Info The Rule of 120

[tursiops]

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

He said there is an equivalent Rule of 120 for the metric system. I asked what was that Rule, but didn't say Rule. I'm sorry that confused you.

 

[oya]

You couldn't even tee me up with "Surely that confused you?"

 

[rddvet]

I ripped it off of GUE. I don't know (or really care that much) what GF they used to figure it. I think, maybe 30/80?
Using average depth as the depth vector it's agreed with my computers within a minute or two for hundreds of dives. I use a 45/85 on my wrist.

It's based off of 20/85 pretty sure

I stopped reading here. Average depth is not a valid construct.
Nitrogen uptake and off-gassing does not depend upon average depth.

Average depth can definitely be used. I will disagree with you, but probably won't say much more than that because history shows me arguing with you is a headache.

 

[rddvet]

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.

You may want to pull in the reins a little. His avatar clearly states he's a Master Instructor. Tough to argue with that.

 

[tursiops]

Average depth can definitely be used.

It would helpful if those arguing for average depth could say why it works....give some information rather than making assertions. I suspect part of the problem. Is what is meant by average depth, for example some say it means the average depth of only the deepest portion of the dive. Is that what you mean?

 

[oya]

You may want to pull in the reins a little. His avatar clearly states he's a Master Instructor. Tough to argue with that.

Lamentably, I am only a lowly normal instructor.

One day, though… as long as I can dare to dream.

 

[Manatee Diver]

I use the rule of 130 the same way @oya does. As a ballpark figure to find out if I need to plan for a staged deco dive or not. In water I listen to my computers unless they are diving me something weird.

 

[inquis]

@oya - I'm trying to understand how you would use such a rule in a cave scenario. Looking at an equivalent "rule" for GF x/80 on EAN32 (i.e., where I have data), I get a Rule of 116 (which is the minimum sum of various NDL times + depth).

So, I'm looking back at one of my dives, and I'm 20 mins in with Davg = 52 ft. As I understand what you're relating, that means I have a turn time of (116-52)/2 = 32 mins. Cool, so I have a bit longer. I do the math again at 24 mins/58 ft avg, yielding turn at 29 mins, so a bit longer. I do the math again at 28 mins/63 ft avg for a turn time of 27 mins. Crap, I'm past the turn time, so I believe you are saying the deco won't clear on the way out.

Am I understanding your logic correctly? Do you really do this math repeatedly as you go along? (or some variant, like 2*T+Davg < 116, since it's probably easier to add)

Meanwhile, my computer says I have another 5 minutes of NDL at that same point. On this particular dive, I turned at 30 mins/65 ft avg (NDL=3 min left), which is way past the Rule-based turn time at that point (116-65)/2 = 26 minutes. And yet, the deco cleared on the way out.

I know the Rule of XX works for a square profile/max depth and is always conservative. However, in a cave scenario when average depth is used, if it ALWAYS resulted in a conservative indicator (as in this example), that would be very interesting.

(This is similar to how I understand that Ratio Deco was developed -- noticing patterns for which conservative rules of thumb could be easily memorized. However, as is seen with RD, there are limits to the applicability of a given pattern, so I'm wondering just where a Rule Of XX/Cave breaks down.)

 

[scubadada]

All these rules of thumb are no longer necessary and no longer needed. I would not use any of them. Use the NDL planner on your computer, for your first dive and any subsequent dives. This is what you want to do.

 

[johndiver999]

@oya -

I know the Rule of XX works for a square profile/max depth and is always conservative. However, in a cave scenario when average depth is used, if it ALWAYS resulted in a conservative indicator (as in this example), that would be very interesting.

I'm not sure why I keep seeing that the rule of 120 is conservative? If you do a square profile, descend fast and do 60 ft for 60 minutes on air for a dive... you are definitely not "safe from bubbles".. In fact you would be well into deco on any computer I use.

The 80-ft/40 min and 90/30 were also pretty aggressive. Of course people got away with it and probably a lot of that had to do with being a little shallower than the square profile,

The 120 rule served a narrow purpose of providing a sanity check and a memory crutch to just make sure you had some clue about the No-deco limit for some common/popular depths when diving air.

My XXX rule is, (now) when I am diving 32 - 36%... I better be watching the computer when I get down to about 800 or 1000 psi, more or less, depending on tank size, depth and workload - for the first dive of course. A rough, simplistic and easily criticized "rule" - not so-different than the rule of 120.

Also, it seems strange for some to accept the rule of 120, while negating a guys anecdotal experience of (apparently) doing a couple of different dives, many hundreds of times where he applies his own rule of thumb. He claims to always getting a nearly identical result as the "actual" prediction from his computer.
I have no reason to doubt him.

Obviously, his SOP is working and serves adequately as a rule of thumb for him.. even if the rounding errors are crude and not mathematically robust?