Figuring thirds....

[Sonofasailor]

Mike, well put. I am glad you cleared that up for me, see my instructor made the rule of thumb you referred to pretty much the only way you could do it. Truly, I have one person in my group who is a hoover, and he regularly uses larger tanks for that. If we were to use the rule of thumb, he would quickly suck down my tank if he were to have an ooa. I now know the correct way after doing a little research, but I guess the only true way to figure it out is to convert to cf.

 

[Jason B]

Jason, this was my concern as well, which is the reason for me starting this thread. My instructor used the very same method.

You gotta be kidding. Do you understand the tank factors rjack spoke of earlier? That is what you want to use right there. It will always work.

 

[Sonofasailor]

yes, those make perfect sense to me. I was talking about the method of using psi only being the method I was taught, not the one rjack was speaking of.

 

[Sonofasailor]

namely, the way dive-aholic was speaking of.

 

[TSandM]

Not to mention that a sac rate difference that big would be addressed prior to the beginning of the dive, by putting the heaviest breather on the smaller tanks in order to have enough gas to get everyone out

Man, I'd be very unhappy if you took my buddy out of his 130's and gave him my 85's, and made me dive the dratted 130's. First of all, you'd have to carry me in and out of the water . . .

There ARE issues with tanks other than volume. Weight and balance are also considerations. Using tank factors allows you to do the gas matching calculations easily, without taking people out of the gear they're familiar with and have learned to weight and balance.

 

[limeyx]

Each diver multiplies their gas in 100s by the tank factor. Divide by 3
Take smallest amount.

Covert smallest amount into psi for respective tanks
Subtract from start pressure
That's your turn pressure

Simple all these "rules of thumb" are bogus and I can't believe people who should know better are advocating dangerous shortcuts. Do the math right everytime.

OW! You're hurting me with pure logic. Stop it.

 

[Cave Diver]

So, I got my cavern cert. back in march and have been figuring thirds the way that I was taught in class. After a recent search online, I have seen in done more than three other ways than I was taught. Whats the best way to calculate thirds in water (without a calculator), once the water temp. has slightly reduced tank volume? I am curious to see the different methods people use.

I didnt like the way I was taught this during my class and I developed my own method. IMO, it's quicker, simpler and safer and I'm less likely to make a mistake in figuring it. If you'd like to know, send me a PM.

 

[Cave Diver]

Man, I'd be very unhappy if you took my buddy out of his 130's and gave him my 85's, and made me dive the dratted 130's. First of all, you'd have to carry me in and out of the water . . .

There ARE issues with tanks other than volume. Weight and balance are also considerations. Using tank factors allows you to do the gas matching calculations easily, without taking people out of the gear they're familiar with and have learned to weight and balance.

Aint THAT the truth!

 

[rjack321]

Don't get me wrong..

I always put the heaviest breather on the smaller tanks.

I'm confused? Why?

Is this human vacuum so unaware that they may use an extra 2/3rds gas without realizing it? That's one way to go OOA.

Its also certainly plausible that the mousy breather on those big *ss 130s could lose a burst disk or have some other catastrophic failure and now the hoover and the mouse are exiting on tiny tanks.

Basically in my mind tank size is irrelevant. You all get the same volume to penetrate. You may have dissimilar volumes for exit, big whoop. You can't compensate by bringing "extra" penetration gas for a hoover or less or whatever. A proper dissimilar tank calculation negates those choices by giving you all the same peneration volume. So teamates should wear reasonably comparable tanks (not AL63s matched with HP130s) that work for them. In most places this ends up being AL80s, LP85s, LP95s or LP108s (or equivelant HP tanks) which have 5,6,7, and 8 cf/100psi respectively.

 

[Charlie99]

I always put the heaviest breather on the smaller tanks.

I'm confused? Why?
.........
Basically in my mind tank size is irrelevant.

If you look at it a bit closer, you will probably come to the same conclusion that I have -- which is that where you put the heavy breather makes no difference in the worst case, but in many failure scenarios having the heavy breather on the big tank gives you additional margin. So putting the heavy breather on the small tank make calculations easier, but increases risk.

Or to put it another way --- if thirds (properly calculated based on smallest tank) are first reached by someone on a tank other than the smallest, then you will have more gas left if the big tank fails.