Shallow Decompression Dives

[Thalassamania]

But see, that's exactly my point. Teach a diver what he needs to know. There's no need to teach a diver in a metric country to do imperial math and understand how equipment in imperial countries are named.

Sure there is, if you are an imperialist.

 

[TSandM]

I don't have any problems with rules of thumb. I think Lamont's article on rock bottom has some rule of thumb gas reserves, and I know my husband has written a little piece for his students that describes what a rock bottom reserve is, and gives a table for various size tanks.

I do think you can teach OW students these things:

1. Maintain a minimum gas reserve. They can be given a table to tell them what that is for a given tank size.

2. Apportion usable gas according to dive conditions -- all available, halves, or thirds (not that new OW divers should ever be doing a dive which requires thirds).

There is very little math involved in doing this, especially if you use the table of rock bottom values. But it's a little more proactive than "back on the boat with 30 bar" -- after all, if you just dive until you get to 50 bar, what if you're a long ways from the boat at that point?

I don't think OW divers have to do their own calculations, but I do believe that the concept of a minimum gas reserve, and some guidelines as to what that IS for a given tank and depth of dive, should be included in OW education.

 

[ArcticDiver]

Stop kidding yourself. DCS just isn't talked about. People are getting bent left and right. 5 divers at Ginnie springs alone were bent last month, all "undeserved" hits according to whatever tool they were using for deco...and we're not even bringing up the "oh I'm not bent, I'll just go to bed or go have a beer" cases.

Look here where 43% of 237 cave divers reported they had been bent
Have you gotten bent? - Cave Diver's Forum

I was told during full cave point blank that I'd get bent eventually. Recently my dive buddy got bent a few weeks back on the same tables (2 minutes different from a previous profile). A 119min dive at Ginnie, about 85ft average, with 35/85GF tables with added time at deep stops, swimming up the spring run on o2 for cushion there, and a generous amount of time on the surface before exiting. Too many people are scared to talk about DCS in my opinion, and it gives divers a false sense of security.

First of all my comments were based on both DAN reports and research. These cover divers in general, not just a small sub-group. In fact the last DAN report I've seen says there were a couple hundred DCS cases out of all the thousands of dives reported to them. So, it seems there is evidence to question the accuracy of the anecdotal claim you post.

If, in fact, your small group has such a high rate of DCS as compared to the overall diving community it would seem to me that common sense would dictate a serious examination of the group's procedures and practice. Mind you, at this point I have no reason to believe the claim is accurate. But, if it were and the cave diving community was as concerned with safety as it loudly and frequently asserts it would be reasonable to think there would be a well publicized effort to change practices to eliminate the excessive DCS. That effort would certainly include research agencies of which DAN is one.

Lets face it, physics is physics. From a hyperbaric point of view water behaves the same way regardless of whether it is in the open or inside a cave or wreck. The same algorithms provide the same level of risk in all three environements. If one small group is actually experiencing a higher DCS rate than the general population the first thought is that the members of that group are not following accepted dive algorithms.

At this point I have seen no evidence to indicate anything other than that DCS is rare and that the best way to minimize DCS risk is to choose an accepted algorithm and follow it.

 

[Thalassamania]

The classic numbers are:

• commercial dives: 1 case of DCS per 1,000 dives.
• recreational dives: 1 case of DCS per 10,000 dives.
• scientific dives: 1 case of DCS per 100,000 dives.

 

[ucfdiver]

The classic numbers are:

• commercial dives: 1 case of DCS per 1,000 dives.
• recreational dives: 1 case of DCS per 10,000 dives.
• scientific dives: 1 case of DCS per 100,000 dives.

I'm guessing that those dives (the recreational portion of DAN's stats) are only saturating the fastest tissues (if any), so a safety stop is pretty effective, and the effects are mostly wiped clean by the next dive. With minimum deco and either a 30minute or 1hr surface interval, agencies like UTD are suggesting divers dive the EXACT profile over again.

Saying the risk between a saturation dive and non saturation dive is the same if you follow the same algorithm just doesn't make sense to me (not saying you-- referring to DAN lumping the statistics together). We're dealing with dives that are saturating slower tissues AND fast tissues vs dives saturating fast tissues only (if any). IMO DAN's statistics don't offer many conclusions for recreational technical dives.

To get a few quotes from Erik Baker's Paper-
http://www.ddplan.com/reference/mvalues.pdf

Even though good data is obtained about the approximate threshold for symptoms of decompression sickness (M-values), this process cannot accurately predict or guarantee an absolute threshold for everyone.

Individual susceptibility can vary on a daily basis as well.

The experience of diving medicine has shown that the established limits (M-values) are sometimes inadequate. The degree of inadequacy is seen to vary with the individual and the situation. Acccordingly, it may be more appropriate to describe an M-value as "a solid line drawn through a
fuzzy, gray area" (see Figure 2). The reasons for this lack of definitude involve complex human physiology, variations among individuals, and predisposing factors for decompression sickness.

Also, if this were so cut and dry, why does the V-Planner website state-

[SIZE=-1]********* WARNING / DISCLAIMER *********[/SIZE] [SIZE=-2] The author and HHS Software Corp do not warrant that this program accurately reflects the Varying Permeability Model algorithms, or that it won't get you bent or dead, or that it will produce safe, reliable results. This dive schedule is experimental and you use it at your own risk. Diving in general is fraught with risk, and decompression diving adds significantly more risk. Deep diving utilizing multiple gasses, including helium, or rebreathers is about as risky as it gets. V-Planner and the decompression schedules it produces are tools for experienced mixed-gas decompression divers ONLY. If you have not been properly trained in mixed-gas decompression diving by an internationally recognized technical certification agency and/or don't have a firm handle on decompression planning and mixed-gas diving, then DO NOT USE THIS SOFTWARE.[/SIZE]

Decoplanner website-

Divers using this program should be aware that decompression is not an exact science and what this program does is attempt to draw a line through this scientific uncertainty whilst providing education such that the user can better understand the results being produced.

I think deco algorithms are reasonably safe, but not the end all.

 

[Kevrumbo]

What's going on then, with the Ginnie Springs Cave Profile that's making saturated cave divers susceptible to DCS type I? Is the flow that bad where you have to hang onto a rock, or continuously fin in place to keep your deep stop depth?

 

[ArcticDiver]

...

I think deco algorithms are reasonably safe, but not the end all.

I think this statement is very much in line with what I and others have posted. Since no one knows what causes DCS in the human body all dive algorithms are, at their core, vehicles for expressing probablilities. In otherwords quantified risk management.

One can use complicated and extensive mathematical expression or concise language to describe any particular DCS theory. But, in the end, it is all theory that seems to work most of the time. But, there has been enough experience with this theory that if any group has results that fall outside the norm the first place to look is the group's practices, not the algorithm.

Oh yes, the disclaimers are there for legal reasons to emphasize that since no one has The Answer the algorithms are just that author's best guess at what works to minimize DCS risk.

 

[LiteHedded]

What's going on then, with the Ginnie Springs Cave Profile that's making saturated cave divers susceptible to DCS type I? Is the flow that bad where you have to hang onto a rock, or continuously fin in place to keep your deep stop depth?

I have some theories...
there are a couple unique features about a typical ginnie profile. not to mention people are working hard (on swim dives)

 

[PfcAJ]

Ginnie is the BENDER. How many DCI cases this year? 5? 6? More? There appears to be far more incidents than other caves of similar depths, but other factors could be influencing these numbers.

However, like LiteHeadded said, there are some real issues with the cave profile/ way people dive it that could contribute to the prevalence of DCI.

 

[ucfdiver]

Ginnie is the BENDER. How many DCI cases this year? 5? 6? More? There appears to be far more incidents than other caves of similar depths, but other factors could be influencing these numbers.

However, like LiteHeadded said, there are some real issues with the cave profile/ way people dive it that could contribute to the prevalence of DCI.

5 last month that I can name.

4 other cave diving related DCI last month according to the chamber operator at Shands Hospital when we were there. A few more OW incidents were sent there as well. Enough people were bent that they were running out of chamber operators.

As you and I have discussed, with as much as the models vary (VPM, Buhl with GF, Computers that "bend and mend", etc), I wonder how much deco really matters in the scheme of things. I know 3 of the divers bent last month think dehydration was the reason.

One of the incidents was certainly a saturation dive, however the diver did almost 20minutes of extra deco over the entire deco schedule according to our conversation we had, and STILL got bent. This is one who thinks dehydration is it.

I've been starting my deco now at 75% depth with short pauses every 10ft, gradually getting longer to until I get to the 30 or 40ft stop where the tables begin, with my end goal being smoothing my ascent compared to the tables...fatigue post-dive seems to be lower, but I'm curious to see if it's a placebo effect, or if I continue to notice it over time.

GUE's decompression sickness dvd The Mysterious Malady: Toward an Understanding of Decompression Injuries | Global Underwater Explorers interviews several experts about decompression. Hearing people who are doing dives that last over 20 hours, at depths of 300ft admit that it's all a mystery is a real eye opener.

Ginnie is also dove a LOT. It's never blown out (well, almost never)...I know I did 3 dives where I hit deco there this weekend, and several others do the same. Lots of people head there when they get off work due to the 24/7 access. I dive it every Friday that I go up for the weekend to cave dive, because it's the only thing open. I'm guessing this distorts the numbers a little, but I'm still very anxious to see what conversation comes up when you post your theory.