Modifying rule of thirds

[AMW]

Within the new CDG manual we give some time to thirds and the application of modified thirds.This is primarily for uneven cylinders sizes and used mainly in underwater digging.

Andrew.

[chimie007]

Hmmm.... Not so sure about statistically impossible!

LOL "statistically impossible" perhaps not the best choice of word. Sludge got it right: statistically insignificant. I mean it's a double failure. Even if you have two free flowing reg, you could feather the valves.

[Wizard]

What about scootering with a buddy bottle?

I wouldn't take off on a long run solo on a scooter. Having 75% of two jacked 104's to cope with exit or emergencies is still a lot of gas so I don't take a buddy bottle. With a scootering buddy it's back to thirds because swimming is less of an option.

That's just me. By all means it's not a rule to base anyones else's life giving air strategy on.

[chimie007]

I wouldn't take off on a long run solo on a scooter. Having 75% of two jacked 104's to cope with exit or emergencies is still a lot of gas so I don't take a buddy bottle. With a scootering buddy it's back to thirds because swimming is less of an option.

Another good thing to do is really know how much you need to exit swimming. Not just numbers but actual proven amount. One of my first scooter dive was to swim back from 3000' at Ginnie. Took me about 40 min and used a bit less than 1/3 of my gas supply. That was pushing the scooter ahead and carrying a stage. I would have dropped both in a real situation.

[MengTze]

I use the rule on '1': How much gas do I need to get out? Other things you have to think about is; do I carry gas for another diver should I come across one in need? How do I carry my gas, in terms of redundant regs and bottles.

Chimie007 is spot on: make sure you DO know where you are in a dive and that given the worst circumstances you do get out. Actually swimming your gear out every now and then does give you an insight into this.

[Line Squirrel]

Another good thing to do is really know how much you need to exit swimming. Not just numbers but actual proven amount. One of my first scooter dive was to swim back from 3000' at Ginnie. Took me about 40 min and used a bit less than 1/3 of my gas supply. That was pushing the scooter ahead and carrying a stage. I would have dropped both in a real situation.

Of course...all of this is nothing more than the experience I mentioned in an earlier post

[aainslie]

Oh what the hell, let's stir the hornet's nest a little.

I posted about this last year when I was doing some exploring in a downstream cave. I did that scariest of things - algebra - and came up with this:

http://www.andrewainslie.com/spreads...s%20Murphy.xls

It wasn't well received. Oh well... Gary and Kelly in particular hated the fact that although I wrote it for downstream calce, by putting a negative number in you can use it for upstream calcs... and it leads you to the conclusion that it's pretty safe when swimming Manatee or similar caves to go WAY over thirds and be safe. Doron Nof, one of the cleverer divers I've ever met, independently verified my math and concurs with my findings - so it's even peer reviewed. Yes I know, we PhD's are mere ivory tower idealists... blah blah... but there are the results if you want to play with them. Here's the original thread.

http://www.cavediver.net/forum/showthread.php?t=6312

For those of you who feel that math is actually useful, have a look through both the thread and the spreadsheet. My suggestion - don't hide behind overly simplistic "rules of thumb". We humans love heuristics - yet there's a massive literature on heuristic bias and how much trouble it gets us into. For example, merely adding an 80 when scootering or when diving solo BM may be overkill, or it may be underkill - you won't know until you do the math.

If you SERIOUSLY want to think about modiflying rules of thirds, start with my spreadsheet and modify it in line with the added contingencies you'd like to add. I have no problem making the claim that this spreadsheet is a more carefully though out approach to safe gas management than anything else out there. yet is still comes with the caveat that you're nuts to use it until you understand every single cell I put together in there.

Two additions that I'm actually going to play with right now - 1) I'd add a "minutes stationary" contingency, i.e. allow for something that stops you for a few minutes completely on the way back AFTER losing half your gas, and 2) if scootering, I'd use a different "speed in" from the "speed out", i.e. assume the scooter speed going in but the swim speed going out.

In all my calculations, I assume you're solo and diving sidemount. I see no other logical way to dive. This "air share" idea is one of the most dangerous ideas in cave diving. By all means train to do it, but IMHO you should never plan your gas management on any assumptions about someone handing you a hose. Just consider yourself really lucky if someone does.

[wingman]

I find that when solo i no longer need that gas reserve i carry for my buddy but rather it is now mine for my own personal consumption. So I modify thirds into .38 instead of .33. I call this the golden rule of gas consumption simply because it is based on the golden ratio (i like to think of that as the ratio of successive fibonnaci numbers F(n) + F(n+1) as n gets large, (1, 2, 3, 5, 8, 13, ....) although there are certainly more artfull ways to get at it (1.618)). So with 3600 I would consume 2225 and reserve 1375 for any untoward events. I certainly add some safety stock if conditions warrant and then I use a gas inventory control model with uncertain demand. I consider the uncertainty to be normally distributed but am now looking at alternative probably distributions. Probability based gas management is certainly the way to go. However, as always, ymmv. Bill

fyi
turns out that an eight followed by a closing parenthesis is 8) so the number above is 1.618...

[Sludge]

Funny, Bill, I've never thought of you as a fibonacci sort of guy. I would have guessed you would lean toward root mean square...

[aainslie]

I find that when solo i no longer need that gas reserve i carry for my buddy but rather it is now mine for my own personal consumption. So I modify thirds into .38 instead of .33. I call this the golden rule of gas consumption simply because it is based on the golden ratio (i like to think of that as the ratio of successive fibonnaci numbers F(n) + F(n+1) as n gets large, (1, 2, 3, 5, 8, 13, ....) although there are certainly more artfull ways to get at it (1.618)). So with 3600 I would consume 2225 and reserve 1375 for any untoward events. I certainly add some safety stock if conditions warrant and then I use a gas inventory control model with uncertain demand. I consider the uncertainty to be normally distributed but am now looking at alternative probably distributions. Probability based gas management is certainly the way to go. However, as always, ymmv. Bill

fyi
turns out that an eight followed by a closing parenthesis is 8) so the number above is 1.618...

This is a beautiful example of a silly heuristic. Sorry, but I just have to point it out.

Fibonacci sequences are elegant. But, for crying out loud, what do they have to do with gas consumption?

Pi is sort of cool - why not reserve 1/pi of your gas?

Oh actually so is e. what about 1/e?

Etc etc etc.....