that we are rigid with respect to standardized gear configuration, but are more flexible respecting decompression theory. Those that can't distniguish the reason why have either missed the boat, or just want to argue semantics. I'm happy to help those that don't see the point, but I'm loathe to engage debates for self-serving purposes..

Anyway I'll see you in 2 weeks..

Later

PS. did you get my PM??

 

[Charlie99]

Ross -- thanks for actually addressing my question as to what would be a good ascent profile from a minimum deco aka NDL aka recreational dive.

As a reminder, I started this thread with some questions:

The same GUE-trained individual that posted the linear ascent method also says that the dir-diver tables are not considered the DIR approach to minimum deco.

It appears to me that the series of 1 minute stops at all depths is an approximation of a 10fpm linear ascent, while the dir-diver.com stops are more in line with an efficient "shape of deco" ascent.

What really is being taught in DIR-F for the optimum ascent profile?

Do different instructors teach different methods?

What is the consensus as to the better ascent profile?

Your comments about 50', 60' or 2ata off of max depth being a good spot to slow the ascent from 30fpm to 10fpm makes a lot of sense to me.

The remaining thing I'm trying to puzzle out has to do with the length of 20 and 10' stops compared to the overall time to ascend. I can see some logic in replacing a 30fpm + 3 minutes at 15' with a 30/20/10' set of 1 minute stops if you are just going to allocate 3 or 4 minutes to the ascent.

But once one starts doing longer total ascent times with 1 minute stops (or 10fpm average ascent) starting deeper (either 80% ata, 75% d3pth, or what appears better --- 50' to 2ata off of max depth) it appears to me that one should lengthen the time in the 20'-10' range. Again, the fixed stops as shown on dir-diver.com match up with what appears to me to be the better ascent strategy.

Any comments on 20' and 10' lengths?

Charlie Allen

p.s. Yes, all of the discussed ascent strategies will work. So did a 60fpm pop to the surface with no stop of any sort. As long as I'm going to spend X amount of time in an ascent, there isn't any reason to use an ascent strategy that doesn't get me out of the water as clean as possible.

 

[Soggy]

As a reminder, I started this thread with some questions:

I think all of your questions have been answered.