Recalculating Thirds

[floridabob]

I'm still not seeing it.

1-if it took 1000 psi in and at your current rate of exit I will take you 1000 psi, how can you jump down a side tunnel at 500 psi into the exit? So, you went in 1000. Came out 500. Jumped a side tunnel for 100psi in and out totaling 200 on that jaunt. You will then have 1300 when you begin your direct ascent again...which is 500 psi from exit. So you finish at 800 psi. Not 1000..ergo you broke thirds.

Essentially, if you were with a buddy and all was equal, if you had an OOG immediately when you left at 1300 psi and had to air share...you'd have 200 psi less than thirds at that point. I see the thought of discarding the successfully covered previous 500 feet but it's not thirds. 1/3 of your gas was not reserved.

Essentially you are rewarding yourself for the exited cave without a failure whereby you will grab that bonus gas to go in a side tunnel. Bit of a hair split, but that's not truly thirds.

Full disclosure, I never said I didn't understand the concept of scavenging that gas but it's in no way reserving 1/3 of the original gas plan. It's a bastardization of a reserve 1/3 plan. Which is fine if you choose to do it.

Utilizing "found" gas from a faster exit than entrance is a no brainer.

Bob

[floridabob]

The math will never elude me. How people choose to apply it will.

It's not thirds. Period.

What it is is a variant of allowable and tolerable gas reserves that you/we compute as we go.

Sure it's probably safe but so is cave diving.

Again, I get it, have done it and can see the logic.

But, it's not thirds. As math or reserves.

[Nitrogenius]

but it's in no way reserving 1/3 of the original gas plan.

That is correct. It is in no way reserving 1/3 of the original gas plan. That is why it is "recaclulating" as well. At the Moment of recalculating you develop a new gas plan. In that new gas plan you MUST not break the rule of thirds for this new plan.
Remember you also set a very diffferent target than in you Initial plan..


What does reserving 1/3 of the original gas plan get you?
The logic behing thirds is that at Maximum Penetration you still "safely" make it out of the cave by either yourself (loosing one cylinder or 50% of your gas) or in a Buddy System one of the diver losses all gas and the other diver carries the reserve.
At that Point obviously with rule of thirds you have no additional safety margin whatsoever, which means to me that thirds are not very advisable at most times, so I very much prefer fourths..
But using thirds, fourths, fiths etc. etc. all Comes down to the Overall conservatism you want to apply. You always can use that same conservatism in recalculating or even better when recalculating even notch conservatism up..
In no circumstances when I recalculate I would Approach it from the angle that I would want to maintain my intitial reserve for my Initial plan.
BTW you can also include recalculating into your dive plan to begin with and align it with whatever gasmanagement rule you wish to apply ..

[floridabob]

Hmmm.


I think I see. You are explaining this to me as if you think I dont understand the concept. I do.

I'm failing to get my point across: we are taught to reserve 1/3 of our starting pressure/volume for entrance. 1/3 for exit and 1/3 for our buddy.

That is the rule of thirds.

IF you choose to adjust your reserves in the water: FINE. But, its not thirds. You cant set aside 1000psi to start as a reserve third, then exit with 800 (my example) and say you didnt violate your own rule of thirds.

Like, DUH. Forgive me, as I'm sure you cant argue that point.

NOW. If you adjust in water. So be it. No worries. But it is in no way 1/3 of your starting pressure. Which is what we were all taught. To teach anything else is a violation of thirds. But not necessarily an unsafe gas reserve, mind you.

Again, maybe splitting hairs in one sense but you cant change the number from the beginning of the dive and call yourself leaving one third of your starting pressure in reserve.

The concept of "recalculating thirds" is a perversion of the rule and be thought of as a gas usage/reserve adjustment.

Again, this was just mental masturbation to pick at the term recalculating thirds.

[Nitrogenius]

Well I guess it all boils down to the Definition of the rule of thirds..
If to you the rule of thirds is related to your starting pressure and your Initial dive plan. yes than you are right it would be a Violation of that rule then.
To me the rule of thirds is what it is meant to be to reserve exactly as much gas as you need to "safely" Exit the cave at any Point during the dive assuming that your SAC would remain stable also with a catastrophic failure.
This is what I am maintaining with recalculating thirds.
Again I have to say thirds does not cut it for me as I do not believe in a stable SAC with a catasprohic failure, so I do want extra reserve. I do even want more extra reserve when recalculating, however once recalculate I might end up with less gas than my Initial calculated gas reserve..
Simply following the Initial calculation without reflecting the sense and logic behind it simply does not cut it for the german engineer
We want the logic preserved not a static value..


And no in no way this is violating the rule of thirds..

[floridabob]

As an engineer you understand the original data set cant be changed, it just is.

No worries on a fluid use of a gas reserve plan from me. It makes sense.

Cant teach that, though. Least not initially.

This was enjoyable, thanks for keeping engaged and not getting emotional. Not that emotions ever show up in a forum!

bob

[twaldo]

Ok... explain to me (yes very much a newbie) why if you are going to do something as stupid as breaking one of the MAJOR rules of cave diving why you don't just carry MORE (or staged) gas?? I don't get it... I understand the idea of making a trip into Little and using my first 3rd just getting to the Chimney and THEN recalculating... but I also understand that this is NOT what I was taught....

Tim

[Nitrogenius]

Nobody wants to break any rule here.. This is what it is all about.
Just don't recalculate and all is well..

[RN]

I don't advocate recalculating thirds. Never have. Not even for the Grand Traverse. The Grand Traverse can be done sticking to a conservative thirds (meaning subtracting 100-200 psi from your actual thirds) with LP85s and a single stage. There's simply no reason to have to recalculate thirds during this dive. Do you really want to exit out of Challenge Sink? What if you're counting on that recalculation and something happens but a boulder has dislodged and is now preventing your exit?

The only time I believe recalculating is okay is someplace like Peacock I and only for sixths. Dive the Olsen line, turn on sixths and get back to the beginning of the gold line in the cavern. Rather than get out, simply recalculate sixths and dive the Peanut Line. Same entrance/exit, sixths not thirds, no point in surfacing just to recalculate.

[jj1987]

For those of you who are against recalculating thirds, how do you feel about a commercial flight where the further into the flight the less and less fuel reserves you have?