Question on 3rd's

[shellmoundal]

I had a discussion with a very experienced cave diver and this is what I was told. When diving in a system like Peacock always turn on 3rd's (Low Flow). When diving in a system like Ginnie you can dive beyond 3rd's. (High Flow).
Personally I have experienced the fact that I am more tired on a long dive in Peacock compared to the same distance in Ginnie. I am strictly speaking of swimming and not using a DPV in Ginnie.
What is you opinion? Does the rule of thirds apply to you when diving no matter what system?

[tbone1004]

I believe the rule of thirds was pointed at high flow caves, Ginnie/LR/JB etc and was mentioned that thirds wasn't conservative enough for low/no flow caves because it doesn't allow you any time to figure anything out. My rule in no flow, with mixed teams, with just backgas is thirds+2. If in a full sidemount team I will dive to thirds.

[bullfrog]

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.

[bullfrog]

Another point some people don't realize is if you fill your tanks to 3600-3900 psi, when you turn you actually have more than two thirds to get out because gas volume is not linear at those pressures.. Try this experiment . make a dive to thirds and return. after a surface interval use the same tanks without topping the off and make the same dive, and see how far you get. You will be surprised to see how far you can go. You should only be able to goo as third as far. But try it!

[Kelly Jessop]

I will break thirds in Ginnie because the flow helps you get out.

I have heard the flow rationale, and there is always good viz because of flow. I have seen large sections of main conduit in Ginnie get blown out by poor scooter divers, and reduce viz to 3'. I have seen some rock movement at JB that required me to swim from 2000ft on the line into the cavern because of 0 viz. I will never forget the team at Ginnie that compromised their 3rds due to flow, and if it wasn't for encountering a team near the Hill 400 jump, they would have drowned.

[Jack Hammer]

Diving beyond thirds is a dangerous and stupid habit. Depending on flow to get you out should not be part of the gas management plan and sure as hell shouldn't be the emergency contingency plan.

Sent from my SM-T810 using Tapatalk

[oya]

Another point some people don't realize is if you fill your tanks to 3600-3900 psi, when you turn you actually have more than two thirds to get out because gas volume is not linear at those pressures.. Try this experiment . make a dive to thirds and return. after a surface interval use the same tanks without topping the off and make the same dive, and see how far you get. You will be surprised to see how far you can go. You should only be able to goo as third as far. But try it!

I'm not sure that's quite how compressed gas works, man.

[tbone1004]

it actually is.... The compressibility isn't linear, so there are more cubic ft in the first 1200psi than the last 1200, so the third we plan on breathing is less than a true third. The actual amount is not something that I have calculated, but have chosen not to violate that rule and choose to use it as an extra buffer

[rddvet]

I had a discussion with a very experienced cave diver and this is what I was told. When diving in a system like Peacock always turn on 3rd's (Low Flow). When diving in a system like Ginnie you can dive beyond 3rd's. (High Flow).
Personally I have experienced the fact that I am more tired on a long dive in Peacock compared to the same distance in Ginnie. I am strictly speaking of swimming and not using a DPV in Ginnie.
What is you opinion? Does the rule of thirds apply to you when diving no matter what system?

This is why being a "very experienced cave diver" or a "big name" doesnt mean squat.


Sent from my iPhone using Tapatalk

[oya]

it actually is.... The compressibility isn't linear, so there are more cubic ft in the first 1200psi than the last 1200, so the third we plan on breathing is less than a true third. The actual amount is not something that I have calculated, but have chosen not to violate that rule and choose to use it as an extra buffer

I'm going to be thinking about this quite a lot for the next few days and having a really hard time understanding it the whole time.

Glad I went closed circuit where if I use more than 10 cubic feet of anything it's weird. Things are just simpler that way and I'm not that smart.