Figuring thirds....

[KMD]

Couldn't agree more - convert to Cubic Feet and then turn is when either diver has used 1/3 of the smaller cubic feet number. In this case 57cuft.

I wouldn't count on making sure to put the diver with the highest SAC in the smallest tanks. Or other non foolproof methods. Do the math, live the math, and have enough to make it back.

Ok lets work this out:

Diver A has double 85's with 2700psi.
Diver B has double 95's with 3600psi.

Diver A has 172 cuft of gas
Diver B has 259 cuft of gas

Diver A has the smaller amount of gas so penitration gas is set at 57cuft (172/3)

Diver A's turn is 1800psi (2700 - (57/6.4*100)*
Diver B's turn is 2800psi (3600 - (57/7.2*100)*

Both of them have a cool dive and reach their turn pressure at the same time.

Diver A has used 57cuft of gas and has 115 cuft left
Diver B has used 57cuft of gas and has 202cuft left

Then at max penetration Diver B looses his gas. The team needs 114cuft of gas to exit.... There is 115 cuft in Diver A's tanks so they just make it out.


So there you have it folks. The OP talked about three different meathods of calculating 3rd. Which plan do you want to use?


*note in normal cases I would be rounding down. I didn't here just to bring the example right to the edge.

 

[rjack321]

So there you have it folks. The OP talked about three different meathods of calculating 3rd. Which plan do you want to use?

I plan on all using the same size rental tanks to dispense with all this cf hassle

 

[Jason B]

So lets work this rule out:

Diver A has double 85's with 2700psi.
Diver B has double 95's with 3600psi.

Diver A's turn is 1800psi (2700 - (2700/3))
Diver B's turn is 2500psi (3600 - (3600/3 - 100))

Both of them have a cool dive and reach their turn pressure at the same time. (diver B has a poorer sac rate)

Diver A has used 57cuft of gas and has 115 cuft left
Diver B has used 79cuft of gas and has 180cuft left

Then at max penetration Diver B looses his gas. The team needs 136cuft of gas to exit.... too bad they only have 115 in diver A's tanks.

You are not following what he is saying.

Diver A has double 85's with 2700psi.
Diver B has double 95's with 3600psi.

Diver A's turn is 1800psi (2700 - (2700/3))
Diver B's turn is 2800psi (3600 - 800) The 800 is diver A's thirds minus 100psi as Diva-aholic stated.

So now
Diver A has a usable volume of (TF x PSI/100) 6 x 900/100 = 54cu ft
Diver B has a usable volume of (TF x PSI/100) 7 x 800/100 = 56cu ft

If that's not close enough for you, let's use the real tank factors, not the rounded ones we were all taught by GUE.

Diver A has a usable volume of (TF x PSI/100) 6.4 x 900/100 = 57.6cu ft
Diver B has a usable volume of (TF x PSI/100) 7.2 x 800/100 = 57.6cu ft

 

[KMD]

You are not following what he is saying.

I think what he is saying is not clear. he said to subtract 100 psi from 1/3rds but didnt specify which 3rds.

And besides

if diver A had double 85's with 3600psi
and diver B had double 95's with 2700psi

Would you now ask diver B to penetrate to 1100psi? 900 psi? or 800psi? Where does the rule break down? Do you really want to be far back in a cave before finding out "oops, dont have enough gas to make it back."?

My answer would be to do the damn math and avoid the whole rule of thumb mess.

 

[Jason B]

I hope he made a mistake in his description of his method as the method I laid out is used all day long in cave country and is basically what is called the ratio method.

85/95 = .895 or close enough to say .9 or 90%

This means the 95's can use 90% of the gas used by the 85's

If the 85's can use 900psi (from your example above)
The 95's can use .9 x 900 = 810psi which we round to 800psi.
Same numbers.

I usually use these "rules of thumb" to check my math.

If I remember correctly from my GUE-f class, this method can not be used when smaller tank is grossly overfilled and the larger one is grossly under-filled.

 

[Meng_Tze]

Some people may be at risk of loosing credibility here...... While there are different calculations you can do to arrive at the same outcome, the outcome should be the same.

 

[Tigerman]

Isnt it easier to just go metric and be done with it?

 

[Jason B]

The way we teach it is:

Diver A has 3600psi in 130's - turn at thirds = 2400psi
Diver B has 3000psi in 120's - turn at thirds = 2000psi

Now we adjust turn pressure in team to the member with the lowest pressure. Therefore, since diver B can only use 1000psi of her gas, diver A will also call thirds at 1000psi usage or 2600psi.

Hypothetically, If diver B could only use 500psi before turn, then diver A would turn dive when he used 500psi, even if his PERSONAL thirds were 1200psi gas use.

I hope I didn't confuse anyone...

1000psi in a set of 130's does not equal 1000psi in a set of 120's as far as gas volume goes. Do you really teach this method?
Volume is our concern when diving thirds, not pressure.

 

[Sonofasailor]

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

 

[Mike Edmonston]

1000psi in a set of 130's does not equal 1000psi in a set of 120's as far as gas volume goes. Do you really teach this method?
Volume is our concern when diving thirds, not pressure.

Don't get me wrong..

I always put the heaviest breather on the smaller tanks.

And yes, I teach all my students to do the math by converting to cf. What I was describing is a simple rule of thumb for our team. We know our sac rates, and they are very close. Do I teach this rule of thumb as law to new students? of course not. As KMD has pointed out, in dissimilar volume/sac situations, a rule of thumb will get you in trouble really quickly. Now, if your team has similar sac rates, it works out well. But yes, you are right, doing the math is the only truly accurate way of Doing It Right

Cheers