High SAC rate and low SAC rate

[Pavão]

That won't work. If he calls it on thirds, and the other diver is near thirds, the high-SAC rate guy had better not have an OOA - there won't be enough.

But that would be the same scenario with equal sac and equal tanks between 2 divers no? They both would use the same amount to go in, (1/3), then, if one goes OOA, there's only 2/3 left for both, 1/3 for each.
Is that what you meant?

[Sludge]

Is that what you meant?

No. The way Sabado first stated it, it sounded like he meant the diver with smaller cylinders called the dive when he "reached thirds."

The scenario I mentioned goes like this: suppose you dive LP-108s and I dive LP-80s. We dive, and right before you hit thirds, I hit thirds. We turn the dive. You need one-third of 104s to exit, but I only have one-third of 80s times two. If you have an OOA, I don't have enough for both me and your hoover self.

I used to dive with a buddy all the time who hit thirds on 104s exactly when I hit thirds on 80s. Rather than cut our dives short by calculating new turn pressures, I just carried a buddy bottle instead.

[gschaut]

See if I can make the math simple:

Low SAC diver has cave filled 85's with a total of 255 cu ft at start of dive. His 1/3 is 85 cu ft and turns with 170 cu ft. left in his tanks.

High SAC diver has cave filled 120's with a total of 360 cu ft at start of dive. His 1/3 is 120 cu ft and turns with 240 cu ft left in his tanks.

Due to difference in SAC rates, BOTH divers hit 1/3's at the same time/point in dive.

5 seconds after turning the dive, the "High SAC" diver (using manifolded doubles) suffers a catastrophic air loss. Total air supply failure. Both divers must exit on air in the low SAC diver's tanks.

Low SAC diver has 170 cu ft in his tank. Low SAC diver consumed 85 cu ft on entry, High SAC diver consumed 120 cu ft on entry.

Both divers together require 205 cu ft (85+120) to exit PROVIDED that nothing happens to slow them down, or increase their SAC.

But the Low SAC diver only has 170 cu ft left. Both divers run out of air and drown while still 1/5th of the way back inside the cave.

Ok, what am I missing?

[Pavão]

To answer the OP's question:

You put the larger tanks on the person who is most comfortable diving them. Period.
I could dive LP121's but the gain in cu' of gas would be lost in me being upside down on my back looking like an idiot.

You are spot on, and I believe you're the first instructor who I've heard say that.
I'm impressed, it's like you were there great job.
Now, I think we can all develop the skills to use almost any size tanks though, just takes a "little" adjusting.

Now that you're comfortable in the tank you like, figure out when to turn the dive. If you don't know how to do that, I can book a class with you for $300/day hehe.

Sure, and I would probably need about 10 to 15 days right?
My next class will hopefully be cave-1 GUE

[bpcfire1]

your missing the rest of the air and Edds phone number

[Sludge]

Gary, exactly how high are you filling these cylinders?

[Pavão]

Yes it does eliminate this as an issue.

As noted above, with proper gas matching, big tanks or small tanks does not matter, everyone on the dive will turn the dive when/if they arrive at the predetermined pressure equal to the "third" in the smallest set of tanks on the dive. That ensures everyone is diving the same size third and that any third will provide an adequate reserve for anyone else on the team.

Personally, I think thirds is pushing it too far in a low flow cave, so I'd scale it back another 100-200 psi from true "thirds" anyway.

Respectfully disagree...thirds is calculated based on all of the above...no matter who calls it, there will always be enough gas for both divers to make it out if either diver has an OOA situation.

The penetration distance is also the same no matter which diver has the larger tanks or smaller tanks. That's the whole purpose of calculating thirds based on dissimilar tank sizes. Penetration distance is the only thing sacrificed with higher sac rates...not safety.

That is exactly what I thought!
But you know, there are plenty of people doing this far longer than me, so, I would definitely listen to what they have to say

[Sabado]

See if I can make the math simple:

Low SAC diver has cave filled 85's with a total of 255 cu ft at start of dive. His 1/3 is 85 cu ft and turns with 170 cu ft. left in his tanks.

High SAC diver has cave filled 120's with a total of 360 cu ft at start of dive. His 1/3 is 120 cu ft and turns with 240 cu ft left in his tanks.

Due to difference in SAC rates, BOTH divers hit 1/3's at the same time/point in dive.

5 seconds after turning the dive, the "High SAC" diver (using manifolded doubles) suffers a catastrophic air loss. Total air supply failure. Both divers must exit on air in the low SAC diver's tanks.

Low SAC diver has 170 cu ft in his tank. Low SAC diver consumed 85 cu ft on entry, High SAC diver consumed 120 cu ft on entry.

Both divers together require 205 cu ft (85+120) to exit PROVIDED that nothing happens to slow them down, or increase their SAC.

But the Low SAC diver only has 170 cu ft left. Both divers run out of air and drown while still 1/5th of the way back inside the cave.

Ok, what am I missing?

You're missing this: Both divers with different tanks don't both dive thirds for their respective tanks. The one with the larger tanks "corrects" his tanks to the smaller tank size so in fact both divers are diving the smaller sized tanks...then calculate thirds. The turn pressure for the larger tank diver will be less than thirds...so your scenario should never happen. The higher sac rate diver will always turn the dive in this scenario. Even if he has the larger tanks because they are corrected down to the smaller size and the turn pressure is based on the 1/3 volume of the smaller tanks.

[Sabado]

In short, you always calculate thirds turn pressure using the 1/3 volume of the smallest sized tanks on the team.

[Pavão]

See if I can make the math simple:

Low SAC diver has cave filled 85's with a total of 255 cu ft at start of dive. His 1/3 is 85 cu ft and turns with 170 cu ft. left in his tanks.

High SAC diver has cave filled 120's with a total of 360 cu ft at start of dive. His 1/3 is 120 cu ft and turns with 240 cu ft left in his tanks.

Due to difference in SAC rates, BOTH divers hit 1/3's at the same time/point in dive.

5 seconds after turning the dive, the "High SAC" diver (using manifolded doubles) suffers a catastrophic air loss. Total air supply failure. Both divers must exit on air in the low SAC diver's tanks.

Low SAC diver has 170 cu ft in his tank. Low SAC diver consumed 85 cu ft on entry, High SAC diver consumed 120 cu ft on entry.

Both divers together require 205 cu ft (85+120) to exit PROVIDED that nothing happens to slow them down, or increase their SAC.

But the Low SAC diver only has 170 cu ft left. Both divers run out of air and drown while still 1/5th of the way back inside the cave.

Ok, what am I missing?

I think your math is correct, not the dive practice I would use though.
On your exemple, they ran into trouble because they did not match gas, if they had done that, the turn pressure would be the equivalent of 85cu ft for BOTH divers, high SAC diver should have never consumed any more than 85cu ft past the turn point, on your exemple, high SAC would have been the one turning the dive, and low sac would not even reach his third.