Info The Rule of 120

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oya

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Dan

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This exception… a solid quarter of the population have a PFO. Which means one in four divers have one. A minority… sure… but a pretty strong one.

The vast majority of them don’t cause problems. Staynong class to No deco limits obviously helps. Hydration. Not jumping right into the hot tub after a dive. Lots of stuff helps.

But the point cuz was trying to make is that just because there are no symptoms doesn’t mean DCS doesn’t exist. It’s simply sun-clinical.

The point I was trying to make is that when people are afraid they’re going to be treated like idiots who obviously messed up to deserve to get DCS, they’re less likely to come forward to talk about symptoms and seek treatment. These are usually people who already scared and in denial. Stigmatizing them on top of that is laughing at someone who just broke their ankle taking a bad stair.

I personally know no fewer than four people who have had PFO. I know a couple of others who just started getting random DCS hits even during no-deco dives but tested negative for a PFO. They just… keep getting bent. One of them a well-known and deservedly well-regarded instructor who had to walk away from their career.

In any case: back to the whole question…
I always dive with two computers. I don’t surface until they’re both clear. It seems the point most people are missing is that I use the 130 thing as a subsidiary tool during dives, not in place of dive planning. And while I’ve tried to explain why it is quite useful to the type of diving I’m doing, I’m either not explaining it properly, or folks are having a hard time understanding the diving I’m doing.

But the thing I’m finding particularly perplexing is that I’ve said over and over and over that while I use the 130 the way I use it, average depth and all… it always comes out to, if not the exact same as my computer, then within a minute or two.

So for all the fact that I am, apparently, a complete moron who doesn’t understand anything about decompression theory… every single time I compare the results of the very simple subtraction I’ve done in my head to the final dive time of the very complex calculations my computer has made using 10second sampling of the actual ambient pressure over the entire dive…

It winds up being the same number.
What GF do you set your dive computer at?
 

EFX

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I answered your question.
The answer was “right here in this thread.”

And now on to my question:
Using average demonstrably works even though you say it doesn’t. Explain.
Based on @rddvet and the above response I feel I might have been misunderstood. I am not criticizing anyone for getting bent. I was simply pointing out that saying something negative regarding something you are trying to persuade someone about works against you. Paraphrasing @oya, "I believe using average depth and modified rule of 130 works and, oh BTW, I've been bent twice." Imagine a one armed man standing by an open gate to an alligator pond and he says, "Go on in and pet the alligators. It's safe." How much confidence does that inspire for anyone, except a suicidal daredevil, to go in!? Now, to be fair to the one armed man (and to @oya by comparison) the one armed man may have have lost his arm for other reasons other than curious and/or hungry alligators. The alligators may very well have been friendly and docile. But, perception is reality and the juxtaposition of simplified deco procedures with getting bent twice did not sit well with me.
 

oya

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I tried to be pretty clear that the two times I’ve experienced any issues were WAAAAAAAAAAAAAY outside the scope of the rule of 130 as I use it to ballpark what my computer is going to tell me in an hour or so.

The one time was a 90 minute dive to about 300’
The other was after a week of two dives to 150-200’ per day for days.
There was no averaging of anything. And 130 never came up. There was planning on the computer and then dive computers guiding.

The reason I brought them up was specifically as a point that despite my predictive tool being, apparently, misinformed, stupid, ignorant, horrible, and idiotic… it works. All the time. And has never, ever, once caused a problem.

(And, to try and clear up any misconceptions… again… I do NOT actually plan dives this way. I simply use it to predict why my computer… which I dive… because I own dive computers… like divers do… is going to say. If my computer says, “Stay here,” I am going to stay here. If there is a difference between what I expected the computer to say and what the computer says, I do what the computer says. It is just rare that there is a difference. But here’s the messed up thing… both times I’ve gotten any symptoms… it was after my computer had cleared. Chilling, huh?)
 

oya

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Also: by the way…

Way to keep the stigma of DCS alive.

“He probly did something stupid.”
 

oya

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What GF do you set your dive computer at?
In warm water: 45/85
In cold or knowing I’ll be working hard: 35/75
 
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BlueTrin

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It does not take a long Internet search to see that the picture is routinely believed to depict the use of inflation to assist in swimming, something that was apparently a reasonably common practice in many parts of the ancient world.
Though I heard that his pops told him how, in his time, people used to swim without a buoyancy device because they had better training in his time which translated in superior in-water skills. :)

I'm not a tech diver, but I am a bit of an algo nerd, and I think a lot of this can be pretty easily visualized and demonstrated.
There's an open source tool called "DIY ZHL" that measures n2 saturation in the 16 tissue compartments.

Using a 40 minute dive time, one with constant values from surface to final descent depth of 90 feet (left) and one using the average depth (which came out to 47ft) of the previous profile (right), you can see that the averaging slightly overestimates N2 loading.

View attachment 731513

However, if you then take a profile that aims to spend more time at max depth (100ft, average of 72ft), you can see the averaging slightly underestimate N2 loading compared to constant polling.

View attachment 731514

Obviously, using max depth on a non-square profile is going to give you a much reduced overall dive time.

The value of the computer though is that it is not depending on average depth nor max depth - it is constantly reevaluating on the fly, to create the most accurate value according to the algo in use, at that moment.


If youre familiar with Jupyter Notebooks, you can play around with it:

This is only measuring N2 loading, not NDL or deco time, but it's closely related.
I think the "rule of thumb" worked fairly well as far as providing a ballpark idea. There's definitely variation, but there's not any massive difference. The only caveat is, these are just two profiles - the variation will rise the further the profile gets from a square. I

@dmaziuk
 

boulderjohn

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Though I heard that his pops told him how, in his time, people used to swim without a buoyancy device because they had better training in his time which translated in superior in-water skills. :)
Whether that was true or not, that certainly would have been the overwhelming response if they had had social media at the time.
 
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