Average Depth Diving?

[JimC]

As long as you stay within the parameters of the system, it works out fine. Same holds for ratio deco. Same holds for every model used in diving. You can go blasting off PADI RDP just the same as you can go outside the limits and intent of the 120 rule just the same as you can make you computer not give you good info.

"Depth averaging" works within its, all be it narrow, range very well giving NDL times +/- a few minutes of any computer I have dove beside or any numbers I have run on my lap top with all of the multitude of planners out there. It was nether the least or most conservative and honestly the spread of numbers you'll get is quite impressive. On a simple 60 foot dive on air NDL varies 18 minutes from 50 minutes to 68. Which one is righter? It only gets worse when you start doing multi level dives with various tools.

IMHO its a lot more important to do the accent part right than nail your average or bottom time exactly, much as Mike has eluded to.

 

[lamont]

Another way of turning this argument on its head is....

If I can make an estimation error in my head which gets me bent, then the model isn't sufficiently conservative.

 

[limeyx]

The luck is in the fact that the math worked out. It's like Uncle Pug's, "add the depth the time" formulas. The point is that you can't work even a small fraction of the possible combinations and permutations, this is true for any model, but more so for a mathematical structure that is not really integral to the model (itself being an abstraction of at least one level from reality), but that has been fit to it ex post facto and is thus an abstraction carried out yet one additional level away from reality.

This is not to say that the system will not work within a narrow suite of endpoints. It may well do so, but you have no idea what the actual endpoint are or when the model will fail to work, and that is the problem. Confidence grows as is successfully used and thus the universe to which it is applied is expanded and confidence grows, etc. But the very real possibility of a catastrophic (in mathematical terms, at least) failure sits there waiting in unexpected and unpredictable loci, one which may be entered as a result of nothing more than not padding the model enough with a subjective safety margin.

The depth-averaging numbers I (and rjack and probably lamont and many others) use are not just plucked from thin air.

Neither is the technique used to calculate the average as simple as some might suppose (although once you get used to using it, it's quite easy to use underwater).

The numbers come from looking at patterns in well-known deco algorithms and generalizing and adding in some conservatism so that within certain ranges of bottom times, at certain (average) depths with certain gases, you can indeed do this safely.

Can you throw away the tables/computer one day and do it the next? Of course not, it takes time to build up confidence.

Used properly, this technique applies easily for everything from recreational diving to 300 foot deco dives.

However, you have to be willing to understand it (and take the appropriate education) and to work with the techniques and find out to some extent how they apply to you personally as decompression can be very individual.

is it safe for everyone? absolutely not, but it works well for a lot of us.

I have not used a computer since dive 40 or so, and have done over 170 dives since then in singles, doubles, recreational and decompression without needing to refer to a printed set of tables or a computer.

 

[TSandM]

I've been reading this thread with interest, as I have a foot in either camp.

What fascinates me are the dives that are routinely done in Monterey. These are significant decompression dives, up to 200 feet or more, but they are shore dives. The dive profile is often not entirely known when the divers go into the water, which is quite different from the typical deco dive on a wreck, where the depth is known and the profile is square. In these dives, the divers are exploring the terrain and constrained by its topography.

Planning the dive is not at all like the typical deco dive profile. You don't have a rapid descent, a period at depth, and then a deco curve shaped entirely by your pre-dive calculations. You have a slow descent, a period at depth determined in part by terrain, and an ascent along terrain which has to end up where you started -- on shore.

These dives are not done using computer pre-planning, as far as I know, so they have to be done using a type of depth averaging/DOTF. And they are done routinely and repeatedly, and to date safely.

It is a very small sample, in terms of the number of divers doing this, but the number of dives is mounting up. It's very interesting to see what is being done, and how different it is from the fairly limiting structures of more mainstream decompression diving.

 

[gcbryan]

I've been reading this thread with interest, as I have a foot in either camp.

What fascinates me are the dives that are routinely done in Monterey. These are significant decompression dives, up to 200 feet or more, but they are shore dives. The dive profile is often not entirely known when the divers go into the water, which is quite different from the typical deco dive on a wreck, where the depth is known and the profile is square. In these dives, the divers are exploring the terrain and constrained by its topography.

Planning the dive is not at all like the typical deco dive profile. You don't have a rapid descent, a period at depth, and then a deco curve shaped entirely by your pre-dive calculations. You have a slow descent, a period at depth determined in part by terrain, and an ascent along terrain which has to end up where you started -- on shore.

These dives are not done using computer pre-planning, as far as I know, so they have to be done using a type of depth averaging/DOTF. And they are done routinely and repeatedly, and to date safely.

It is a very small sample, in terms of the number of divers doing this, but the number of dives is mounting up. It's very interesting to see what is being done, and how different it is from the fairly limiting structures of more mainstream decompression diving.

Even though you could approximate something like this on V-Planner this is where most any dive computer can be used. If you are willing to do deco on the fly (rather than in advance) then this is what you get from most any dive computer.

Most dive computers are as conservative (if not more so) than V-Planner for shorter deco obligations. In most cases the dive computer may not start the stops deep enough to suit many people but it's not hard to add those in yourself.

Again, I'm just addressing the deeper shore diving deco situation you address. If you are talking about longer deco obligations or even greater depths then I'm not sure if using a dive computer is practical. It may add too much time and in the wrong places.

The interesting thing to me is that most of us do most of our dives in a remarkably similar way from dive to dive as this is dictated by our location and interests in diving. Therefore most of our dives will fall within certain boundaries each time. I'm sure anyone that dives to 140 fsw looking for marine life in the PNW (for example) can do most of their dives with or without a computer, or DOTF, or using V-Planner after awhile. It's only when you change your diving profile that you really need to recheck things.

I'm not suggesting that you should disregard your computer or preplanning just that the range that most of us dive within usually doesn't change that much.

 

[grazie42]

Originally posted by gcbryan
The interesting thing to me is that most of us do most of our dives in a remarkably similar way from dive to dive as this is dictated by our location and interests in diving. Therefore most of our dives will fall within certain boundaries each time. I'm sure anyone that dives to 140 fsw looking for marine life in the PNW (for example) can do most of their dives with or without a computer, or DOTF, or using V-Planner after awhile. It's only when you change your diving profile that you really need to recheck things.

If I´ve understood the posts correctly that is exactly what UP and others are doing...
a) do a bunch of dives with computer/tables/whatever
b) compare how you feel after those dives to how you want to feel
c) start making minute adjustments until you feel the way you want to feel
d) find patterns in the profiles that make you feel good.
e) come up with a equation/model/rule that produces the pattern you´ve observed
f) go out and dive according to your "model" while comparing it with computer/tables/whatever.
g) if "f" shows that the comparison gets you out of the water after having satisfied whatever rule you´ve found and tables/computer/whatever then that rule is valid for those kinds of dives.

Voila!

Is that more or less it? I´m not "for" or "against" any of the methods suggested in this thread, just trying to understand how they´re developed...

 

[SubMariner]

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[Thalassamania]

The depth-averaging numbers I (and rjack and probably lamont and many others) use are not just plucked from thin air.

No, but they are abstracted from another system that was not designed to support that abstraction. You can always fit a new model to any set of data points that are produced by a previous model, but when you do that you can only have confidence in the performance of the new model between the end points that you used from the old model and, in reality, only at the precise values that you used to fit the new model to, data points outside your set are anyone's guess and values between your data points are assumed to fall on a line that has been fit (in come manner) between the data points. That assumption is a natural one for humans to make and can get us into all kinds of trouble with statistics and potentially decompression.

Neither is the technique used to calculate the average as simple as some might suppose (although once you get used to using it, it's quite easy to use underwater).

It's been discussed and revealed, if not here then other places on the web, it requires little more than elementary school math.

The numbers come from looking at patterns in well-known deco algorithms and generalizing and adding in some conservatism so that within certain ranges of bottom times, at certain (average) depths with certain gases, you can indeed do this safely.

If you add in enough conservatism than any decompression model, no matter how bizarre, will work, to wit: make a dive to 60 feet on 36, stay there for 25 minutes, and then do a one minute stop every ten feet on the way up. Safe? Yes. Conservative? Yes. Efficient? Not really.

Can you throw away the tables/computer one day and do it the next? Of course not, it takes time to build up confidence.

You can only build confidence in the actual dives that you do. How is this any different from making a given dive a bunch of times and then not using tables or a computer, because you know the dive? Same thing. The problem comes in when you decide that the next dive is similar, but not exactly the same. If you have a large degree of conservatism built in, similar should be OK, but, then it is not efficient and why bother? If you do not have a large degree of conservatism built in, then you must carefully evaluate the risk level of your abstraction, something that you've got absolutely no information about. All you know is that similar dives (and you have a passing small sample size of those) were OK.

Now let me give you an example of the problem. There are no-D limits (we’ll use Navy for the moment) that were developed first with a model, then with animal tests, then with human subject testing, then with field testing that continues to this day. From 40 to 200 they are respectively: 200, 100, 60, 50, 40, 30, 25, 20, 15, 10, 10, 5, 5, 5, 5, 5 mins. If we fit the 440 rule (440/depth)Squared, we get for NDLs: 121, 77, 54, 40, 30, 24, 19, 16, 13, 11, 10, 9, 8, 7, 6, 5.

So we dive the shallow ones first, clearly we will have no problem all the way down to 120 feet, our dives will be much shorter, in many cases, then need be … but that’s the price we pay for preferring to carry a calculator rather than a set of no-D limits. But at 130 we’re a minute over, chances are we won’t be bent on that dive, but there's a chance, then we’re fine at 140, our biggest problem will be at 150, with a diminishing potential for disaster down to 190 where we’re OK again.

Now, I’m not advocating the 440 rule, it clearly has problems that are easy to demonstrate, and knowing them we could use it, inefficiently, to 120 and above. Is your averaging technique a better fit? In my estimation … yes it is. How much better? I don’t know. Are there any holes in it like the 150 hole in the 440 rule? I don’t know, and that’s what concerns me. And on the flip side, how efficient is it? We don’t know.

Used properly, this technique applies easily for everything from recreational diving to 300 foot deco dives.

That’s what is not known. Can you prove that? Can you verify that through trials? No. Simulation and anecdotal is the best that you can do.

However, you have to be willing to understand it (and take the appropriate education) and to work with the techniques and find out to some extent how they apply to you personally as decompression can be very individual.

Now that’s the insult that started the problem last time. Now I will just point it out and move on.

Is it safe for everyone? absolutely not, but it works well for a lot of us.

So how do we tell whom it will work for?

I have not used a computer since dive 40 or so, and have done over 170 dives since then in singles, doubles, recreational and decompression without needing to refer to a printed set of tables or a computer.

That’s great, and I hope that the next one is as good for you as the previous 130, but you have no real way of being sure of that, except a passing small sample and simulation and anecdote.

 

[rjack321]

1st: there's no way to do "trials", that would require having someone push the envelope and that would be medically unethical IMO.

2nd: the method requires an understanding of both dissolved gas and bubble mechanics

3rd: the method requires the same gasses for the same depth ranges. "best mix" nitrox in contraindicated, although not necessarily impossible.

4th: the Navy tables were developed by anecdote. Some got bent - some didn't, a couple thousand dives later and some negotiation between various Navy units and the tables emerged. Just amongst those knowledgable posting in this thread, there are probably a thosuand DCS free dives in aggregate. Why does anecdotal evidence not satisfy you that is works - way beyond recreational depths too.

5th: it is also taught by certain instructors/agencies, you don't just pick it up and start winging it. Its in the Koolaid.

 

[Charlie99]

2nd: the method requires an understanding of both dissolved gas and bubble mechanics

Why?