Question on 3rd's

[tbone1004]

https://en.wikipedia.org/wiki/Compressibility_factor

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we live in a world of around 300k in temp, which is around 80f. Up to about 2200psi or 150bar, the gasses behave about 1:1. As in, in a 1l volume cylinder, at 150bar, there will be 150 liters of gas in there. Cave fills are 250bar, which means the compressibility factor is around 1.07, so you have 7% less gas than you think you do, so that tank at 250bar, doesn't hold 250 liters of gas, it only holds about 235 liters. So let's say the tank is filled to 240bar for easy math. Instead of 240, it actually holds 225. Since we don't factor that into gas planning, when we go "in" for a third, in this case 80 bar of pressure, instead of breathing 80 liters on that 1l tank, we actually only breathed 65. So, on the way out, where we have "2x the gas needed for exit", you actually have about 2.4x the gas needed to get out. Gives a bit of an extra safety buffer.

Oh, and this is for air, different gasses compress differently, which should be explained in a blending class, especially when you test the gas you blended, and the O2 percentage is off, O2, N2, and He all compress at different rates.

[FW]

The biggest problem breaking thirds in a high flow cave is that the flow itself can seriously get in the way in a low/no viz situation. Sure Devil's is usually clear, but what about about breaking or loosing your mask?

[oya]

Gives a bit of an extra safety buffer.

I like this part.

I'm gonna stick with that and stay within thirds on the annoying occasions I'm blowing bubbles.

[bullfrog]

if you will not break thirds in Ginnie,,Perhaps you should not break sixths in peacock.

[PfcAJ]

Ugh. Thirds isn't just about enough gas to share. It's about time to solve problems.

Breaking thirds doesn't give you much time to sort stuff out. Depending on the cave diving to thirds doesn't, either, but cutting reserve gas to go a tiny bit farther is dumb. Saddle up and take a stage and do it safely.

[tbone1004]

Ugh. Thirds isn't just about enough gas to share. It's about time to solve problems.

Breaking thirds doesn't give you much time to sort stuff out. Depending on the cave diving to thirds doesn't, either, but cutting reserve gas to go a tiny bit farther is dumb. Saddle up and take a stage and do it safely.

I've never understood why we don't calculate adjusted thirds like we tell OW divers to calculate rock bottom... You have gas required to get you back+gas required to get your buddy back+however many minutes you have decided is required to sort things out.

In JB, call it 100ft, 3 minutes to sort out, combined sac of 2.0, need 24cf to sort things out. On HP100's, that's 400psi... On LP121's, it's 250psi. Something we tend to not think about....

[rddvet]

if you will not break thirds in Ginnie,,Perhaps you should not break sixths in peacock.

And thank you for reaffirming that not picking up instabuddies on cdf or at the dive site is a good choice.

[TaraR]

I will break thirds in Ginnie because the flow helps you get out. People here will argue you never break thirds but in my opinion that is safer than diving thirds in peacock. I am not talking a lot, maybe 3-400 psi. it has been argued here before but I am sure it will be argued again.

Maybe you should ask Agnes how well this worked for her in Ginnie when she was borrowing random stages to get her ass out.

Oh wait, you can't...

[ffdiver5597]

Is there any other details of that story about Agnes at Ginnie?


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[Sh0use]

Is there any other details of that story about Agnes at Ginnie?


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Id really like to report the IRAP for the incident if someone has it.