xiSkiGuy
Contributor
So from my Cavern and Basic Cave classes I feel like I understand how to match thirds with dissimilar tank volumes. For example:
Diver A has LP-85s with 3400psi
Diver B has E8-130s with 3500psi
To prevent the team from penetrating beyond a point where Diver A's gas supply could get both divers back to open water if Diver B lost all his gas, we limit Diver B's third to match the volume of the third of the smaller tanks:
LP-85s: 170cf / 2640psi x 100 = 6.4 cf per 100psi.
We round 3400psi down to 3300 and divide by 3 to get thirds = 1100psi
In LP-85s 1100psi = 70.4 cf (6.4 x 11)
So we now limit Diver B's penetration volume to 70.4cf:
E8-130s: 260cf / 3442psi x 100 = 7.5 cf per 100psi
70.4 / 7.5 x 100 = 938psi.
So rounding down we limit Diver B's penetration to 900psi.
3500 - 900 = 2600psi turn pressure.
Now each diver has enough gas to exit the cave on a third and has a a big enough third in reserve for their buddy (or themselves if the had to isolate and exit alone).
It all makes sense to me, but I have been wondering: Can you do it another way that gives Diver B (who has a higher SAC rate) more penetration volume and is as safe as if both divers had the same size tanks?
Does it work to have Diver A keep a reserve that is equivalent to Diver B's third?
Using the same example: Diver B's penetration volume is 1100psi. In E8-130s 1100psi is 82.5 cf (7.5 x 11). So Diver B needs 82.5cf to exit the cave if he loses all his gas right as he turns on thirds.
So what if Diver A finds his turn pressure by reserving 82.5cf and halving the remaining supply:
82.5cf / 6.4cf = 12.89cf x 100psi = 1289psi.
Call it 1300psi.
3400psi - 1300psi = 2100psi.
2100psi / 2 = 1050psi (round down to 1000psi).
3400 - 1000 = 2400psi turn pressure.
This gives Diver A 154.5cf to exit the cave. This allows 82.5cf for his buddy and 72cf for himself. He used 64cf getting to thirds.
Diver B uses thirds and has 180cf to exit the cave. This allows 82.5cf for him and 97.5cf for his buddy.
Both divers have enough volume at two thirds to get themselves and their buddy out. Both have enough to isolate and exit alone on one tank.
Am I missing something that make this unsafe?
Before anyone comments, this academic exercise assumes the dive is a cave where thirds are appropriate (into highflow). And yes, currently I only have a Basic Cave cert, so I limit my all my cave dives 1/6 of doubles.
Diver A has LP-85s with 3400psi
Diver B has E8-130s with 3500psi
To prevent the team from penetrating beyond a point where Diver A's gas supply could get both divers back to open water if Diver B lost all his gas, we limit Diver B's third to match the volume of the third of the smaller tanks:
LP-85s: 170cf / 2640psi x 100 = 6.4 cf per 100psi.
We round 3400psi down to 3300 and divide by 3 to get thirds = 1100psi
In LP-85s 1100psi = 70.4 cf (6.4 x 11)
So we now limit Diver B's penetration volume to 70.4cf:
E8-130s: 260cf / 3442psi x 100 = 7.5 cf per 100psi
70.4 / 7.5 x 100 = 938psi.
So rounding down we limit Diver B's penetration to 900psi.
3500 - 900 = 2600psi turn pressure.
Now each diver has enough gas to exit the cave on a third and has a a big enough third in reserve for their buddy (or themselves if the had to isolate and exit alone).
It all makes sense to me, but I have been wondering: Can you do it another way that gives Diver B (who has a higher SAC rate) more penetration volume and is as safe as if both divers had the same size tanks?
Does it work to have Diver A keep a reserve that is equivalent to Diver B's third?
Using the same example: Diver B's penetration volume is 1100psi. In E8-130s 1100psi is 82.5 cf (7.5 x 11). So Diver B needs 82.5cf to exit the cave if he loses all his gas right as he turns on thirds.
So what if Diver A finds his turn pressure by reserving 82.5cf and halving the remaining supply:
82.5cf / 6.4cf = 12.89cf x 100psi = 1289psi.
Call it 1300psi.
3400psi - 1300psi = 2100psi.
2100psi / 2 = 1050psi (round down to 1000psi).
3400 - 1000 = 2400psi turn pressure.
This gives Diver A 154.5cf to exit the cave. This allows 82.5cf for his buddy and 72cf for himself. He used 64cf getting to thirds.
Diver B uses thirds and has 180cf to exit the cave. This allows 82.5cf for him and 97.5cf for his buddy.
Both divers have enough volume at two thirds to get themselves and their buddy out. Both have enough to isolate and exit alone on one tank.
Am I missing something that make this unsafe?
Before anyone comments, this academic exercise assumes the dive is a cave where thirds are appropriate (into highflow). And yes, currently I only have a Basic Cave cert, so I limit my all my cave dives 1/6 of doubles.