Average Depth Diving?

[Thalassamania]

The span of one's C.V. does not make one right or wrong. The extent of one's C.V. can, however, sometimes be use to add authority to an assertion and to squelch naysayers. If that be the case, I'll happily invite BiggDawg to stand upon mine, which runs to about 12 pages now; though the acuity of his insights suggest that he does not need to.

 

[Charlie99]

I'm not a fan of the average depth method either. I believe in redundancy rather than relying solely upon my fallable and forgetful brain to keep a reasonably accurate running average depth during a dive. I prefer to let a computer do the detailed tracking while I just keep a rough sanity check going in my head.


OTOH, most of your critique of average depth method is off base.

That is exactly the problem.
average calculations may not have much to do with pressure groups, but pressure groups have everything to do with nitrogen loading! :icosm12:

Pressure groups are simply a method of calculating nitrogen loading. They are, of course, mathematical models that have eventually been empirically tested.

Are you aware that pressure groups track ONE and ONLY ONE halftime compartment? (60 minute on PADI RDP, 120 minute on USN table and derivatives.) They are a very limited and partial method of tracking nitrogen loading.

Dive tables and computers use a minimum of twelve different groups, with different rates and different halftimes.

Dive tables use one and only one halftime compartment to track N2 loading. NDLs are determined by different compartments, but the table only tracks one compartment.

Averaging only estimates based upon loading for One (i.e., an "average") Tissue group As a result, averaging is not likely to be calculating the one tissue group that is controlling the dive (the one being closest to saturation)..

The Average Depth method, not surprisingly, tracks average DEPTH, not any specific compartment. Time and average depth is then compared to an NDL. Since the NDLs are determined by the different halftime compartments, this means that the average depth method of dive tracking does indeed rely upon multiple compartments.

If this doesn't make sense, read the last two sentences again, think about how dive time limits are set on a table, and then think about how they are set in average depth tracking. Both are similar, in that they only track one parameter, but have NDLs set by other factors.

(BTW, your assumption that the controlling compartment is the one closest to saturation is incorrect. It usually is NOT the one closest to saturation. On a 60' dive for example, the 5 minute compartments will saturate long, long before you reach NDL.)

Even if the averaging method did, in fact, watch the correct tissue group, averaging is a linear estimate. Since tissue saturates with nitrogen at an exponential rate, a linear estimate is going to be in error. And, that error will increase the greater the difference between the deeper and shallower depth. (Of course, if there is not much difference between top and bottom, the estimate won't be far off, but there won't be any need to average, either.)

Result is, the more you have use for multilevel profile calculations, the less accurate averaging will be.

The fact that nitrogen uptake is exponential has no relevance to whether or not one should use linear, exponential, or geometric calculations for determining average depth. Two different things. Two different ways to track them.

There are different ways to handle depth averaging, and in particular, different rules on how to average depths according to the sequence in which one dives different depths. 15 minutes at 100' followed by 10 minutes 60' is a lot different profile than 10 minutes at 60' followed by 15 minutes at 100'.

As I posted above, there can be very large differences between what level of saturation averaging will estimate, and what level the diver actually has.

And depending upon the method of "averaging", the result will be further and further from the actual loading, but in the conservative (i.e. "safer") direction.

 

[Thalassamania]

Dive tables use one and only one halftime compartment to track N2 loading. NDLs are determined by different compartments, but the table only tracks one compartment.

No actually tables and computers both use multiple compartments. Tables reduce themselves to a single compartment only when you take a letter designation for repetitive dives.

The Average Depth method, not surprisingly, tracks average DEPTH, not any specific compartment. Time and average depth is then compared to an NDL. Since the NDLs are determined by the different halftime compartments, this means that the average depth method of dive tracking does indeed rely upon multiple compartments.

But each time you average you throw away all the information in everything but one compartment. Yes, that is the compartment that is most likely to be on interest, but as a stupid exercise I’m sure that you could come up with a series of depth changes that would result in the loss of controlling tissue data as a result of averaging, even with all the if-this-then-that rules.

(BTW, your assumption that the controlling compartment is the one closest to saturation is incorrect. It usually is NOT the one closest to saturation. On a 60' dive for example, the 5 minute compartments will saturate long, long before you reach NDL.)

Compartments are only theoretical constructs to make the models work. You can establish any compartments that you want. The two-second compartment is fully saturated in twelve seconds, but will never be the controlling tissue since it offgasses faster then you can ascend. In a similarly absurd vein, the four-week tissue will not be the controlling tissue unless your on a very long habitat mission, and even then it will not be of any importance because it releases its gas too slowly to do any damage.

The bottom line is that averaging seems to work across a limited suite of exposures rather by chance then by design. There is no theoretical basis for it and no controlled human trial for it. It makes about as much sense as the summing the depth and time to 120 (which similarly works, in English units only, by a quirk of the math).

Do what you want with your spine, I’m not averaging mine.

 

[lamont]

If you go look at what I posted earlier in this thread, I don't depth average in the way that you suppose. I depth average when the exponential function can be approximated linearly during the beginning of offgassing.

I do actually have enough years of college level math and physics courses dealing with exponential functions that I actually understand what I'm doing. I've also got enough diving experience that I treat dives near the NDLs as deco dives and don't pop to 15 feet for 3 minutes and get out. And I ran my suunto vyper and my brain side-by-side for over 100 dives before I was confident that my brain worked just as good as the computer did without annoying me (my brain, for example, is always set on 32% or 30/30 and doesn't need to be reset from 21% every time I go diving)...

 

[Charlie99]

No actually tables and computers both use multiple compartments. Tables reduce themselves to a single compartment only when you take a letter designation for repetitive dives.

Exactly. You are saying the same thing I stated. All of the numbers you see on a table are from one compartment only, except for the NDL numbers, which depend upon a variety of compartments. This is quite different than saying that tables keep track of 12 different compartments. The difference between tracking one variable, average depth, and one variable, one compartment, it not as great of a difference as BigDawg would have us believe.

But each time you average you throw away all the information in everything but one compartment. Yes, that is the compartment that is most likely to be on interest, but as a stupid exercise I’m sure that you could come up with a series of depth changes that would result in the loss of controlling tissue data as a result of averaging, even with all the if-this-then-that rules.

If you take "depth averaging" very literally, you can put together profiles that are obviously bogus. Those profiles are most likely to be ones that have a shallow segment followed by a deeper segment.

A far as "throwing away information", that is a problem with any simplistic tracking method, whether mental or table-based. For example, on a multilevel dive, tables throw away all depth information other than maximum depth. That makes tracking very poor. Conservative, but poor.

Average Depth tracking does a better job of tracking a multilevel dive than does a square profile table.

Compartments are only theoretical constructs to make the models work. You can establish any compartments that you want. The two-second compartment is fully saturated in twelve seconds, but will never be the controlling tissue since it offgasses faster then you can ascend. In a similarly absurd vein, the four-week tissue will not be the controlling tissue unless your on a very long habitat mission, and even then it will not be of any importance because it releases its gas too slowly to do any damage.

Agreed. I called attention to that point simply to show that BigDawg's understanding of decompression was somewhat askew when he states "the one tissue group that is controlling the dive (the one being closest to saturation)."
The correct statement is that the controlling compartment is the first one to reach the model's limits, rather than the first one to reach saturation. As a general rule of thumb, the controlling compartment halftime for a square profile dive will be in the vicinity of 2/3rd of the NDL for that depth.

The bottom line is that averaging seems to work across a limited suite of exposures rather by chance then by design. There is no theoretical basis for it and no controlled human trial for it.

That is like saying that there is no theoretical basis for dive computers and no controlled human trials them.. One can spend as long as you want coming up with different potential schemes to track decompression, and then "test" that method against an underlying decompression model. If you trust your decompression model, then you should trust that "test" of "average depth" or "120 rule" or "Charlie's Rule of 440" or whatever. (If you can't remember NDLs for various depths, just remember that (440 / Depth in Feet) ^ 2 = NDL. Doubles as a narcosis check when you calculate it at depth. :banana: )

Blind rejection of a tool is as dumb as blindly accepting it without recognizing the limitations. Look at the method. Try it for some typical dive profiles. Poke it to see where it falls apart.

 

[rjack321]

So UP, with about 1.5 more years of experience with "depth averaging" under your belt are you bent yet?

 

[Rick Murchison]

So UP, with about 1.5 more years of experience with "depth averaging" under your belt are you bent yet?

Pug's on sabbitical from SB, but I believe I can answer for him that he's quite well and safe in the envelope he uses for his dive planning and execution.
Rick

 

[SDAnderson]

The span of one's C.V. does not make one right or wrong. The extent of one's C.V. can, however, sometimes be use to add authority to an assertion and to squelch naysayers.

Fair point. Nonetheless, a CV (or just knowing something about the speaker) is a tried and true shortcut on the path to deciding whether or not the speaker knows *** or has merely had too much practice with a shovel in the barnyard.

BiggDawg is new to the board and he's challening some folks that think they know of what they speak. Based upon what he has said in this thread, he knows something about decompression but it also doesn't seem that really understands ratio deco. Who knows? Certainly nobody who only knows him from his six or seven posts on this board. Which brings up the point about his bona fides: maybe he's a truly knowledgeable diver with a lot to contribute; maybe he's a semi-knowledgeable troll looking to stir up the bottom. Some information about who he is and what he's done would go a long way towards answering those questions and towards establishing his thoughts and opinions as being worth listening to.

His choice but I tend to place new members, especially those with strong contrarian opinions, in the "troll" category until they establish themselves.

As to DOTF, I have only limited experience with it and certainly don't endorse it for any but the experienced, trained and brave. My dive profiles are pretty square so there hasn't been much need to really experiment with it but, to the extent that I've used it, it has worked well. It requires some learning and some thinking - one or both of which can be a problem for some divers. It isn't something I would ever teach to someone over the internet, which may support BD's position that it's too risky for most.

 

[lamont]

The problem is that he's not looking at the whole picture of how to do depth averaging, which usually goes beyond the simple averaging of depth and includes making sure that your profile starts deeper and pads it a little. Also, everyone that does depth averaging on recreational dives usually does it with 32% or 30/30 and once they hit the limit of the "deep portion" of the dive in the 90-110 ft range get off the bottom and above 50 feet where the NDLs get extremely long so they're not depth averaging over large depth differentials. In other words, I would never hammer on my compartments betwen 90-110 and then try to depth average that with part of the dive at 70, I'd just consider that "deep" time and add it directly onto the NDL for 100 fsw -- if you're not going up sufficiently to start decompressing at that point, you're just adding time on the bottom.

Now as a more concrete example, what do you get if you do 5 minutes at 110, followed by 10 minutes at 100, followed by 10 minutes at 90 on EAN32? And how does that compare to 25 minutes at 100?

And it would be interesting to actually implement and test zhl16b compartments for those two examples, but i currently have copious-free time issues so i'm not likely to get around to it...

 

[Thalassamania]

Sure you can simulate an exponential with a series of short linear segments of ever decreasing slope, but that’s not a tested and proven methodology. I could likely hip shoot most of my decompression needs without looking at a computer or a set of tables, just from experience. But I would no more do that than dive your series of linear approximations. Since it has worked for you so far, its clear that the math is accidentally close in the range that you’ve used it, you’ve been damn lucky that you’ve not stepped outside the range that it works for and you’re bright enough to make your errors on the favorable tail of the model. But frankly that does not stand the test of modern science, it’s just voodoo. One experimental subject, even with 100 trials is not the way its done.