turn pressure calculation

[WarmWaterDiver]

Our ANDI training used RMV, not SAC, and we were taught to use rule of thirds for enough gas for both divers.

 

[WarmWaterDiver]

Double-posted - deleted

 

[theskull]

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.

True, but with some buffer as well. If a cave dive, a majority of the time you will be exiting with the flow having swam in against it, and therefore have an additional margin of error. If open water, you most often will have descended, swam around a little, and if you must now share air you can make a direct ascent--getting to the surface being more important than returning over exact route or getting back to the descent line. Also, in both cases you are done sight-seeing and are focused on the task of exiting/ascending without dawdling.

That said, thirds is still the limit--not the goal. There is no penalty for ending the dive with more than the planned amount of breathing gas!

theskull

 

[wedivebc]

Our ANDI training used RMV, not SAC, and we were taught to use rule of thirds for enough gas for both divers.

So how does ANDI define RMV?

 

[Charlie99]

True, but with some buffer as well. If a cave dive, a majority of the time you will be exiting with the flow having swam in against it, and therefore have an additional margin of error. If open water, you most often will have descended, swam around a little, and if you must now share air you can make a direct ascent--getting to the surface being more important than returning over exact route or getting back to the descent line. Also, in both cases you are done sight-seeing and are focused on the task of exiting/ascending without dawdling.

That said, thirds is still the limit--not the goal. There is no penalty for ending the dive with more than the planned amount of breathing gas!

I agree with all of the above. I just threw out my comment because I've seen many posts that sound like the poster thinks rule of thirds is really conservative. It isn't if you are using it in a case where you really do need to make it back to an exit point. There definitely is no allowace for increased SAC like there is in rock bottom calculations.

 

[Jason B]

True, but with some buffer as well. If a cave dive, a majority of the time you will be exiting with the flow having swam in against it, and therefore have an additional margin of error. If open water, you most often will have descended, swam around a little, and if you must now share air you can make a direct ascent--getting to the surface being more important than returning over exact route or getting back to the descent line. Also, in both cases you are done sight-seeing and are focused on the task of exiting/ascending without dawdling.

That said, thirds is still the limit--not the goal. There is no penalty for ending the dive with more than the planned amount of breathing gas!

theskull

True and also keep in mind that when calculating our thirds, we did a crap load of rounding down which also puts more gas in our tanks that we don't have included in our plan.

 

[Diver0001]

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?

Maybe I'm simple, but I don't understand what's so complicated about it...

If we're diving 1/3rds then I have to turn at 132 bar, Michael, who has a totally other config has to turn at 200 bar..... We all know this.

When we plan a particular dive then all the divers on the team email the other divers with their preferred bottom times for that particular dive. We all make plans for the shortest bottom time plus 25% longer times (for example, what's common for us is 60 and 75 mintues).

R..

 

[WarmWaterDiver]

So how does ANDI define RMV?

I find this a bit odd, considering what you list in your Scubaboard profile ("Certification Level: ANDI level 5 trimix instructor sport kiss and inspiration diver, intro cave
Dive Classification: Instructor/Assistant Instructor/DM"), but I'm game if you are.

I'll reference the ANDI text Advanced Diving Activities, Third Edition, September 10,1999 (available for purchase through your neighborhood ANDI Instructor - perhaps a more recent edition is available now). This was the text for our TSD-3 certification course.

I'll not do too much copy & paste, I'll just list the formulae themselves.

For RMV,

Surface RMV = [(Working Volume / Working Pressure)] * [psig per minute scr]

where scr was defined on page 107 as

scr = [psig used / t] / [(D / 33) + 1]

where
t = minutes elapsed time at depth
D = depth in fsw
psig used = difference in cylinder pressure, beginning and end of swim

So, scr is analogous to SAC

Then, on page 108, RMV at depth is defined as

RMV at depth = [(D / 33) + 1] * [Surface RMV]

Page 156 lists the cylinder data, and the rest of page 108 and continuing on through pages 109-121 teach gas volume matching using the Imperial system. For how to apply this methodology using the metric system, use pages 122-131 for this same information starting with scr and RMV.

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?

 

[ItsBruce]

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?

 

[ItsBruce]

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.

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