Rule of 1/3s and different tank sizes

[bax737]

dissimilar thirds is VERY easy to do in excel. I have a sheet that I wrote in college that does it for me as well as cascade calculations and all sorts of other nifty scuba math to have it as convenient. You can either set it up for a "smaller" and "bigger" side, or just use an if statement to where it doesn't matter.

You have it much easier in metric for running the calculations since you measure your tanks directly.
essentially you have inputs of
tank size
tank fill pressure

the output is then going to be your turn pressure which is fill pressure minus a third of the smaller tank. Super super simple formula to write in excel if you know how to do it.

Third option is to print a table with the common tank sizes on each axis. The bit in the middle will be their turn pressures assuming they're filled to working pressures. Print them, laminate them, stick them in your wetsuits. Then you run the smaller tanks down the y-axis, and then over to the bigger tanks along the x-axis, and you have the turn pressure for the bigger tanks *smaller will always just dive to thirds*. Easy peasy

Thank you !!this is what im after

 

[bax737]

Dissimilar tank sizes - rule of thirds - excel calculations - that really was what i was after

 

[Kevrumbo]

dissimilar thirds is VERY easy to do in excel. I have a sheet that I wrote in college that does it for me as well as cascade calculations and all sorts of other nifty scuba math to have it as convenient. You can either set it up for a "smaller" and "bigger" side, or just use an if statement to where it doesn't matter.

You have it much easier in metric for running the calculations since you measure your tanks directly.
essentially you have inputs of
tank size
tank fill pressure

the output is then going to be your turn pressure which is fill pressure minus a third of the smaller tank. Super super simple formula to write in excel if you know how to do it.

Third option is to print a table with the common tank sizes on each axis. The bit in the middle will be their turn pressures assuming they're filled to working pressures. Print them, laminate them, stick them in your wetsuits. Then you run the smaller tanks down the y-axis, and then over to the bigger tanks along the x-axis, and you have the turn pressure for the bigger tanks *smaller will always just dive to thirds*. Easy peasy

Thank you !!this is what im after

Dissimilar tank sizes - rule of thirds - excel calculations - that really was what i was after

Here's an attached old Excel file (Imperial Units though and not Metric -nor figuring in dissimilar tank sizes or Rule of Thirds), that can get you started with Rock Bottom/MGR. . .

 

[CptTightPants21]

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. And All Usable and Half Usable. Then he taught me not to rely on simplistic rules and to actually do the math, instead.

When I plan a dive, I calculate the Rock Bottom gas requirement as the amount of gas needed to get myself and my buddy from the furthest point in the dive to the surface or first gas switch. I use RMV #s that reflect our actual RMV #s adjusted for stress (I.e. Usually I use a high estimate of each of our normal RMVs and then double the total). I allow time at max depth for resolving issues (usually 1 to 3 minutes depending on anticipated conditions). I allow estimated time for swimming back to the anchor line at max depth (if the dive will require it, and based on the size of the wreck or whatever other factors would be relevant to the specific dive). Then, I allow for the time required to ascend at 30 fpm, and including any required deco stops.

The result is the amount of gas (in cubic feet) that my buddy and I each need to have as our Rock Bottom volume. Each person then determines their own RB pressure based on the size of tanks they are carrying.

At that point, the RB is also the Turn Pressure. As should be obvious, if I'm using different size tanks, my TP will be different than my buddy's.

As should also be obvious, a properly planned and executed deco dive would (almost) never result in either diver actually hitting their RB as their cue to begin the exit/ascent. It's a back stop to the planned run time. If you hit your RB before you get to your planned bottom time, then something is definitely wrong (though possibly just with your planning) and it's time for an early exit/ascent.

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

So run me through one of your calculations for a deco dive at 130ft. You can pick run time and tank size.

 

[victorzamora]

Dissimilar tank sizes - rule of thirds - excel calculations - that really was what i was after

One thing that trips most people up is that "thirds" isn't really on pressure but on useful volume. To run through the basics with an example:

Diver1 has twin HP100s at 3600psi. That's ~210cu.ft of gas. Thirds means Diver1 can use 70cu.ft of gas.
Diver2 has twin LP121s at 3600psi. That's ~330cu.ft of gas. Thirds means Diver2 can use 110cu.ft of gas.

If they're diving together, Diver1 and Diver2 can each only use 70cu.ft of gas before turning. Diver1 has smaller tanks, so can turn at 2400psi. Diver2 has to turn short of thirds, at 70cu.ft used. That pressure is ((330-70)/330)*3600) 2840psi.

The reason for that? If Diver1 and Diver2 swim away from their exit point (something like a cave, no flow, etc) using 70cu.ft each and Diver2 magically loses all gas, they'd have theoretically exactly enough gas to swim back. Diver1 would use 70cu.ft on the way in and be left with 140cu.ft for the exit. Diver1 and Diver2 would each use 70cu.ft for exit, and they'd exit with exactly empty tanks. That's assuming the same SAC or Diver1 reaches 70cu.ft first.

If we do the "normal" thing and give the diver with the higher SAC the bigger tanks, and they both hit 2400psi at the same time, then Diver2 has used 110cu.ft of gas to get there and would need at least 110cu.ft to get out plus Diver1's 70cu.ft needed to exit. Diver1 only has 140cu.ft in reserve. At least one of them is going to be left short on gas.

The keys to take away: Gas matching means using the volume of the smallest tanks' "thirds"....not at similar pressures. Remember also that gas volume is mostly linear at these pressures (less so at 300bar) but that volumes and pressures are nearly direct ratios of each other.

Also: I just noticed that you spoke of metric in the beginning. That makes this entire thing easier as you lose one step in your math...but is otherwise the same. Just figure out total liters useful for the smallest tanks and work out the pressure that volume reflects out of the bigger tanks.

 

[kensuf]

nevermind

 

[Kevrumbo]

One thing that trips most people up is that "thirds" isn't really on pressure but on useful volume. To run through the basics with an example:

Diver1 has twin HP100s at 3600psi. That's ~210cu.ft of gas. Thirds means Diver1 can use 70cu.ft of gas.
Diver2 has twin LP121s at 3600psi. That's ~330cu.ft of gas. Thirds means Diver2 can use 110cu.ft of gas.

If they're diving together, Diver1 and Diver2 can each only use 70cu.ft of gas before turning. Diver1 has smaller tanks, so can turn at 2400psi. Diver2 has to turn short of thirds, at 70cu.ft used. That pressure is ((330-70)/330)*3600) 2840psi.

The reason for that? If Diver1 and Diver2 swim away from their exit point (something like a cave, no flow, etc) using 70cu.ft each and Diver2 magically loses all gas, they'd have theoretically exactly enough gas to swim back. Diver1 would use 70cu.ft on the way in and be left with 140cu.ft for the exit. Diver1 and Diver2 would each use 70cu.ft for exit, and they'd exit with exactly empty tanks. That's assuming the same SAC or Diver1 reaches 70cu.ft first.

Agreed with an added comment: The above shows to a first approximation, that they can egress the cave at turnaround max penetration point while emergency gas sharing using the smaller Third's Turn Volume of Diver 1's 70cu.ft -->all usable of Diver 1's entire gas supply of 210cu.ft though, and the problem then becomes no gas remaining to get both divers to the surface from the cave's exit.

Now you have to figure in a Minimum Gas Reserve (MGR): This is a separate reserve volume based on depth and ascent strategy to allocate some amount of Diver 1's 210cu.ft in order to get them both sharing gas from the cave exit/entrance to the surface (or a first intermediate deco stop and deco gas switch). Then re-calculate a new Third's Turn Volume based on this revised usable amount of Diver 1's volume supply after subtracting out the MGR. (The same MGR value applies to Diver 2's gas supply of 330cu.ft for a lower revised usable amount as well). With cave conditions of flow, or complex navigation & restrictions, deteriorating viz considerations etc, you may have to factor in and use even a Sixth's Turn Volume for greater conservatism & margin (or as another option use Stage Cylinders ).

. . .If we do the "normal" thing and give the diver with the higher SAC the bigger tanks, and they both hit 2400psi at the same time, then Diver2 has used 110cu.ft of gas to get there and would need at least 110cu.ft to get out plus Diver1's 70cu.ft needed to exit. Diver1 only has 140cu.ft in reserve. At least one of them is going to be left short on gas.

The keys to take away: Gas matching means using the volume of the smallest tanks' "thirds"....not at similar pressures. Remember also that gas volume is mostly linear at these pressures (less so at 300bar) but that volumes and pressures are nearly direct ratios of each other.

Also: I just noticed that you spoke of metric in the beginning. That makes this entire thing easier as you lose one step in your math...but is otherwise the same. Just figure out total liters useful for the smallest tanks and work out the pressure that volume reflects out of the bigger tanks.

Agree, which is why I prefer metric system because the "Tanks' Factor" nomenclature is always identified and referred to upfront: that is for example, "D11" refers to Double 11L/bar cylinders (Tanks' Factor of 22L/bar total) --which is an AL80's twinset in US Imperial. Max fill pressure 200bar.

A "D12" is Double 12L/bar cylinders (TF of 24L/bar total) --which is a steel HP100's twinset; a "D15" is Double 15L/bar cylinders (TF 30L/bar total) --a Faber HP120's twinset or a Faber HP117's twinset; a "D17" is Double 17L/bar (TF 34L/bar total) --Faber HP133's twinset, etc. All fill pressures to 230bar, or a cave fill to 300bar.

 

[stuartv]

So run me through one of your calculations for a deco dive at 130ft. You can pick run time and tank size.

I'm actually working on my plan for diving the Oriskany next month, so how about my plan for that?

I want to go to the hangar deck. Pending better info, I'm planning for a max depth of 170'. I got my tanks filled before I tried to go to the O back at Thanksgiving, and they've just been sitting, so I'll still use them. When I got them filled, I requested TX21/20, but after settling they test at 20/24. That is acceptable to me. I'll be diving with 1 deco gas and the shortest total run time comes from using EAN70, so I'll go with that. Double HP120s on my back and an AL80 for deco gas.

The plan is for 23 minutes @ 170'. Using GF50/80. My usual RMV at depth is around 0.7, in a drysuit, doubles, with a deco bottle. For conservatism, I'm planning for myself at 1.0 on the bottom and 0.6 during deco. I expect conditions to be "better" than what I usually dive, so I expect my actual RMV to be a little better than 0.7. Somewhere around 0.55 - 0.6.

Total gas required is 158 cu-ft of 20/24 and 26 cu-ft of 70%.

The Oriskany is big and we're going to try and swim to the bow, going along at the hangar deck level - not planning for penetration. That would be my furthest point of the dive. Based on the size of the wreck and the position of the anchor line at the top of the tower, I'm allowing 10 minutes to resolve a problem and swim back to the tower to begin an ascent. Because our normal SAC rates are 0.7 (or less) and because we could easily ascend to the flight deck and swim back to the tower at 140', I'm planning for a combined OOA SAC of 1.5 and still planning the exit at 170' back to the tower. If I knew I had to stay at max depth for the swim to the exit, then I would plan with a combined SAC of 2.0. (And yes, I realize that 1.5 @ 170' is not as conservative as 2.0 at 140', but right now I'm feeling like it is adequately conservative based on the dive site and anticipated conditions)

Running the #s in Multi Deco, I need 125 cu-ft in my tanks to make that swim from the furthest point back to the tower and ascend to 40' (after already having 13 minutes of BT at 170, and while sharing air), where we can switch to our 70% deco gas. In my tanks, that works out to 1800 psi of gas. Considering the last 200 psi to be unusable, I would go with a TP of 2000 psi. The 125 # is calculated by Multi Deco for me. It's roughly 92 cu-ft for the swim and the rest is for the ascent to 40'.

Once you run a plan in MD, there is a tab that will show you every leg of the dive and ascent and the gas required for each leg. To get it to calculate my reserve required, I change the dive parameters to set the RMV to be my emergency combined RMV (in this case, 1.5). And I break the bottom time apart into 2 legs at max depth. The second leg is the amount of time I am estimating for combined emergency resolution and swim to the anchor. In this case, 10 minutes. The first leg of bottom time is whatever remaining time there is. So, in this example, the first leg is 13:00 at 170' and the second leg is 10:00 at 170'. The first leg is calculated by MD to include an initial descent of approximately 3 minutes. I run the plan this way and it tells me a total amount of gas required (234 cu-ft of TX20/24), and the gas required for each leg. I subtract the gas required for the descent and the first leg (15.7 and 93.7) and what's left is the gas required for the "emergency" leg and the ascent (~125 cu-ft).

My buddy would dive the same gases as I am and also reserve 125 cu-ft of back gas for an OOA event. He would calculate his TP based on what actual tanks he's using.

I have used the term Turn Pressure here, but I don't feel like that term is well-used. It is not the pressure at which I would be planning to turn around and start my swim out. It is the absolute lowest pressure I would allow myself to get to before heading for the anchor line. But, if the dive is proceeding normally, I should and would be turning to start heading back towards the anchor line way before I get there. Probably somewhere around 2700 psi. If I actually used 1500 psi out of my tanks (which started at 3500psi) before I made the turn and started to swim back from the furthest point, then that would indicate I am somehow using way more air (around twice as much) as what I had planned for. If my buddy went OOA right at the turn, and my excessive consumption continued, we would not have enough gas to get to our first gas switch. So, on a dive like this, with a planned out and back, I would also calculate a way point of my turn around which would be around 13:00 elapsed dive time and whatever tank pressure I calculate that I should be at, at that point. (Quick calculation says I should actually plan to turn around before I get below 2500 psi, and no later than 13:00 EDT)

This plan is not final. It's just my current iteration. All the #s are subject to change pending review by my buddy, more info on the dive site, etc.. I am definitely still debating adding a bit more conservatism to the gas reserve plan, but want to talk it over with my buddy before I spend any more time on it.

I know I am coming across as a know-it-all and I don't mean to sound that way. If there is something wrong with the process I'm using to determine my numbers, I would LOVE to know it! It is my life on the line, after all. If you see something I am doing wrong, please explain it to me, if you're willing to give me the time.

 

[stuartv]

ps. I will probably also calculate the required reserve for being at the anchor line, at 170', and starting the ascent. That way, if I am heading back to the anchor line, my waypoint and TP from the bow are not my only data point. If I get back to the anchor line early, I'll have those additional numbers I can use to support hanging around on the bottom a little longer, if I want to.

EDIT:

pps. This plan is also based on the knowledge that if we HAVE to, we CAN shoot a bag and do a direct, free ascent.

 

[victorzamora]

Agree with added comment

I agree with your comments and do not feel that thirds is conservative enough in most cave situations. I was simply trying to show how one would go about calculating idealized thirds with dissimilar tanks. SAC rate doesn't matter, it's just volumetric reserve.