Info The Rule of 120

[oya]

Do you really do this math repeatedly as you go along?

Yes and no.

I do it occasionally. When I care. As an informed prediction of what my computer is probably going to say.

If it's a question of "I don't want to do any deco because I'm lazy or cold or need to get home early," then I'll keep an eye on it to keep deco to non-existent or, at least, less than a few minutes. If I've got an accumulated run-time of, say, 100 minutes then I obviously need to turn somewhere near minute 50 or so.
Give or take.

And then, despite the gainsaying of some people about how this planning couldn't conceivably ever work because it's illiterate nonsense, turns out it actually does work.

But it's not like I'm spending the whole dive doing constant math. I have a computer on each wrist to do that for me far more accurately than I could ever do in my head on my own.

 

[oya]

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.

What you want to do is understand why your computer is telling you what it wants you to do.

So that if it tells you some dumb ****, it piques your attention.

 

[inquis]

I'm not sure why I keep seeing that the rule of 120 is conservative?

The 120 value maps to some particular level of risk at a particular depth. It is conservative in the sense that following it at other depths yields less risk. You're right that it's still pretty risky (assuming air). OTOH, the rule of 116 on EAN32 I mentioned was based on Buhlmann+GF x/80 near 80 ft (I forget exactly), which is pretty middle of the road risk-wise, but at other depths would give an NDL time commensurate with a lower GFHigh (more conservative).

 

[johndiver999]

The 120 value maps to some particular level of risk at a particular depth. It is conservative in the sense that following it at other depths yields less risk. You're right that it's still pretty risky (assuming air). OTOH, the rule of 116 on EAN32 I mentioned was based on Buhlmann+GF x/80 near 80 ft (I forget exactly), which is pretty middle of the road risk-wise, but at other depths would give an NDL time commensurate with a lower GFHigh (more conservative).

The 120 rule refers to the navy tables on air. If you want to change the name of the 120 rule and apply it to things other than air, then we have a moving target with respect to nomenclature. It is simply the old navy table numbers from long ago that pertains to a few depths. 60-90 feet only).

 

[oya]

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.

One of my favourite bits of dialog from any movie ever is from Animal Crackers. It's an old Marx Brother's movie.

Groucho, as Captain Geoffrey T. Spaulding is talking to Roscoe W. Chandler, famous art patron, about building a new opera house near Central Park in Manhattan. Groucho suggests putting it in Central Park at night when no one is looking. Maybe right in the reservoir. And then screenwriter George Kaufman drops this gem:

"Well, Art is Art, isn't it? Still, on the other hand, water is water. And east is east and west is west and if you take cranberries and stew them like applesauce they taste much more like prunes than rhubarb does. Now you tell me what you know."

I am very much reminded of this scene by your XXX rule. There are a handful of bits of information, which are generally related, but don't exactly fit together.

Why are you watching your computer when you get down to 800-100psi? Is it AI? Why is tank size important? Is it a single HP120? Is it a set of doubled LP50s? 800psi in those two different rigs is pretty goddamn different. The deco models from 32 (the most useful gas possible) and 36 (some crap that dive resorts sell because they can charge more for) is pretty different, too. "For the first dive of course," ... of course? I don't know why "of course."

So many questions.

But I'd like to ask you one. Specifically about 800psi. Let's go with an AL80 because... well... why the hell not.

800 psi in an AL80 is ~20 cubic feet. If you are at, say, 100 feet of depth (we are completely ignoring all sorts of discussion about NDLs or deco at this point because... well... If you take cranberries and stew them like applesauce, they taste much more like prunes than rhubarb does) and you look down to see that you're at 800 psi... you are actually, possibly, completely screwed.

To get from 100 feet to the surface at a safe ascent rate should take AT LEAST 4 minutes at 30 feet per minute, excluding any safety stop or slowed shallow ascent rate. So let's call it 5. For luck.

OH DIP, WE'RE GONNA TALK AVERAGE DEPTHS!!
100 feet -> 0 feet in five minutes. Average depth = 50 feet over those five minutes.
If you're breathing rate is, say, .7cf/minute, which is on the mid-high side of average... but you're at and average of 50 feet, or 2.5ATA... you're burning through about 1.75cf/minute. So you need at least 9 cubic feet to get safely to the surface. But you've only got 20. Which means...

Oh. Hm. That by the time you hit the surface you've still got 11 cubic feet (or 440psi) left in your tank. Which is nice, I guess.

Assuming your pressure gauge is accurate at those lower pressures and your heart rate/breathing rate/workload never escalated at all or that you never had to share air with a buddy who was also running on fumes.

 

[oya]

The 120 rule refers to the navy tables on air.

Wait....
Just..........
Seriously, hold up........
You can dive using air?

Sounds gross.

 

[johndiver999]

One of my favourite bits of dialog from any movie ever is from Animal Crackers. It's an old Marx Brother's movie.

Groucho, as Captain Geoffrey T. Spaulding is talking to Roscoe W. Chandler, famous art patron, about building a new opera house near Central Park in Manhattan. Groucho suggests putting it in Central Park at night when no one is looking. Maybe right in the reservoir. And then screenwriter George Kaufman drops this gem:

"Well, Art is Art, isn't it? Still, on the other hand, water is water. And east is east and west is west and if you take cranberries and stew them like applesauce they taste much more like prunes than rhubarb does. Now you tell me what you know."

I am very much reminded of this scene by your XXX rule. There are a handful of bits of information, which are generally related, but don't exactly fit together.

Why are you watching your computer when you get down to 800-100psi? Is it AI? Why is tank size important? Is it a single HP120? Is it a set of doubled LP50s? 800psi in those two different rigs is pretty goddamn different. The deco models from 32 (the most useful gas possible) and 36 (some crap that dive resorts sell because they can charge more for) is pretty different, too. "For the first dive of course," ... of course? I don't know why "of course."

So many questions.

But I'd like to ask you one. Specifically about 800psi. Let's go with an AL80 because... well... why the hell not.

800 psi in an AL80 is ~20 cubic feet. If you are at, say, 100 feet of depth (we are completely ignoring all sorts of discussion about NDLs or deco at this point because... well... If you take cranberries and stew them like applesauce, they taste much more like prunes than rhubarb does) and you look down to see that you're at 800 psi... you are actually, possibly, completely screwed.

To get from 100 feet to the surface at a safe ascent rate should take AT LEAST 4 minutes at 30 feet per minute, excluding any safety stop or slowed shallow ascent rate. So let's call it 5. For luck.

OH DIP, WE'RE GONNA TALK AVERAGE DEPTHS!!
100 feet -> 0 feet in five minutes. Average depth = 50 feet over those five minutes.
If you're breathing rate is, say, .7cf/minute, which is on the mid-high side of average... but you're at and average of 50 feet, or 2.5ATA... you're burning through about 1.75cf/minute. So you need at least 9 cubic feet to get safely to the surface. But you've only got 20. Which means...

Oh. Hm. That by the time you hit the surface you've still got 11 cubic feet (or 440psi) left in your tank. Which is nice, I guess.

Assuming your pressure gauge is accurate at those lower pressures and your heart rate/breathing rate/workload never escalated at all or that you never had to share air with a buddy who was also running on fumes.

LOL, my XXXX Rule. what was your question? Specifically?

 

[Brett Hatch]

In this thread

 

[inquis]

then we have a moving target with respect to nomenclature

Agreed, so it would be good to state the "qualifiers" like Rule of 120/Cave or Rule of 120/Avg or Rule of 116/EAN32+GF80. Or any of the other rules in the OP that were qualified with introductory remarks.

It is simply the old navy table numbers from long ago that pertains to a few depths. 60-90 feet only).

The Rule of 120 is most accurate at 80 ft. (It gives 40 mins NDL, the Navy table gives 39 mins.) However, it "pertains" wherever you wish to apply it with increasing conservatism. At 100 ft, you would come up 5 minutes sooner when using the Rule of 120 than the Navy NDL. There is obviously a trade-off between accuracy and simplicity, and one may wish to use a different approach when accuracy degrades sufficiently. The whole point of using the tangent line (which all the Rules mentioned in the OP do) is that the errors are all one direction -- in the conservative direction.

 

[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.