Figuring thirds....

[limeyx]

...
My point? Familiarity with the location and the experience play a huge role. With the right buddy, I don't believe that diving a double staged scooter dive in Jackson Blue to 1/3s is 'asking for trouble'. (granted the plan actually gives me about 7 cuft more than 2/3 reserve). But that confidence absolutely has to do with personal knowledge of the cave, understanding of -actual- transit times, and distance limitations.

True, I did make something of a blanket statement, and I am a new enough cave diver that no cave is that familiar to me at this point !

 

[rjack321]

True, I did make something of a blanket statement, and I am a new enough cave diver that no cave is that familiar to me at this point !

And of course since you're saving your virginity you're not bringin home "some woman" in the dark either

 

[Dive-aholic]

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.

My mistake. I forgot to mention that the turn pressure is always based on the lowest turn pressure. So in your example, the turn pressure for diver B is 2800, not 2500. I guess the fills we get are always in the 3500 + range. I haven't had to deal with such a big difference in tank pressures. If 85s are filled to 3500 and 95s are filled to 3600, then the turn pressure will be 2400 for 85s and 2600 for 95s (1100 psi - 100 psi). Does that make more sense? My fault for not realizing that some people will get crap fills and still dive. I'm just used to getting decent fills and similar pressures.

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

The OP asked to calculate 1/3s in the water without a calculator after the tanks have cooled down to the water temperature. Do you actually hang in the water calculating 1/3s on actual tank pressure or do you go with the hot pressure? My tanks cool about 200psi. I want to go with the actual tank pressure at spring temperature, not post fill temperature.

Jeesh what so hard about converting to cf using tank factors?
AL80s = 5cf/100psi
lp85s/hp100 = 6cf/100psi
lp95s/hp119 = 7cf/100psi
lp100/hp130 = 8cf/100psi

And what are tank factors? The numbers you provide are not actual tank factors, they're estimations, or rules of thumb. Actual tank factor for 85s is 6.4, for 95s 7.2...you get the picture. Is there much of a difference between your method and mine?


Whenever I'm diving a new size tank, I will do the calcs before the dive. I'll know what the difference is going to be. But once I'm in the water, I use my rule of thumb because I know it works. I'm not going to hang there with a calculator in a housing or doing math on my wet notes when I can quickly do it in my head and get on with the dive. I believe tank factors need to be taught and used, but I also believe that students should be told about the various "short cuts" and use them once they become comfortable with them. It's only a disservice to not give them all the options available. The well informed diver should know his/her limits and make informed decisions.

 

[Meng_Tze]

My mistake. I forgot to mention that the turn pressure is always based on the lowest turn pressure.

A deadly mistake that can be.....

(and I assume you mean volume, whether directly calculated, or indirectly)

 

[Dive-aholic]

Apparently, but it's something that just comes 2nd nature now. Volume matching should have been covered in the OP's class. I always base turn pressure on volume.