Disturbing 'advice' from DAN in Alert Diver

[ianr33]

A computer will probably be quite happy with a 100' dive followed by surfacing to change tanks and a continued dive to some depth, perhaps even including 100' At least the GEO doesn't seem too concerned about it, other than the usual nitrogen loading.

Agreed,most computers are simple "nitrogen logging devices" They could not care less if you see sawed 50 times between 100 feet and 10 feet during a dive ,but you would most likely get bent even though the computer would still be in the green.
Its up to the diver not to do stupid stuff.

Having said that I have noticed that my Aeris computer gets massively conservative on the odd occasion that I have done an NDL dive followed by a deco dive an hour later. It obviously has some algorithm to try and warn divers off from doing this but what that algorithm might be is anybodys guess.

 

[Blackwood]

First, I hope your wife escaped without issues. Secondly, it is a real eye-opener that a single bounce like that generated a type II hit. Instructors doing multiple ESA's are indeed at risk.

Thanks. Yah, she's fine. A couple of chamber rides did the trick. It was a few years ago. She no longer dives.

 

[Barracuda2]

Man, after having read all these posts, especially the technical ones on bubbles, lungs, and arteries, I wonder how I'm still alive and didn't cash in back in the early 60's. Never heard of a safety stop; never did safety stops, and lots of resurfacing and then heading back to depth, up to 3 or four times on one dive, (looking for the boat, or shore marker, or to verbally communicate with my buddy, etc.) This also included repetitive diving and no computers; just the tables. I do safety stops now, and the only time I'll surface during a dive is when I have to. So, bottom line, are safety stops really that important or just a "it can't hurt" idea?

 

[lowviz]

... So, bottom line, are safety stops really that important or just a "it can't hurt" idea?

You off-gass most rapidly when you are breathing a gas that presents a much lower partial pressure of the inert gas you are trying to get rid of. That is why deco divers often breathe pure oxygen at its maximum operating depth. The inert gas sees a partial pressure of zero surrounding the lung capillaries and floods out of the blood resulting in much shorter hang times.

Safety stops are usually at 15 to 20 feet. They are this shallow because the reduced water pressure reduces the effective 'concentration' of inert gas that you are breathing, even without switching breathing gasses. The reduced 'concentration' increases the differential concentration between the inert gas in the blood and the inert gas in the lungs. It is a non-equilibrium state where the inert gas in the blood efficiently leaves the blood for the gas in the lungs to re-establish eqilibrium.

Now to your question,
You can easily come up with a dive profile that will kill you. You can easily come up with one that won't. Somewhere in between, near the edges for your computer or tables, statistics presents itself. Some people will get hurt, others won't. So we place trust in the profiles and take comfort in an 'undeserved hit' when we are doing everything right and still get hurt. This thread is about a nasty dive profile that can rapidly put you into harm's way. It can easily happen to anyone. The practice of doing safety stops also trains you to control your ascent and this helps greatly in avoiding this issue. Your question seems to speak to being over-cautious. One can keep relaxing one's discipline until one gets seriously hurt.

 

[TBURKE]

, are safety stops really that important or just a "it can't hurt" idea?[/QUOTE]

We would surface, swim back to the boat, then go down the anchor line to do our deco.
We extended our shallow stops because we felt "more was better". It also made us feel less tired and less fizzy.
It's amazing we survived.:shocked2:

BTW does the Navy still pull their divers out of the water to strip their gear and put them in a chamber to decompress?

 

[Dan Weidert]

Dan,

I can understand why you are interested in what might happen to the gas in your blood if you should lose control of your buoyancy and end up unintentionally on the surface.

Just kidding!!

Rick,
You're just sore 'cause I let you die
Dan

 

[Big Toes]

Um, this pretty much looked like a pretty simple post from the OP but now looks like it's gotten to be a question for a diving doc to answer. so maybe some insight...

as an ex-USN diver/chamber supervisor/spec ops guy i can tell you that any shallow omitted-d done in a short timeframe usually requires the diver get back to depth (this absent a hostile environment i.e. minefield, etc). Depth and stop times change depending on the depth left from and the time on the surface, and your computer usually won't figure this out for you.

Now deep stuff is another story; an asymptomatic diver might be able to get back to that stop and complete it along with his/her other stops with needed adjustments (not a good idea if you've got an available chamber), but a symptomatic diver is not going to want to do that as it's far quicker and safer to get chamber evacuation. the general rule is that if you're within 24 hours of a chamber don't put them back in the water. and you're almost ALWAYS going to be within 24 hours.

(NOTE: NOT a suggestion to throw a deep omitted-d diver back to depth! even if he's asymptomatic! if you have an available chamber that's where he needs to go!)

now as i'm getting a bit older i usually consider my 15' stop for :05 mandatory So if i had to escort a buddy to the surface for whatever reason, i'd certainly return to the safety stop depth, add a minute or two if surface time was <:01, and finish out the stop.

so while i'm not a diving doc and can't argue with some of the issues that have been made here as some of you sound much smarter than me, i do know what works and what doesn't. big gas bubbles coming out of solution on the surface=bad; smaller bubbles coming out of solution at a shallow depth=good! Almost 30 years of diving and not bent/embolised yet despite doing some really stupid things, mandatory or not...

These theories have worked for many years and many people; hope they help. Most of you guys have some very complex theories about this topic and i was hoping that this might help the average guy/gal understand what to do with something like this.

 

[lowviz]

Depth and stop times change depending on the depth left from and the time on the surface, and your computer usually won't figure this out for you.

Agreed.

So it seems that if you blew an ascent and you lungs are now actively scavenging bubbles you'll likely be fine if you remain on the surface. If you descend, however, you promoted your old safety stop to a mandatory deco stop whose duration won't be found on any computer or table.

How about this:
Bad ascent in rough seas, surface swim to equipment line, descend 20', drop gear, and re-ascend. This behavior [that I'm guilty of] might need some re-thinking.

 

[Blackwood]

as an ex-USN diver/chamber supervisor/spec ops guy i can tell you that any shallow omitted-d done in a short timeframe usually requires the diver get back to depth (this absent a hostile environment i.e. minefield, etc). Depth and stop times change depending on the depth left from and the time on the surface, and your computer usually won't figure this out for you.

Yah. I have three or four different omitted-deco procedures that I'm happy to discuss, but as you noted it's sorta... beyond the scope of this (Basic Scuba) thread.

While I'm not opposed to threads finding their own path ("you must keep on topic" suppresses some of the value of message boards, in my opinion), I will re-iterate the point I wanted to make:

It is not a sure thing that re-descending to perform a safety stop won't cause any problems. Since it won't solve any problems, why do it?

 

[wve]

Re-descending after a dive during which bubbling has occurred ... introduces the potential for those bubbles to be compressed and passed from the venous side over to the arterial side in just enough time to re-ascend, turning a benign bubble into a potential type II neurological hit.

Let me stipulate that I agree with Blackwood's warning that we don't really know whether re-descending is helpful or harmful. Just because people get away with it may only prove that some people are lucky. But ALL of this stuff is only theory combined with experience -- not hard science.

So I am not disputing the claim about "bubble pumping." I am asking: Can this really happen? If so, how does it happen in the absence of a shunt, such as a patent foramen ovale?

With respect to re-descending: Re-descending has been called "a poor man's recompression chamber." Well, like the rest of life, you may get what you pay for. While the theory is fine, in practice you are more likely to run out of air or run out of time long before those bubbles go back into solution. And the conditions are a whole lot less controlled than in a chamber. If the worry is that strong, why risk it?

"Well," one might reply, "we aren't talking about treating DCS, but preventing it." Okay, so re-descending is not to try to reverse what has already happened, but to try to lower the risk by decreasing the nitrogen gradient, thereby slowing the dissolution of nitrogen from the tissues, and hopefully keeping an asymptomatic condition from turning into a symptomatic one.

But now we have raised the question: If you re-descend, do you now trade one risk for another? What happens to those bubbles which have already formed? Do they compress? Do they break up into smaller bubbles? Can they really cross the pulmonary circulation to enter the left side of the heart? Can they really cause a stroke?

I don't know the answer, but Gene Hobbs might.

In the meantime, if a dive profile seriously leaves one at the surface worrying whether s/he is going to get bent, why not just get on some oxygen and call it a day?