Rule of 1/3s and different tank sizes

[Kevrumbo]

I think to a small degree you are also over complicating it. Rule of thirds works perfectly well in a cave system and when you figure out your turn pressures between you and your buddy it does away with the whole SAC this and SAC that. You turn when the first of you hits their turn pressure. This would mean the one with the highest SAC will more than likely be the one to turn the dive. The only way this is complicated to the degree that you are talking about is if you have a specific target to achieve and your high SAC guy simply can not make it with a reasonable amount of gas. I just finished my Intro so I am not uber experienced admittedly but my buddy can go in with double AL80s and I will go in with HP100s bumped to 4000, my 1/6th STILL turns the dive prior to my buddy hitting his 1/6th because his SAC is better. Not to be nonchalant about it but sometimes you can't see the trees for the forest.

Am I overcomplicating it?

Imagine that you and your buddy are diving a cave, using the Rule of Thirds. He's got double AL80s and you've got double HP100s. You "do away with the whole SAC this and SAC that" and go in with your buddy having a TP of 2000 and you have a TP of 2400.

Your buddy's SAC is 0.6 cu-fit/min and yours is 0.8. You both hit your TP at the same time. Also at that moment, as you turn around in the cave, you whack the crap out of your isolator knob on the ceiling and totally break your manifold so that you lose all the gas from both your tanks.

Does your buddy have enough gas left in his double 80s for you both to get out?

Spoiler alert:

No, he doesn't. Even if you both maintain your same SAC. Much less if either of you is suddenly breathing hard.

That is why just using the Rule of Thirds is no good. It does work if you both have the same size tanks (and your breathing doesn't change when you start air sharing - good luck with that!). But, that's such a serious limitation that, as I have said, I don't see why anybody would use the Rule of Thirds in lieu of proper gas planning. If you're complicating it more than saying "my TP is 2/3 of my starting pressure", then it's not the Rule of Thirds anymore and anything other than a full gas plan is a hack that doesn't seem worth the time, to me.

1/3's usablewith RB includes Sac rates, tank factors and other considerations......gas matching is still included.

Yes, and working with a very poor understanding. People have tried very patiently to give you some basics without teaching you a whole course.

Yes it does. Everything is taken into consideration, especially different tank volumes, typical SAC's, and elevated SAC's, and applied to the type of dive environment.

All of this and much more is taught in a GUE Fundamentals course. It is certainly a real gas plan and many people would see the resulting turn pressure as very conservative, especially in a single tank and even in an o/w basic dive. Overhead and deco dives are conducted differently with even more stringent rules.

If an instructor gets an inkling that you are not actually interested in GUE dive planning and are just trying to pass the course, they would be hard pressed to pass you.

I am very open to learning something new! Please tell me where I'm going wrong.

You're calculating min gas as the gas to get you and your buddy from the base of the anchor line to the surface. That factors in SAC and tank sizes. . .

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.

@stuartv , per your example above:

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

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

Let's use an overhead cave with an average depth parameter of 100fsw (4ATA).

You cannot apply straight-on One-Third's of the fill pressures above because of the dissimilar tanks and Sac Rates.

The basic starting solution and approximation which proper initial gas planning "roughly" takes this all into account is to use a simple one-third of the Diver with the smaller volume of backgas, which is Diver A above with the double AL80's (total 154 cuft, because one AL80 really only has 77 cuft of gas at 3000psi; 77 x 2 then is 154); so one-third of 154 cuft is 50 cuft for a TP of 2000psi actual SPG reading for Diver A.

Now you apply this limiting third's turn volume to Diver B and his twin hp100's: 50 cuft corresponds to a TP of 2739psi or round it to 2800psi actual reading SPG. (@stuartv , your example above has Diver B at a lower TP of 2400psi, and Diver A with TP of 2000psi: these third's TP values will not work, and leaves Diver A without enough reserve turnaround pressure if he has to donate and emergency gas share with Diver B --they both will go OOG and not make it out of the cave.)

Strategically, here is the important point on how this limiting third's turn volume works: comparing the given Sac Rates, for every minute, Diver A nominally consumes 0.6 cuft of gas versus 0.8 cuft for Diver B. So Diver B consumes at a higher rate more than Diver A, and therefore will reach his 2800psi TP earlier in time than Diver A. (Another way of interpreting this is that Diver A uses 0.6 cuft per min divided-by 0.8 cuft per min, or 75% the nominal volume of gas used by Diver B over a given interval of time.)

So for example at 100fsw (4ATA), Diver B will reach TP in 15 minutes (Diver A can go to 20 minutes, but Diver B controls & calls the dive at his TP turnaround earlier at 15 minutes because of his higher Sac Rate). Diver B has used 50 cuft from 200 cuft total equals 150 cuft available for an emergency gas share at that point. In 15 minutes, Diver A has used up only 37 cuft of the allotted 50 cuft third's turn volume; 37 cuft from Diver A's 154 cuft total equals 117 cuft available for an emergency gas share at that same turnaround point called by Diver B.

Diver A needs 37 cuft and Diver B needs 50 cuft in an emergency gas share to exit the overhead, for a total of 87 cuft (round it up from 87 cuft to 100 cuft for conservatism). Do they each have enough turnaround reserve volume to make it out of the overhead in an emergency gas share contingency? Yes! Diver B has 150 cuft and Diver A has 117 cuft available, both at the turnaround point called by Diver B.

Lastly and other considerations though, is there enough Minimum Gas Reserve (MGR) in this example after exiting the overhead at 100fsw for both Divers in an emergency gas share contingency to make it to an open water deco stop? For a Nitrox50 bottle switch at 70fsw, it's possible but close to if not completely going OOG on backgas (Diver A may only have 17 cuft left to share with Diver B after using a combined 100 cuft for exiting the overhead), taking into account factors like stress Sac Rate levels; egress delays due to complex navigation/restrictions and zero viz/silt-out conditions; how much usable gas did the team consume in open water just to initially get to the overhead entrance; retrieving deco bottles etc. I would re-calculate for more margin MGR and/or greater conservancy of usable gas volume as well by using Sixth's (or Fifth's, or Fourth's), instead of Third's. . .

 

[AlexL]

Anyway, the OP specifically says "Rule of Thirds". This is the Adv Scuba forum and he did not say "rec thirds".

this got way off topic, and it was not my intent...

OP said 'rule of thirds', both variants are called 'rule of thirds' and which type you're talking about is very context dependent, and gave an example of the non obvious variant. figured OP would jump in details on what exactly knows the math for. i just called it 'rec thirds' to differentiate.

to be explicit if you're actually using a plan with min gas + thirds, i would not consider it a recreational dive.

is 'rec thirds' valid for out and back, basic open water depths, sunny nice weather, warm water, pretty fishes, dives? i said sure, its close enough, and presented how the gas planning system that is freshest in my memory, GUE open water min gas, correlates that statement, with the caveat that its considered extremely conservative by many. what i did not make explicit is that i was comparing min gas + halves vs recreational thirds and did not show the work because 900psi min gas is close enough to 1000psi it would be obvious.

diving with different SAC rates? air hog? different tanks? deeper then 60'? don’t pay attention to gauges? easily spooked? cold? hungover? mars rides at dawn in the company of the seven sisters? all bets are off, and you need to run the numbers in volume vs recorded SAC as mentioned by others.

 

[decompression]

Here's a similar thread.....Kevrumbo also answers with some posts.....

Gas Matching Unequal SAC Rates Along With Dissimilar Tanks

 

[Jack Hammer]

@stuartv , perhaps you should ask your AN/DP Instructor about thirds and how it's actually applied. Typically it involves gas matching which accounts for dissimilar tank sizes and breathing rates. We do this quite frequently in our heads and adjust (reduce usable gas) for several factors. SAC rate becomes irrelevant as the thirds is typically done for cubic feet of gas usable. Also everything is rounded for more conservatism.

In a simplistic example what that means is if we dive together and you breath gently on your doubled al80s filled to 3kpsi and I'm an air hog with lp121s filled to 4kpsi my turn pressure at "thirds" is 3500psi while yours is 2000psi. That's about 50cf each. The first to hit their pressure turns the dive, so you may be at 2600 when I hit 3500 and we turn together. In reality we would add conservatism for several factors and our planned turns would more likely be 2200psi for you and 3600psi for me.

 

[Kevrumbo]

@stuartv , perhaps you should ask your AN/DP Instructor about thirds and how it's actually applied. Typically it involves gas matching which accounts for dissimilar tank sizes and breathing rates. We do this quite frequently in our heads and adjust (reduce usable gas) for several factors. SAC rate becomes irrelevant as the thirds is typically done for cubic feet of gas usable. Also everything is rounded for more conservatism.

In a simplistic example what that means is if we dive together and you breath gently on your doubled al80s filled to 3kpsi and I'm an air hog with lp121s filled to 4kpsi my turn pressure at "thirds" is 3500psi while yours is 2000psi. That's about 50cf each. The first to hit their pressure turns the dive, so you may be at 2600 when I hit 3500 and we turn together. In reality we would add conservatism for several factors and our planned turns would more likely be 2200psi for you and 3600psi for me.

Sac Rates are in fact directly proportional to the gas volume consumed of each diver, and will determine who reaches turn volume with corresponding turn pressure first. Further conservatism is inherently added by taking third's of the diver with the smaller total bottom gas volume in case of a dive team with dissimilar tanks, and applying that limiting third's volume to determine the turn pressures of the other teammates with the different larger sized volumes of bottom gas cylinders.

So SAC Rates are really not irrelevant per se (as @stuartv argues), even though they apparently seem transparent, extraneous or "not needed" to calculating proper limiting Third's Turn Volumes/Turn Pressures. . .

 

[Jack Hammer]

...

So SAC Rates are really not irrelevant per se (as @stuartv argues), even though they apparently seem transparent, extraneous or "not needed" to calculating proper limiting Third's Turn Volumes/Turn Pressures. . .

Why over complicate things?

SAC rate is irrelevant to the initial gas matching calculations for thirds, which is just a baseline starting point. The only relevance it may have is if some one decides they want to take it into consideration for additional conservatism.

 

[Kevrumbo]

Why over complicate things?

SAC rate is irrelevant to the initial gas matching calculations for thirds, which is just a baseline starting point. The only relevance it may have is if some one decides they want to take it into consideration for additional conservatism.

Because @stuartv has to understand, learn and "see" by rote calculation that the Diver with the higher Sac Rate will reach proper limiting third's turn volume/turn pressure first, thus convincing himself there is enough reserve turnaround volume in an initial baseline approximation for both divers to egress an overhead in an emergency gas sharing contingency. You then allot more margin gas volume for conservatism to account for factors such as current/flow, delays due restrictions & complex navigation, possible silt-out, finding & clipping on deco bottles etc.

For initial allocation of MGR volume -which you should actually calculate first and then apply Rule of Third's above to the remaining usable gas- you will first need to figure in a Sac Rate (usually stressed of 1 cuft per min or 30 liters per min), Average Depth in ATA and Time using an ascent rate of 30fpm or 9mpm, to get you from the overhead exit in an emergency gas sharing contingency to the surface, or alternatively your first deco stop depth/deco gas switch. . .

 

[Jack Hammer]

For initial allocation of MGR volume -which you should actually calculate first and then apply Rule of Third's above to the remaining usable gas- you will first need to figure in a Sac Rate (usually stressed of 1 cuft per min or 30 liters per min), Average Depth in ATA and Time using an ascent rate of 30fpm or 9mpm, to get you from the overhead exit in an emergency gas sharing contingency to the surface, or alternatively your first deco stop depth/deco gas switch. . .

It can really seem complex for some one new to it. Minimum gas is a separate calculation from thirds (though in cave they can be somewhat similar (that's a whole 'nother mess)), even though they are related in the gas management family and may be used together, but not always.

Again, why over complicate it for some one trying to grasp the concept? It's just gonna make it seem more confusing. I'm in the start it simple so people can grasp the concept, then expand upon that concept camp. I agree minimum gas should be accounted for and is not part of usable thirds. For most new to the concepts though it adds more math and makes it more difficult.

@stuartv , Kevin brings up a great point that the gas needed to get the team to the surface needs to be accounted for, not just the gas for penetration. You may find that as you get deeper into tech diving that you need A LOT of gas to start, only a little is usable, and that you end dives with a bunch of it remaining, yet you dove "thirds".

 

[Kevrumbo]

It can really seem complex for some one new to it. Minimum gas is a separate calculation from thirds (though in cave they can be somewhat similar (that's a whole 'nother mess)), even though they are related in the gas management family and may be used together, but not always.

Again, why over complicate it for some one trying to grasp the concept? It's just gonna make it seem more confusing. I'm in the start it simple so people can grasp the concept, then expand upon that concept camp. I agree minimum gas should be accounted for and is not part of usable thirds. For most new to the concepts though it adds more math and makes it more difficult.

@stuartv , Kevin brings up a great point that the gas needed to get the team to the surface needs to be accounted for, not just the gas for penetration. You may find that as you get deeper into tech diving that you need A LOT of gas to start, only a little is usable, and that you end dives with a bunch of it remaining, yet you dove "thirds".

@Jack Hammer . . .You have to explain the concept dynamics further -that means doing the math that clearly shows Sac Rates are automatically conserved and implicitly taken into account- because @stuartv does not understand, or misunderstands the use of "Proper Limiting Third's" when he makes the claim in this example (see underline phrases below):

Am I overcomplicating it?

Imagine that you and your buddy are diving a cave, using the Rule of Thirds. He's got double AL80s and you've got double HP100s. You "do away with the whole SAC this and SAC that" and go in with your buddy having a TP of 2000 and you have a TP of 2400.

Your buddy's SAC is 0.6 cu-fit/min and yours is 0.8. You both hit your TP at the same time. Also at that moment, as you turn around in the cave, you whack the crap out of your isolator knob on the ceiling and totally break your manifold so that you lose all the gas from both your tanks.

Does your buddy have enough gas left in his double 80s for you both to get out?
[No because at 100fsw (4ATA) for example, 20 minutes penetration into the cave which is the TP turnaround time, the buddy only has 104 cuft in his double 80's at turnaround, while you both need 120 cuft to get out in this example of the incorrect use of "straight third's." Boldface answer is mine @Kevrumbo ]

Spoiler alert:

No, he doesn't. Even if you both maintain your same SAC. Much less if either of you is suddenly breathing hard.

That is why just using the Rule of Thirds is no good. It does work if you both have the same size tanks (and your breathing doesn't change when you start air sharing - good luck with that!). But, that's such a serious limitation that, as I have said, I don't see why anybody would use the Rule of Thirds in lieu of proper gas planning. If you're complicating it more than saying "my TP is 2/3 of my starting pressure", then it's not the Rule of Thirds anymore and anything other than a full gas plan is a hack that doesn't seem worth the time, to me.

The correct use of the Rule of Third's (or what can also be better called or termed as the "Rule of Limiting Third's" or "Rule of Modified Third's") for this example above is completely explained and justified in post #31 above.

 

[stuartv]

@stuartv , perhaps you should ask your AN/DP Instructor about thirds and how it's actually applied. Typically it involves gas matching which accounts for dissimilar tank sizes and breathing rates. We do this quite frequently in our heads and adjust (reduce usable gas) for several factors. SAC rate becomes irrelevant as the thirds is typically done for cubic feet of gas usable. Also everything is rounded for more conservatism.

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.