Info The Rule of 120

[tursiops]

Yes, it is an obscure, narrow, off-the-wall, nearly-unheard-of example... that I do nearly every day of my life.

But it should not therefore be offered as a general approach....

 

[oya]

Everything in deco is approximation, isn't?

Joe Dituri tells a great parable:
If you ask a brand new open water diver what DCS is they'll probably say, "I dunno, it's got something to do with bubbles."
If you as an open water instructor, they'll rattle off the list of stay in NDLs, don't ascend to fast, don't get on a plane.
If you ask a tech diver they'll start talking at you about Gradient Factors.
If you ask a tech instructor they'll probably start bringing up things like compartments or bubble surface tensions.
But if you ask the top five people in the field of decompression physiology, the people who have committed their entire professional lives to this study what DCS is they'll probably say, "I dunno, it's got something to do with bubbles."

 

[jale]

So you are falling back on those COMEX tables, which do not apparently mean what you think they mean.

The great fall-back response. It is all approximate so let's just make stuff up and use it...
This is in the same category as "my opinion is just as valid as your opinion."
We are not talking about opinions.
We are talking about depth averaging as the input to a calculation of nitrogen uptake, which is exponentially related to depth, not linearly related to depth. Sure you may not be very wrong with your answer, but if that is good enough for you, go for it.

You are arguing with a narrow example that coincidentally works in a narrow set of conditions.

If I cannot trust COMEX, I don't know who I can trust about deco!
It is not a fall-back response is to say that there are a lot of little mnemotechnic tricks related to deco. Your 120 rules is just another one(a good one; we have a similar one in metric )

 

[oya]

But it should not therefore be offered as a general approach....

Random hypotheticals about a barely understood theory trump widely used practical experience.
Got it.

 

[tursiops]

Mathematical arguments I don't understand about a theory I don't understand trump my narrow practical experience.

Fixed that for you.

 

[tursiops]

we have a similar one in metric

What is it?

 

[oya]

Fixed that for you.

Oh, One day I’ll work up to having your vast well of experience.
In the meantime I’ll just have to go about diving and teaching at cave and technical levels nearly every day of my life, making something of a successful living running my own dive operation for years and years.
Hopefully that will eventually open up my narrow experience. Patiently. One dive at a time.
But, even you were a beginner once. Long before you knew everything about the decompression theory that even the world’s experts can’t quite grasp.

 

[tursiops]

decompression theory that even the world’s experts can’t quite grasp.

Not sure why you say this. There are various levels of understanding....and denying.

You seem to object to my statement that dive tables cannot be built -- in general -- based on average depth of a dive, because average depth is calculated with every depth having the same contribution to the average. But nitrogen uptake is not that way; greater ambient pressures have much more impact on driving the process than lesser ambient pressures, so that needs to be taken into account. Are you taking isse with that statement?

You seem to feel that because some details of decompression theory are not agreed, that the entire theory needs to be thrown out. That is a rather black-or-while view of things. Do you not use a laptop or desktop computer with some confidence? Do you understand all the details of how it and its software work? I suspect not, but you still use it.

I take no issue with your cave dives. My issue is using your cave dives to argue that deco theory is hogwash and nobody understands it. My issue is your argument that average depth is a perfectly reasonable way to calculate nitrogen uptake, when there is massive agreement among those who DO understand deco theory that it is not.

 

[oya]

Yeah man... I just dive a computer. Several of them.

I have a couple of little tricks that help me estimate what the computer is going to say along with enough experience to ballpark it. Including modified versions of the rule of 130 for planned decompression.
I've done dives with hours of deco. Many, many times. And some of the team clears sooner than me, sometimes I clear sooner than them.

I have a computer plumbed into my CCR that is monitoring my PO2 real time and an offboard one I just tell it I'm at 1.2 the whole time, sometimes they're off by a few minutes... but 5 minutes difference after 5 hours of deco doesn't bug me much.

A few minutes here and there. And any time I hear people get SUPER worked up about one or two minutes of deco... especially when that deco is less than, say 1/2 hour, or even no-deco limits, all I can think of is the 7 minute abs bit from Something About Mary.

I respect decompression theory and can give a better-than-average explanation of it at a theoretical level... but I admit I can't explain the actual math. Because it's pointless and contentious and constantly evolving. Because it's a working theory with various schools of thought, not established law.

I'm a dive instructor, not a mathematician, an algorithm programmer, or a physiologist.

But what I do know is that I've only ever been bent twice (both semi-deserved... one I started hiking gear back to the car as soon as I surfaced after over 6 hours of deco and got skin bends, the other I was doing long deco dives in frigid cold water for days and days... and had forgotten to back my computer off of my usual warm-water 45/85 to a little more conservative. My elbow hurt for a few hours. I breathed O2 and watched TV for an hour or so and skipped the next two days of diving.)

Other than that, following my looney dive planning that lacks any comprehension of anything neither I nor any of my hundreds of students have ever had a problem with DCS.

I take no issue that you wanna be a knowitall. I wanna be a billionaire. Neither of us is going to succeed.

 

[oya]

What is it?

It's a system of measurment that's used everywhere else on the planet but the US. But that's not important right now.