Recalculating Thirds

[Sludge]

After further contemplation, you may think that is an actual issue. Many times I have had to explain to new open water divers that 2400psi in a LP-95 is more gas than 3000psi in an AL-80. Do you seriously think that someone with cave diver training is that clueless?

[Nitrogenius]

Not by you on this thread! Your calculations and understanding of the problem are correct.

I was reffering to that eastern front comment with that..

[victorzamora]

I believe anybody that has the ability to find the "on" button of their computer can understand that already.

Russell, I knew you knew Ideal Gas Law issues and volume differentials. I guess the point I was trying to make is that (obviously) pressure isn't as simple to work with and volume plays a big part. We cave divers all too often speak purely in pressures, which simply isn't the full story. As has been said, swimming in and swimming back can be simplified into pressures....but restrictions are volumes, plain and simple. Which of course you knew.

To me, this means the safety and sanity of recalculating thirds isn't constant but varies on a bunch of factors....tank size being one. I think this is why pressures AND volumes were being brought up so much.

[stickydeco]

. . .History will show that when we monkey with gas planning it really can bite us,and that is borne in the fact that we lost two world class cave divers who used some voo-doo gas planning.. . .

FINALLY, someone with some freekin common sense speaks. I just hope that all y'all listen up.
Thanks Kel

[stickydeco]

Dude: What are you doing, switching your main gas cylinders in the middle of a dive? Because that's what you would have to be doing if I follow your example! Stated another way: Pressure IS everything!

[twaldo]

Several people are going on and on about pressure versus volume. I have been cave diving for almost nineteen years and have always been under the impression that the relationship of pressure and volume is linear. In other words, at 3600psi my 95s have 260 cubic feet of gas and at 1800 they have 130. If this is not true please enlighten me.

That might have been me.... and you are right as long as your talking about YOUR 95's I was stating... here or another thread that pressure is not volume in the sense that NAUI requirements are 140 cuFt of gas to start a cave dive.... but everyone was talking about recalculating based on pressure... you can have 2000 psi in an AL8 or an AL80 but obviously not the same volume of breathing gas in the two. I dont' know about the other agencies but I find it hard to believe that they require a minimum pressure to start a dive and not a minimum volume.
As only a Cave 1 certified diver I'm sticking to my agencies training requirements of 140 CuFt.... I dive LP85's so I can start with a pumped up set of tanks, do a dive and still have enough volume to do a second dive on the same tanks after a SI....

Rob described the only instance I would ever consider recalculating without a surface interval because I know that at 2200 psi in my tanks I still have 140 cuFt... Without regard to the agency, the standards are there to protect us...and I'm a FULL believer in follow the rules of cave diving and live to dive another day. I have NO desire to prove the standards are to conservative... I LOVE conservative.

And THAT is why I say pressure isn't volume.



Tim

[twaldo]

We cave divers all too often speak purely in pressures, which simply isn't the full story.

THANK YOU!!!!!

-Tim

[RN]

That might have been me.... and you are right as long as your talking about YOUR 95's I was stating... here or another thread that pressure is not volume in the sense that NAUI requirements are 140 cuFt of gas to start a cave dive.... but everyone was talking about recalculating based on pressure... you can have 2000 psi in an AL8 or an AL80 but obviously not the same volume of breathing gas in the two. I dont' know about the other agencies but I find it hard to believe that they require a minimum pressure to start a dive and not a minimum volume.

That's pretty much the standards across most agencies. It was based on diving double 72s, which were the most common cylinder used when the standards were created. Basically the standards required a cave dive be started with a full set of double 72s.

[FW]

I think maybe the airplane guys have the right idea. They don't talk about tank volume, pounds of fuel, or gallons of fuel. The bottom line, most important thing to be concerned with is TIME IN TANKS! The main thing to know is how long/far can you get on the gas you have. That depends on the size tanks you have, the tank pressure, and your SAC rate. It also depends on things like current, passage size, visibility, potential things that can change any of the above.

[gsxr1997]

I have been watching this thread with interest.. As far as I am concerned , the only time to recalculate thirds in the system is in a lost buddy situation..