turn pressure calculation

[WarmWaterDiver]

Sorry if I'm being dense. Maybe I need more experience with some of the technical stuff. Maybe I need more experience with math. Whatever, the case, I want to be a better diver, but here goes...

Isn't all of the calculation the same as each diver taking his or her starting pressure, multiplying by 2/3 (e.g. 3000 * 2/3 = 2000) and everyone turns when the first diver hits that figure?

Not if they aren't using identical cylinders, and even if they are, not if they aren't filled to identical pressures. And even if that were true, not if they don't have identical gas consumption rates.

For example, say I carry twin HP 100's as back gas filled to 3500 PSIG while my dive buddy / team mate / wife carries twin HP 80's for her smaller stature also filled to 3500 PSIG. We would calculate based on my gas consumption which is typically about twice as high as hers, and her carried gas supply, as the smaller volume / mass of the two sets of gas carried.

Going back to Charlie99's point on stress, the ANDI text I referenced above mentions this on page 118 for the Imperial system and more in depth material is on pages 136-137.

 

[ItsBruce]

WarmWaterDiver: Thanks. As soon as I see that, the theory makes perfect sense. I just need to get better with the calculations.

 

[wedivebc]

Can you help me understand why someone who is listed as ANDI level 5 trimix instructor is asking this question? Are these fundamentals not part of higher level ANDI educational materials and exercises?

Thank you for posting the information. I was looking for your feedback on what you were taught. I am presently putting together a lesson plan for a course and I am using material from many agencies. I have a mountain of ANDI, TDI,IANTD,GUE and PADI manuals here and I have been asking questions from members of scubaboard. I appreciate your reply.
My copy of Advanced Diving Activities states on page 85 SAC = [Psig Used / minutes elapsed] / [{D / 33}+1] so when you said

our ANDI training used RMV not SAC...

I was a little surprised. I believe you have the more updated version of the book and I will be calling on monday to check this out.
Thanks again.

 

[WarmWaterDiver]

No problem. We enjoyed both ANDI courses we took. My wife compiled a bit of an errata sheet for this edition during our training which I think our instructor has submitted to ANDI HQ. There were some simple math errors in some of the examples in the third edition - shameful for a text from an agency founded by an engineer!

One twist for us. By basing solely off my gas consumption, there is extra contingency for stress etc. for the two of us. However, we could volume match 'by the book' using RMV ratios etc. for 2/3 gas for our two RMV's. As someone else pointed out, there's no real penalty for ending the dive with more than 1/3 gas left - although there are potentially lots of imaginary penalties.

And, just to be clear, ItsBruce. We would still calculate a turn pressure for my back gas, as well. But the predicted / planned turn point would be controlled by the smaller volume of gas she would be carrying.

 

[wedivebc]

I can see that 2640/170 = 15.5, but what does that 15.5 figure represent? And, how does that help in getting to the 51 cu. ft figure?

I really want to get this and need some help.

(Any recommended texts?)

Thanks

The main point of gas matching is that the extra third of gas doesn't belong to you it belongs to your team member(s). The turn pressure needs to be adjusted so there is sufficient gas to get anyone who may have a problem home. It does not matter who has what SAC rate because it only matters that the dive gets turned when the first person reaches the turn pressure determined by whoever is carrying the least volume.
So pre dive we determine that diver A has dual 85s or 170cuft which are filled to 2650psi diver B has 98s or 196cuft filled to 2650. 1/3 or diver As volume is ~57 so we need to determine when diver B has also used 57cuft.
2650/196 = 13.5 psi/cuft so if we multiply 57cuft * 13.5 = 769.5 round to 750
so turn pressure diver B = 2650 - 750 = 1900 Turn pressure for diver A is 2650/3 = 883 round to 850 so 2650 - 850 = 1800
This method does not require graphs charts or need to consider SAC rate because who ever reaches thirds first turns.
I am sure someone will tell me if I botched the figures:06:

 

[Charlie99]

My copy of Advanced Diving Activities states on page 85 SAC = [Psig Used / minutes elapsed] / [{D / 33}+1] so when you said I was a little surprised. I believe you have the more updated version of the book and I will be calling on monday to check this out.
Thanks again.

Although usually context makes it clear what is meant, some people/ some agencies use one term to indicate consumption (nomalized to the surface) in psi or bar/minute, and another term to indicate consumption (normalized to surface) in volume units such as cubic feet or liters per minute.

If you give units, as in CFM or liter/min, or psi/min, then which definition you use for which term doesn't really matter.

 

[OneBrightGator]

I can see that 2640/170 = 15.5, but what does that 15.5 figure represent? And, how does that help in getting to the 51 cu. ft figure?

That's called a "tank factor" or something similar, it's a way to relate the pressure to volume. LP85s are full at 2640 psi, full being 85 cu. ft. a piece, so doubled equals 170 cu. ft. therefore 15.5 means 15.5 psi/cu. ft., so when you divide the amount of "pressure" in a 1/3 of the usuable gas, 800 psi, by that tank factor you get 51 cu. ft.

[psi]/[psi/cu. ft.] = [psi]*[cu. ft./psi] so the pressures cancel out and leave volume. It's part cave/cavern training and part just unit manipulation, so I can't really recommend any texts.

Dave, the only issue I have with your calcs is your use of pressures, we never calculate below "hundreds" of pressure, and always round conservatively, i.e 2650 rounds down to 2600, then to 2400 to get an even turn pressure of 800, but when calculating Critical Air Supply you would round up to 2700, subtract 800 to get 1900. It makes your thirds and CAS easier to remember, and it's safer by two ways (more conservative planning and use of your SPG).

 

[Rick Murchison]

Dave, the only issue I have with your calcs is your use of pressures, we never calculate below "hundreds" of pressure, and always round conservatively, i.e 2650 rounds down to 2600, then to 2400 to get an even turn pressure of 800, but when calculating Critical Air Supply you would round up to 2700, subtract 800 to get 1900. It makes your thirds and CAS easier to remember, and it's safer by two ways (more conservative planning and use of your SPG).

Hmmm... lessee now... if I were to round up to 2700 (the nearest hundred above 2640 divisible by 3) and figure thirds from there, I'd turn at 2700 - 900 or 1800... and the way I figure it 1800 is more than 2/3 of 2640. Ain't it? This method works for any pressure ... and is even easier to calculate and remember than dealing with the pressure below as well. I must admit, though, that I do go to the trouble of figuring things both ways, so for an actual pressure of 2500 I'd figure 1/3 at 800 - and then subtract from 2500 to get 1700, which I'd use in that case. So, from 2401-2500 I'd use 1700, from 2501-2700 I'd use 1800. There's just no sense in going up (in this case from 1800 to 1900) on turn pressure when you go down (from 2700 to 2640) on start pressure.
Rick

 

[ItsBruce]

Thanks. I'm sitting here ruminating on the math and feel like I'm narced.

With as much open ocean as there is, I'm not sure I'll ever need to go into a cave. However, I'm so intrigued by the planning and math that I'm definately going to learn about it.

 

[Charlie99]

Thanks. I'm sitting here ruminating on the math and feel like I'm narced.

Go grab a bottle of trimix. It sounds like you are narc'd at 0.79ppN2 and greater. :banana:


Of course, if you were doing the calculations using metric method of describing tanks by water volume, then everything would be much easier.