Deep safety stop question

[Charlie99]

Nice to see the "torture factor" factored in. Pleasure:effort ratio, I call it.

I loaded that phrase into my mental notepad also. It's a good shorthand description of why I'll dive air at Molokini, but always dive nitrox in SE Florida. At Molokini you have a nice reef on which you can slowly ascend over the entire span of the dive and start the "ascent" phase at 30' or less even on a dive with 130' max depth. Easy and pleasurable to do on air.

In SE Florida, once you lift of the relatively low relief (all near bottom depth) reef, it's just a boring series of midwater hangs. Nitrox both extends the NDL at the relatively flat bottom, and also speeds up offgassing during the ascent and stops.

 

[limeyx]

I read it all and then do my best to do what Charlie 99 says, because it is usually understandable.

I agree -- what Charlie posted is pretty good for dives down to 100 feet or so.
As long as you plan for emergencies (by reserving enough gas to do the extended stops, or agreeing to do less stops if someone goes OOA), then for recreational diving it works out pretty well.

Once you move out of that range, then other factors apply.

 

[Bowzer]

Wow, lots of interesting reading in this thread. Thanks to all.
One of my reason for starting this thread is that I have a significant difference in gas consumption in cold vs warm water. I normally dive Lake George NY, etc. (cold, dark) and have a SAC of .7 +/-. When in FL waters last summer my SAC went to .45 (cold, low viz and lots of weight/rubber makes quite a difference). The difference leads to a major change in how long I can go on a tank. I dive an Aries computer and use YMCA tables as backup. Even on EAN32, it is possible to get into N2 loading areas that I am uncomfortable with, especially on 3 tank dives with little sit out time. The air divers on the same dives had to be way past what I would consider safe. This got me looking into deco and just where the safe line is between the "recreational safety stop" and "deco". I picked up a copy of Wienke's book and while I can follow it, calculus was too many years back to do the math. I do plan my dives in a general way, but with rec diving the level of planning that tech divers go to isn't justified and often the boat will divert to another site.
What I would like to do is to find a simple, effective, generally agreed to, formula that I can put into table form and keep on my slate for recreational purposes. While every training agency has created recreational tables, most are rather shy on deeper "recreational" stops. I assume it is a liability thing, or else they are loath to admit a safety stop is a decompression stop.

 

[Dr Deco]

Hello Bowzer :

Testing

This idea of deep stops is complicated since, to my knowledge, there have not been released the results of any controlled, laboratory tests. [I understand that the US Navy is conducting some studies.] All advice is based on reports from the field; this is valid but without cross-over controls

Decompression or Not?

There are two aspects to surfacing from a dive.

[a] One involves the controlled release of dissolved nitrogen from “compartments.” This is the Haldane [and Buhlmann] method and expressly causes direct ascents to depths relatively near the surface. This will increase the off gassing gradient and results in the fastest removal of dissolved tissue nitrogen.

Deep stops are not concerned with rapid removal of dissolved nitrogen. They consider rather, controlling the size and growth of preformed micronuclei. Deep stops rely on surface tension (the Laplace pressure) to squeeze microbubbles and prevent their eventual growth. Bubbles must be kept small such that the microbubbles have an internal pressure (hydrostatic plus surface tension) greater than the pressure of the dissolved gases surrounding that particular microbubble.

It is this principle that prevents fizzing when the cap of a bottle of carbonated beverage is slowly unscrewed. When quickly unscrewed, the pop will effervesce violently. Clearly, the slow release of internal pressure I the bottle does not result in degassing the liquid in a few seconds.

Stops

As is true of every decompression procedure, it is to prevent excessive bubble development in the body. However, DCS is not caused by bubble formation/growth in one tissue. Bubbles can form in connective tissue, vasculature, or arterialize. These can lead to joint-pain [the bends], neurological problems, or stroke-like symptoms. What exactly are deep stops controlling? [Although this question can be asked of any stop.]

Nucleation

We have in addition the question of musculoskeletal activity and stress-assisted nucleation. The temporal proximity of physical activity of the first deep stop does not allow for much time for nuclei to resolve [dissolve]. This clouds the picture.

The stop depths and durations become empirical [as is most everything in decompression] and the data are limited.

Dr Deco :doctor:


The next class in Decompression Physiology for 2007 is August 18-19. :1book:
This class is at the USC campus in Los Angeles.
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[Bowzer]

Dr. Deco, thanks for the response. It would seem as I expected, that the notion of deep stops is correct but that the literature has not yet formulated generalized specific guidelines. i.e. within reason any deep stop is better than none.

Perhaps this belongs in a separate thread, but, what of nervous tissue... From all I have read recreational divers suffer from a far greater percentage of nerve hits than other types. As this tissue is reported to correlate with a 12.5 minute compartment and most rec tables are based on 5 and 10 minute compartments, which have larger M values. Doesn't this argue for some kind of modification (or is the problem that rec divers ignore tables and safety stops?)

Personally, I can attest that even the 20 foot safety stop is effective. Back in the 70s in college I used to dive. At the end of a day I was tired... Now, with safety stops I don't have the fatigue I used to have.

 

[Charlie99]

This idea of deep stops is complicated since, to my knowledge, there have not been released the results of any controlled, laboratory tests.

While not ideal, the DAN Europe tests on different ascent profiles from 25 meters are illuminating.

The 5th through 11th articles on the DAN Europe Medical Articles page are all about a series of tests on different ascent profiles from 82' 25minute + 82' 20 minute repetitive dives. A limited number of trials, not the profiles I would have chosen to test, and only done at one depth, but it is a set of data that is real.

-----------------------

A simple review of the effect of deep stops on the various compartments of multicompartment dissolved-gas-only, aka Haldane or neo-Haldanian or Buhlmann, algorithm is Erik Baker's Clearing Up The Confusion
About “Deep Stops”. While many may consider it to be a gross simplification of the problem, his article clearly compares what goes on in the various compartments on an ascent that goes directly to the 1st required decompression stop (or to the surface for an NDL dive) vs one that makes some short stops deeper.

It's all about balancing A) going shallower to offgass dissolved N2 vs B) staying deep to prevent bubble formation.

Eric Baker's graphs show that a direct ascent to the surface or first deco stop will drive the fastest compartments right to their limits. OTOH, judiciously adding in some relatively short deep stops will avoid driving the fastest compartments right up against their limits, while at the same time not excessively adding to the N2 loading of the slower compartments.

Simply by making the assumption that bubble vs no-bubble or DCS vs no DCS is a fuzzy and soft transition with respect to N2 loading (rather then the compartment M-values being a sharp DCS vs no-DCS dividing line) and acting appropriately comes up with more or less the same results that both anectdotal experience and more sophisticated dual phase models such as VPM come up with.

The crude but effective neo-Haldanian model, with the minor tweak of gradient factors, excels in terms of developing an understanding for the rationale behind deep stops, and for providing an intuitive feel for what a good ascent profile would be from any given dive.

 

[Cacia]

Going overboard and starting deep stops super early doesn't add conservatism or safety ---- it simply adds to your loading.

easy mistake to make too. I find I need to stick to a profile that is simple enough to be executed without my total undivided attention. There is a diminished return once it gets too complicated, even if the theory can be intellectually supported, imv.

Thanks guys for taking the time to write all that out, very clearly too.

I always assumed the "stroke like symptoms" were from gas emboli, but you are making it sound like it could be another compartment?

 

[Charlie99]

.. From all I have read recreational divers suffer from a far greater percentage of nerve hits than other types. As this tissue is reported to correlate with a 12.5 minute compartment and most rec tables are based on 5 and 10 minute compartments, which have larger M values. Doesn't this argue for some kind of modification (or is the problem that rec divers ignore tables and safety stops?)

Short answer: the recreational tables DO consider multiple compartments (including 5, 10, and 20 minute halftimes) when determining NDLs, and adding a 12.5 minute compartment would have no signficant effect on the tables.

------
Long winded explanation:

The repetitive dive calculations of most recreational tables are based solely upon the 60 minute compartment (PADI) or the 120 minute compartment (USN-derived tables such as NAUI, YMCA, SSI, etc.). All compartments are considered when setting the NDL limits (both USN and PADI/DSAT models have 5, 10, and 20 minute compartments as the fastest ones). For normal recreational dives, compartments much faster than 60 or 120 minutes limit the NDL. This is why, for example, that you hit NDL before reaching the highest letter group/repetitive pressure group. A rough rule of thumb is that for a square profile dive, the compartment that is hitting the limit first has a halftime about 1/2 to 2/3 of the NDL time.

As you noted, the compartment limits (M-values) decrease as the halftimes increase. If you run throught the math, you'd find that adding in a 12.5 minute compartment with the appropriate limit would not signicantly change the table NDLs --- as you plot NDL vs depth, the handoff of control of the NDL from one compartment to the next is fairly smooth and adding lots of intermediate halftime compartments doesn't change the results much. A result of the interplay between M-value limit vs compartment halftime relationship and the exponential loading curves is that a pretty good estimate of NDL over a wide range of depths is that the [square root of NDL] * depth = a constant. (Hempleman square root law).

As to whether or not divers are getting bent by ignoring tables and safety stops, my guess is that the more common real life problems are things like uncontrolled ascents and people unwittingly making ascents much faster than the recommended rates. Just the opposite of slowing the ascent by doing deep stops.

 

[Dr Deco]

Hello readers :


Nerve Hits and M-values

Recreational divers might be ascending too rapidly for short, deep dives. This question is under investigation by DAN. Thus, it might be advisable to reduce the allowed supersaturation for the fastest compartments. This was done for the PADI Recreational Dive Planner.

I suspect that one reason for the equal percentage of neuro DCS and pain-only DCS is that recreational divers ignore – or miss completely – minor joint pain that would be found in a laboratory setting.:huh:

Some Deco

As Dr Bill Hamilton is fond of saying, “It less important what kind of decompression you do, as long as you do some decompression.” There is a degree of “fuzziness” that can be explained, but it is difficult to do on a written forum. It is much easer with PowerPoint slides that move.

Neurological DCS

Spinal cord DCS appears to be autochthonous bubble formation, i.e., bubbles that form in the tissue and do not migrate.

DCS in the brain appears to be the result of gas bubbles in the venous circulation that migrate into the arterial circulation and embolized the brain tissue.


Dr Deco :doctor: The next class in Decompression Physiology for 2007 is August 18-19. :1book:
This class is at the USC campus in Los Angeles.
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[Bowzer]

I thought that 5 and 10 minute compartments were the primary ones that rec tables were based upon as these were the ones that were libel to reach high percentages of saturation during one tank dives and that the longer compartments were what came into play with double tanks, etc. One hour at depth saturating a 10 minute compartment, etc. That is why I was referring to the 12.5 minute compartment for nerve tissue as being outside the table design.

I have certainly seen my share of less then careful behavior by rec divers....

With the exception of Weinke and Diving Physiology in Plain English, most of my reading has been catch as catch can on the net. Guess I missed something. Can you provide me info on some references to read.

thanx