Rule of 1/3s and different tank sizes

[Doby45]

As I've been saying, I don't see any place for the Rule of Thirds, except maybe in simple, single tank recreational dives.

Well, it is used every single day in cave diving and I would not count those dives as single tank recreational dives.

 

[NAM001]

I think your idea of a program to figure this is moot. you have 3k in a warm tank and then you get in the water (cold) and the pressure drops to 2700 lower. You need to do this on the flyyou do a pressure check prior to entry and gage your 2/3 psi at that time. This is like tryng to figure what psi everyone will turn at. the first to get to 2/3 calls turn point even if everyone else has 80%. Given all else being equal the smallest tank is the controlling psi.

 

[tbone1004]

@stuartv rule of thirds is for penetration dives. For me, that can be actually going into an overhead environment, i.e. wreck or cave, but could also be for shore diving where you can't, or don't want to surface anywhere but the shore. In non-penetration dives, rule of thirds has no real benefit and rock bottom should be used instead.

for penetration dives, we use thirds because it is based on a conservative reality. It is intended to be the most aggressive gas management plan used, reserved essentially for easy, high-flow caves. I.e. Little River, Ginnie Springs etc.
The rationale is that if you have done proper gas matching, whomever hits their third first, will have the higher SAC rate since you have allocated the same volume of gas for each diver to use. Whomever finishes that volume first, turns the dive.
At least in theory, you will have a faster exit than you had entry, assisted by the flow, as well as the lack of time/effort spent tying in primary lines and jumps. That faster exit should reduce your consumption, but you also have a bit of a buffer because the other diver used a full third of that gas you are sharing, but you didn't, so you have a bit more to work with.

If you have different tanks, then there is a chance that you end up together on the larger tanks where you have extra gas buffers, but most divers I know who dive in a regular team together choose to dive the same size tanks for convenience.

 

[Jim Lapenta]

There is also the fact that in a 3000 PSI cylinder you don't have 3000 available. You have 2500-2700 depending on the reserve you want to have in the cylinder to account for SPG inaccuracy or the fact that, if you don't own your own tanks, the op says you better be back with 300-500 PSI.
With my students, and the calculations are all in both of my books, to be conservative they have 833, 833, and 833 available to be precise but rounding to 900 if you get a good fill or 800 if the cylinder has less than 3000 to start, they are told to allow 833 for the descent and swim, 833 for the return, and 833 for emergencies. Which actually results in 1133 - 1333. Now it's true that is very conservative for OW diving but that's ok.
They also get told that if they are back at the mooring line or shallows near the shore, it's fine to look around and see all the things they missed when they started to head out to see the reef, wreck, shopping cart with the naked mannequin, etc.
For myself, having had the experience of an SPG that was off by 300 PSI, I still do not count that 300-500 PSI in my cylinders as part of the supply available to me.

 

[Kevrumbo]

I don't think I need to ask my instructor because I know what he taught me and, as his DM candidate right now, I sat in on a dive planning seminar today with his current Trimix and Helitrox classes. I think I am pretty fresh on what he teaches.

He taught me what Rule of Thirds is. . .

As I've been saying, I don't see any place for the Rule of Thirds, except maybe in simple, single tank recreational dives.

Calculate a basic reference gas plan then -by the way you understand it- and show "as you've been saying" why you think proper Rule of Thirds won't work.

For this initial first approximation, don't figure in other factors just yet (i.e. like stress Sac Rates, allocating Minimum Gas Reserves etc) -all we want to see to start, is if two divers in an emergency gas share contingency have enough to make it out of the overhead using limiting Third's turn volume, by the way you learned it from your AN/DP Instructor.

Use the example and parameters you initially gave above for an overhead:

Diver A has an SCR of 0.6 cuft per minute and is diving on twin AL80's charged to 3000psi.

Diver B has an SCR of 0.8 cuft per minute and is diving on twin hp100's charged to 3600psi.

Use a depth of 100fsw (4ATA), or keep it simple for yourself and just use 1ATA.

[If you don't understand @stuartv , then just admit it. . . we'll show you how to explain it back to your Instructor ]

 

[Ayisha]

In non-penetration dives, rule of thirds has no real benefit and rock bottom should be used instead.

It may depend on where you dive, even if it is technically not a penetration dive. For example, if you dive on a shipwreck in the St Lawrence river here, you may need to exit on the mooring line where you entered or you may end up in boat traffic or the shipping channel and keep flying. On a deeper dive where the exit point is the same as the entry point, again a mooring line, even if there's no current, it can be similar.

While it may be ok to simply use minimum gas and plan that no more than half the remaining gas is usable, it may be prudent to treat such a dive as a virtual overhead and consider that only 1/3 is usable. Some will say that such dives are not really considered open water, but they are simply considered advanced dives here.

 

[Ayisha]

If the answer is that "1/3 Usable" is not as simplistic as I wrote and that you actually calculate how much you can use in a more complicated way, that factors in different SACs, different tank sizes, etc., then I submit to you that at that point you are doing full-on gas planning and calling it "1/3 Usable" doesn't actually make it "1/3 usable". And you're just saying the same thing as I have been saying all along - i.e. that simplistic rules like the Rule of Thirds are pretty much useless and you need to do formal gas planning.

Calculations that take into account all the aspects we've been discussing (tank factors, tank volume, dissimilar tanks, SAC rates of each member of the team, etc.) are required to determine minimum gas. That is where all the math comes in. Once minimum gas is determined, then the remaining gas is determined to be all usable, half usable or one-third usable. Like we've said before, most people would consider minimum gas to be very conservative. The original rock bottom calculations were not nearly as conservative as minimum gas is. Then rock bottom was revised in the mid-2000's or so IIRC, but it still wasn't as conservative as minimum gas. Not sure how it compares now.

 

[NAM001]

I don't use 1/3 unless I can not go directly to the surface. Open water I use more of a rock bottom process. 100 ft I can stay till 1000 I know that's not RB but the point is that I my return path does not require another 1/3 till I get to a place I can directly ascend. 2 cases 2 methods both correct for their purpose. Also if I were in a cavern and was a minute or so form the exit I would not leave at 2/3 either.

 

[tbone1004]

@Ayisha those would fall under the shore diving rule for me, but it definitely falls under advanced diving with specific gas planning requirements and arguably some training involved for actually moving around while air sharing. Most agencies don't teach you to actually travel when you are sharing air, and most rigs don't really allow you to do it comfortably or really at all, so it definitely doesn't fall under "recreational" diving to me

 

[Kevrumbo]

Calculations that take into account all the aspects we've been discussing (tank factors, tank volume, dissimilar tanks, SAC rates of each member of the team, etc.) are required to determine minimum gas. That is where all the math comes in. Once minimum gas is determined, then the remaining gas is determined to be all usable, half usable or one-third usable. Like we've said before, most people would consider minimum gas to be very conservative. The original rock bottom calculations were not nearly as conservative as minimum gas is. Then rock bottom was revised in the mid-2000's or so IIRC, but it still wasn't as conservative as minimum gas. Not sure how it compares now.

Ok, @stuartv , this is where all the canonical math comes in:

Here's a comprehensive Open Circuit gas plan/turn pressure exercise for a planned wreck penetration at 30 meters depth (4ATA) for 40 minutes bottom time on Nitrox32 and O2 deco. In this case, the plan is complicated by dissimilar tank sizes and unequal breathing rates between the Buddy Pair.

Given Diver A:
12L twinset (total 24L/bar); 230 bar fill; nominal SCR (Surface Consumption Rate; also known as SAC rate/RMV) of 9.9L/min;

Diver B:
15L twinset (total 30L/bar); 230 bar fill; nominal SCR of 19.8 L/min (twice as much SCR as Diver A).

Solution:
First start with Minimum Gas Reserve (MGR) Volume -a "Rock Bottom" Calculation Sum from 30 meters or 4ATA depth- (assume emergency stressed 30 L/min SCR for both divers with a 9m/min ascent rate), to reach an Open Water O2 Deco Stop at 6 meters, after exiting wreck:

4.0 ATA x 2 min x 30L/min = 240 Liters. [Two minutes to retrieve O2 deco bottles];
3.7 ATA x 0.3 min x 30L/min = 33.3 Liters
3.4 ATA x 0.3 min x 30L/min = 30.6 Liters
3.1 ATA x 0.3 min x 30L/min = 27.9 Liters
2.8 ATA x 0.3 min x 30L/min = 25.2 Liters
2.5 ATA x 0.3 min x 30L/min = 22.5 Liters
2.2 ATA x 0.3 min x 30L/min = 19.3 Liters
1.9 ATA x 0.3 min x 30L/min = 17.1 Liters
1.6 ATA x 2 min x 30L/min = 96 Liters. [Two minutes to switch to O2 Deco bottles].

Total Liters: 512 L for one diver.

Total MGR backgas volume for 2 Divers: 512 x 2 = 1024 L;

Diver A Pressure MGR reading: 1024 L divided-by 24 L per bar = 43 bar; Round this up to and use 50bar actual reading instead on the SPG for conservatism.

Diver B Pressure MGR reading: 1024 L divided-by 30 L per bar = 34 bar; Round this up to and use 50bar actual reading instead on the SPG for conservatism.

Diver A Usable Gas: 230bar - 50bar = 180 bar. So 180bar x 24L/bar = 4320 Liters usable;

Diver B Usable Gas: 230bar - 50bar = 180 bar. So 180bar x 30L/bar = 5400 Liters usable.

Now Apply Proper Limiting Rule of Thirds:

Tank Match Starting Reference is to Diver A since he has the lowest and limiting amount of usable gas (4320 Liters). One-Third of 4320L is 1440L which is 60bar for an actual SPG Turn Pressure reading of 120bar for Diver A (180bar usable minus 60bar = 120bar).

Calculate Diver B turn pressure: now apply limiting Third's turn volume of 1440L to Diver B, divided-by his tank value of 30L/bar. So 1440L divided-by 30L/bar = 48 bar; 180bar usable minus 48bar = 132bar actual SPG Turn Pressure reading for Diver B.

Diver B will reach the turn volume of 1440L in 18min; (1440L divided-by 19.8L/min, and then divided-by 4ATA equals 18 minutes.). Because of having a lesser SCR that is 50% of Diver B, Diver A uses only half of his allotted 1440L turn volume in 18min, which is 720L

They both need a combined sum of 1440 plus 720 equals 2160 liters total needed to exit in an emergency gas sharing contingency at the turnaround: Check -->Do they have enough usable Third's turnaround reserve gas volume?

Diver A: 4320L usable minus 720L consumed equals 3600L available for an emergency gas share (2160L is needed, check -yes).

Diver B: 5400L usable minus 1440L consumed equals 3960L available for an emergency gas share (2160L is needed, check -yes).

Finally, if more conservatism is required, then use Fourth's, Fifth's or even Sixth's Volume of the diver with smallest total usable gas (Diver A), and/or use an intermediate Nitrox50 deco gas switch at 21m --instead of a delayed traveling ascent on backgas from 30m to an O2 deco gas switch way up there shallower at 6m-- in order to provide more "Rock Bottom margin" for a MGR backgas volume of 1024 liters.