Calculating turn pressures (gas matching) for a pair of cave divers.

[CaveSloth]

What are the formulas for calculating standard / accepted practice turn pressures for 2 divers A & B with starting pressures and volumes of Pa, Pb, Va, Vb in PSI and CF?

 

[Lorenzoid]

Is this a quiz?

 

[rddvet]

Are you asking about gas matching when two divers have different sized tanks.

 

[CaveSloth]

Are you asking about gas matching when two divers have different sized tanks.

Yes. Sorry, updated the title.

 

[Storker]

The formulas won't help you much unless you know where they're coming from.

If you don't know where the formulas are coming from, don't use them. Because you aren't able to check if they turn out the right answer. And if you know where they're coming from, you shouldn't have to ask.

 

[rddvet]

The formulas won't help you much unless you know where they're coming from.

If you don't know where the formulas are coming from, don't use them. Because you aren't able to check if they turn out the right answer. And if you know where they're coming from, you shouldn't have to ask.

that’s not entirely true. You have no idea how many cave divers with years of experience I know that have no clue how to gas match.
Personally, I have a cheat sheet from an excel soreadsheet I pull out of my wet notes so I don’t have math errors if distracted.

 

[Lorenzoid]

that’s not entirely true. You have no idea how many cave divers with years of experience I know that have no clue how to gas match.

I hope that's because they took their cave courses back in the stone age. Nowadays, I think gas matching is taught in all the intro-level textbooks. One of the few things about cave diving you actually can learn from a book. Mine didn't have formulas, though, which is what the OP is asking for. Rather, we were taught to go through it in a step-by-step way. But of course there are also cheat sheets, as you point out.

 

[rddvet]

I hope that's because they took their cave courses back in the stone age. Nowadays, I think gas matching is taught in all the intro-level textbooks. One of the few things about cave diving you actually can learn from a book. Mine didn't have formulas, though, which is what the OP is asking for. Rather, we were taught to go through it in a step-by-step way. But of course there are also cheat sheets, as you point out.

Some are older cave divers who weren't taught. Some just don't see the point and choose not to gas match. Others were taught the math and just never use it, so have forgotten it (and could care less). I personally can't easily do the math on the fly since I rarely need to (rarely dive unmatched team), so I use my cheat sheet.
Years ago when I did my cave course we didn't even have a textbook. It was all taught via the instructor.
I can give the formulas, but I figure if a student isn't given the formula by their instructor/book, there's a reason.

 

[O-ring]

Meet back at the entrance with 500 psi. Same as wreck diving but use entrance instead of anchor line.

 

[Manatee Diver]

The diver with the lowest available gas calculates their thirds in cubic feet. Everyone calculates their turn pressure by taking that cuft turning it into PSI for their tank, and subtracting it from their starting pressure.

So for example I have LP85s @3,600 and everyone else has 108s @3,600.

6.4 (tank factor for LP85s) * 36 = 230cuft

Which means a third is 76cuft.

I give that to my buddy diving 108s who has a tank factor of 8, which means he has 900psi to work with. So with a starting pressure of 3,600 he will turn at 2,700 psi. I will turn at 2,400psi so we both turn at 76cuft.

This is covered pretty early in the NSS workbook.