Place of dive tables in modern diving (Split from the basic thread)

[boulderjohn]

I never heard of the rule of 120 until I had completed well over 1,000 dives. I didn't know what it was when I first heard it. I still don't understand how knowing it would have affected my diving.

 

[Kevrumbo]

While the rest us, following tables or PDCs get bent so rarely as to DAN referring to these incidents as being mere "noise", it's not so with ratio deco divers. They get bent to the point of it being alarm. . .

To put this in perspective, I know of only 4 people who got bent diving within their NDLs. All of them were on Ratio Deco. All of them. None of them blame ratio deco... they blame themselves for not averaging correctly, making mental mistakes and what not. This is why PDCs are so good. They never get distracted from their one job of making sure you're not a DCS statistic. Within NDLs, they are amazingly effective. Get into deco and especially multiple days of deco and then things start to slide a bit. But not nearly as bad as Ratio Deco. I love you Kev, but that RD crap scares the bejesus out of me. I won't go there. I won't.

Oh hey Pete! I luv ya too!

What scaring the bejesus out of me, are unknown latent long term pathological effects of years of loading/unloading inert-gas-decompression-stress on my body as I get older (i.e. dysbaric osteonecrosis for example). Kind of an analogous concern akin to the syndrome of CTE, and other physically debilitating disorders & diseases in retired NFL veterans.

All 8 cases I know of personally were on Ratio Deco. Those cases lead me to point out two interesting uses of logic related to the explanations of them.

1. The reason to use Ratio Deco, we were told, was because it is possible that the computer could make a mistake,so it is better to trust "the computer between the ears." In several of the cases I know, divers made mistakes, mistakes that were revealed because they had a computer in gauge mode for a bottom timer and were able to check the log for the dive and find what they actually did rather than what they thought they did. When people make mistakes calculating average depths, miscount their deco times, etc., then it is their fault, not the fault of Ratio Deco. So apparently Ratio Deco is better than a computer because the human brain never makes mistakes, but when it does make mistakes, the brain is suddenly no longer an integral part of Ratio Deco.
2. All the cases I know of occurred at altitude--not quite 5,000 feet. We were told not to adjust Ratio Deco in any way for altitude, because altitude does not matter for decompression. Since that contradicts what everyone else believes, I asked how they knew it was safe to use RD at altitude without adjustment. I was told two reasons: 1) Andrew dives at Lake Tahoe without adjusting, and he is fine. 2) No one has ever been bent at altitude using RD. I responded that all the people in our group who had gotten bent were using RD at altitude. I was told those did not count, because there was some other reason for their being bent. I asked what those reasons were. They didn't know--maybe PFOs, maybe something else. How did they know it was not Ratio Deco? Because no one gets bent diving at altitude using RD, so therefore it had to be something else.

I find it interesting that when some people follow procedures that are different from everyone else and unsupported by any scientific studies, they are automatically assumed by some to be superior to everyone else. I was recently part of a FaceBook discussion in which PADI was mocked because it was still teaching traditional decompression approaches and not teaching this "more sophisticated" approach. When I pointed out that there was no science supporting it, I was removed from the discussion.

Sounds like one of the worst cases of normalisation of deviance that I have heard off. Pick any thing as the cause other than the one core idea. The reason for the belief that RD is safe to use at altitude might be something as simple as Andrew is one of those individuals who has a higher resistance to being bent, has developed some resistance to DCS or manages to stay just on the subclinical side of bent. Picking a methodology on the basis in item 2 seems like a huge gamble - you are going on the basis of a sample size of one who no-one verifiably knows how close he is to bent he actually is. This is especially true if (and I have no reason to doubt your views) any cases of DCS that do occur are effectively swept under the carpet due to "human error".

Surely the best situation is to do as much as humanly possible to minimise the chance and effect of human error. Pick an algorithm that has scientific basis (using informed sources to make your choice) and dive it conservatively (adjusting depending on physical factors such as age, fitness, altitude,cold, hydration as required).

Me, personally, I will stick to using a PDC and NDL diving. If my computer dies on me I will call the dive and ascend at a safe rate with my buddy (as indicated by his PDC) or, if he is somehow not available, at the rate of my bubbles and do a stop as near to 5m as I can for as long as I can. Then, depending on if I have a spare, I will probably sit a couple of dives until I am "clear" or carry on diving if I can work out my profile and residual gas (based on my buddies profile with a safety margin - I know where I have been in relation to him during the dive so it should be possible to get a reasonable "guesstimate").

If your intermediate/slow tissues have high surfacing supersaturation tensions along with:

• A sequence of consecutive dive days with multiple-dives-per-day further loading those intermediate/slow tissues with residual inert gas;

And/or:

• Ascending at altitude where there is a diminished ambient surface pressure.

You will increase your chances of a type I DCS hit.
Worst case, if you're unlucky enough to have a PFO or other major cardio/pulmonary venous to arterial shunt, your chances of a type Ii DCS or AGE pathology will increase. At the very least, you will have a measurable increase in the level of decompression stress as detected by post-dive Doppler VGE scores. . .

You compensate by either:

• Taking a day-off from diving after three or four consecutive dive days;
• Pad your Oxygen schedule with extra stop time if doing mandatory staged decompression;
• Account for altitude with the appropriate computer setting or table cross correction;

And finally consider an elective PFO screening test.

 

[boulderjohn]

If your intermediate/slow tissues have high surfacing supersaturation tensions along with:

• A sequence of consecutive dive days with multiple-dives-per-day further loading those intermediate/slow tissues with residual inert gas;

And/or:

• Ascending at altitude where there is a diminished ambient surface pressure.

You will increase your chances of a type I DCS hit.
Worst case, if you're unlucky enough to have a PFO or other major cardio/pulmonary venous to arterial shunt, your chances of a type Ii DCS or AGE pathology will increase.

You compensate by either:

• Taking a day-off from diving after three or four consecutive dive days;
• Pad your Oxygen schedule with extra stop time if doing mandatory staged decompression;
• Account for altitude with the appropriate computer setting or table cross correction;

I am afraid there is a whole more to diving at altitude than this. I just completed a detailed explanation of issues related to diving at altitude. It is sitting in the staff backroom awaiting the time when ScubaBoard unveils coming forum for dive knowledge. You do not even mention the 2-3 top reasons that altitude matters in diving.

My theory is that, like many controversies in diving, it all goes back to George Irvine III. His statements about why altitude doesn't matter are still posted on some DIR sites, and he was just as ill-informed and wrong then as now when people still follow his beliefs. His understanding o altitude issues was limited to what you wrote above, and he dismissed it for those reasons.

 

[CT-Rich]

I’ve used the rule of 120 since I was certified in 1980. I had to look up what it was when mentioned here... I had never heard the term before. At least never remembered hearing it.

 

[Kevrumbo]

If your intermediate/slow tissues have high surfacing supersaturation tensions along with:

• A sequence of consecutive dive days with multiple-dives-per-day further loading those intermediate/slow tissues with residual inert gas;

And/or:

• Ascending at altitude where there is a diminished ambient surface pressure.

You will increase your chances of a type I DCS hit.
Worst case, if you're unlucky enough to have a PFO or other major cardio/pulmonary venous to arterial shunt, your chances of a type Ii DCS or AGE pathology will increase. At the very least, you will have a measurable increase in the level of decompression stress as detected by post-dive Doppler VGE scores. . .

You compensate by either:

• Taking a day-off from diving after three or four consecutive dive days;
• Pad your Oxygen schedule with extra stop time if doing mandatory staged decompression;
• Account for altitude with the appropriate computer setting or table cross correction;

And finally consider an elective PFO screening test.

I am afraid there is a whole more to diving at altitude than this. I just completed a detailed explanation of issues related to diving at altitude. It is sitting in the staff backroom awaiting the time when ScubaBoard unveils coming forum for dive knowledge. You do not even mention the 2-3 top reasons that altitude matters in diving.

My theory is that, like many controversies in diving, it all goes back to George Irvine III. His statements about why altitude doesn't matter are still posted on some DIR sites, and he was just as ill-informed and wrong then as now when people still follow his beliefs. His understanding of altitude issues was limited to what you wrote above, and he dismissed it for those reasons.

No John, you're mistakened:

What George Irvine quoted is not at all similar to what I posted above. . .

 

[boulderjohn]

No John, you're mistakened:
View attachment 447367
What George Irvine quoted is not at all similar to what I posted above. . .

Sure it is. You both think the only big deal is ascending to altitude during the dive. Neither of you mention the real reason diving at altitude is a concern--the difference in the pressure gradient upon ascent at the end of the dive.

 

[Kevrumbo]

If your intermediate/slow tissues have high surfacing supersaturation tensions along with:

• A sequence of consecutive dive days with multiple-dives-per-day further loading those intermediate/slow tissues with residual inert gas;

And/or:

• Ascending at altitude where there is a diminished ambient surface pressure.

You will increase your chances of a type I DCS hit.
Worst case, if you're unlucky enough to have a PFO or other major cardio/pulmonary venous to arterial shunt, your chances of a type Ii DCS or AGE pathology will increase. At the very least, you will have a measurable increase in the level of decompression stress as detected by post-dive Doppler VGE scores. . .

You compensate by either:

• Taking a day-off from diving after three or four consecutive dive days;
• Pad your Oxygen schedule with extra stop time if doing mandatory staged decompression;
• Account for altitude with the appropriate computer setting or table cross correction;

And finally consider an elective PFO screening test.

Sure it is. You both think the only big deal is ascending to altitude during the dive. Neither of you mention the real reason diving at altitude is a concern--the difference in the pressure gradient upon ascent at the end of the dive.

No John. . .

It's the deep stops issue and the NEDU Study once again that George wasn't aware of (or apparently along with AG was physically immune to its effects), and the consequence of loading those intermediate/slow tissues to supersaturation upon surfacing and later post-dive -->further exacerbated by the lower ambient surface pressure at altitude.

 

[The Chairman]

(or apparently along with AG was physically immune to its effects)

That's the problem with RD. It's based on conjecture and two guys not getting bent: not on science.

 

[Kevrumbo]

That's the problem with RD. It's based on conjecture and two guys not getting bent: not on science.

Actually Pete, it was anecdotally based on these two anatomical cyborgs designated as "AG" and "GI3" . . . maybe even a third called "JJ" as well.

Seriously though, I wonder what kind of problems they might be having now as they're getting older. . .

 

[The Chairman]

Seriously though, I wonder what kind of problems they might be having now as they're getting older. . .

I'm closing on 49 years of diving in June. I never did the dives they did: I never wanted to. Until the turn of the century, I didn't even use tables. But then, I owned a single 72 and had to get it filled between dives. As best I can tell, I've never been bent, had any type of DCS or other diving injury. I hope to make it 50 years without an incident. Being in the automotive industry for 30 years did more harm to my body than diving ever did. Getting old is not for wimps. I do have joint issues, but then so did my mom.