turn pressure calculation

[wedivebc]

After wading through 20 pages of posts unrelated to this topic I thought I would ask:
When I was taught technical diving I was taught turn pressure based on buddy's air consumption, tank volume, volume matching. When I took my cave training we were taught a much simpler method using only buddy with least volume controlled turn pressure regardless of SAC rate. It actually made sense.
I was wondering what agency taught what procedure. Anyone care to comment?

 

[Soggy]

SAC rate does not affect turn pressure calculations. When diving thirds, you determine who's thirds is the smallest number in cubic feet, then you both turn when that *volume* has been reached. The diver with the higher SAC rate will just end up reaching turn pressure earlier. My cave training is through NACD, but I believe this to be the standard practice with most agencies these days.

 

[mwpowell]

IANTD calculates turn pressure based on RMVs. Besides being unnecessarily complicated, the method assumes that both you and your buddy's RMV won't change. The method that Soggy described for calculating turn pressure is both simpler, and doesn't make any assumptions on RMVs.

Mike

 

[Charlie99]

The method that Soggy described for calculating turn pressure is both simpler, and doesn't make any assumptions on RMVs.

Actually there IS one assumption that rule of thirds makes. It assumes that the remaining 2/3 gas is enough for both divers to make it back to the exit point. That assumes that the combined SAC of the two divers under what could be a stressful situation will not be more than 2 x the normal SAC of the higher SAC diver.

Rule of thirds isn't all that conservative, particularly when you consider that incidents tend to happen at transition points, and turning the dive is a transition point.

 

[wedivebc]

SAC rate does not affect turn pressure calculations. When diving thirds, you determine who's thirds is the smallest number in cubic feet, then you both turn when that *volume* has been reached. The diver with the higher SAC rate will just end up reaching turn pressure earlier. My cave training is through NACD, but I believe this to be the standard practice with most agencies these days.

Care to give an example. Say diver A has dual 85s (2650 psi) diver B has dual 119s (3500 psi)

 

[OneBrightGator]

Alright... 2650 rounds down to 2600, which is not divisable by 3, so it goes to 2400, divided by 3 is 800 PSI, if the tanks hold 170 cu. ft. @ 2640 (2640/170=15.5) then 800 PSI equals 51 cu. ft. which is the amount of usuable gas.

3442/238=14.5, 14.5x51=740, rounds to 700 PSI.

So Diver A has a Critical Air Supply (CAS) of 1800 PSI (2600-800)
Diver B has a CAS of 2800 (3500-700)

Now, what I really really do is take the diver using 80/85s (HP100s) and subtract 100 PSI from their third to find thirds for someone using 95s (HP119s) or 200 PSI for someone using 104s (HP130s).

So...
Diver 85s third = 800 PSI
Diver 95s third = 700 PSI
Diver 104s third = 600 PSI

 

[OneBrightGator]

Now, I use Rock Bottom or something similar sometimes (not cave diving), where I calculate the amount of gas needed to get me and my buddy to the surface. I used it alot DMing in the Keys, to "stretch" my gas because using Thirds would have been overly conservative.

 

[Soggy]

Ben beat me to it....

 

[Soggy]

Actually there IS one assumption that rule of thirds makes. It assumes that the remaining 2/3 gas is enough for both divers to make it back to the exit point. That assumes that the combined SAC of the two divers under what could be a stressful situation will not be more than 2 x the normal SAC of the higher SAC diver.

Rule of thirds isn't all that conservative, particularly when you consider that incidents tend to happen at transition points, and turning the dive is a transition point.

I wholeheartedly agree.

 

[wedivebc]

I wholeheartedly agree.

I would too except nothing assures low SAC rates like knowing you have enough gas to get home