One dive or two?

[jj1987]

Better think again. If you made it in on thirds, and don't re-calc. You will be planning the return trip on half of your remaining gas.

Howard

I'm not following what you're saying here, how is this any different than if I turn my dive at 1/3 in a cave that isn't a traverse?

Let's say if you go from manatee to sue sink on 1/3, surface with 2/3, and then turn around and complete the dive back downstream, how is this different than if you hit 1/3 5ft before you ran into open water, and turned at that point without recalculating?

[metaldector]

It's one dive.

[Howard]

you start the dive with 3000# you dive 3rds and make it to your destination ending that segment with 2000# If you turnaround and come back on the other 3rd, you will end up with a 1000#.
the post was, is this 1 or two dives. If you treat it as one dive, all should be OK. But if you are looking to plan this as 2 dives, I am saying if you turnaround at 2000# recalc 3rds your new turn pressure is ~1400# you will be hitting 3rd and keeping on going toward your intended exit.

[cavediver256]

you start the dive with 3000# you dive 3rds and make it to your destination ending that segment with 2000# If you turnaround and come back on the other 3rd, you will end up with a 1000#.
the post was, is this 1 or two dives. If you treat it as one dive, all should be OK. But if you are looking to plan this as 2 dives, I am saying if you turnaround at 2000# recalc 3rds your new turn pressure is ~1400# you will be hitting 3rd and keeping on going toward your intended exit.

Ah yes,but at that point you are heading to a known exit with your second third....same as a circuit.

[aainslie]

Don't dive thirds. Dive doubles.

Always have twice the gas that you need to exit.

In a traverse where you KNOW EXACTLY where the midpoint is and how to navigate it and there is no flow, if you hit thirds at or after the halfway point you're OK. At that stage you ahve double the gas needed to exit - on the other side.

So - on the first traverse you'd better use WAY under thirds to get to the midpoint. When you get to OG, you should recalc thirds. If you hit the midpoint before hitting that third, you're OK.

Key principle - ALWAYS HAVE DOUBLE THE GAS YOU NEED TO EXIT.

I hate "thirds". It's such a bad name as it detracts from the true intent of this rule.

And you're nuts not to recalculate thirds properly just because you don't surface. Knowledge is your friend. Knowing that you have tons of gas or the return will help you prevent panic in an emergency. Let's say you start with 3000 psi, and get to OG with 2000 psi. You don't bother to recompute so you're thinking "Sheesh! That was close! But I'm juuuuust OK to go back on thirds". You head back, and you get tangled or something, and suddenly the gremlins start muttering, "this is gonna be close... this is gonna be close...". But if you'd recalculated thirds you'd know you have plenty of time to sort it out. Since it only took 500 psi to get to the halfway point on the way there, and you have 670 psi to get there with this time through (OK 600 if you insist on doing that rounding thing), you've just bought a nice time margin and know there's less need to panic.

Your high school teacher was right... Math is your friend.

(Oh and for this exercise let's forget all those little holes you stick your head up along the way... each of which once more gives you the opportunity to recalc thirds...)

[OFG-1]

If you stick your head up at pothole each way, is it then 4 dives?

[guru caver]

If you stick your head up at pothole each way, is it then 4 dives?

hey, it's as many dives as you need to feel like a man...