Rule of 1/3s and different tank sizes
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AlexL
Contributor
sorry, snuck in an edit about 'thirds' vs 'thirds' in my prior post. posted in adv and not basic, so it could go either way.
min gas gets you and a buddy to the surface, min gas + thirds gets you and buddy back to the line and to the surface, 'thirds' (reserve 1/3 for min gas, turn on remaining halfs) gets you and a buddy to the surface without a return.
Exactly.
As we now see, there seem to be a number of different ideas of what the Rule of Thirds is. To me, the Rule of Thirds is simple: 1/3 of your tank to go out, 1/3 to come back, and 1/3 for your buddy... and a completely piss poor substitute for actual gas planning, that should only MAYBE be used for simple, single tank recreational dives. It doesn't allow for elevated SAC rates and it definitely doesn't allow for mandatory deco stops.
You may as well just do a real gas plan if you're going to try and account for 2 divers with different size cylinders and different SAC rates.
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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
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
AlexL
Contributor
when we crunched the numbers in fundies last week, 'rec thirds' works well enough to about 60ft/20m (mg=900psi) with an assumed .75 cfm SAC for two divers [ in matched single AL80s ]. any deeper or higher SAC and it quickly falls short.
[edit] this is comparing 'rec thirds' of 1000psi out, 1000psi back, 1000psi reserve - 500psi back on the boat but its there, to min gas of 900psi rounded up to 1000psi + halves, turn on 1000psi.
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And what if the diver with a smaller tank also has a very different SAC than the other diver?
Like I said, I don't think simplistic rules and assumptions about SACs are an acceptable substitute for real gas planning.... unless maybe you're planning for a simple, single tank recreational (i.e. no deco) dive.
And I still don't see a point of making up some Min Gas number and then applying Rule of Thirds on top of that. At that point, you're making it way more complicated than just "Rule of Thirds". Why would you not just do a formal gas plan? Min Gas + Rule of Thirds and still no allowance for an elevated SAC from the turn point on? Seems like a complete waste of time, to me.
Do it right or don't do it at all. Definitely don't fudge something together that results in an unmerited sense of confidence.
Like I said, I don't think simplistic rules and assumptions about SACs are an acceptable substitute for real gas planning.... unless maybe you're planning for a simple, single tank recreational (i.e. no deco) dive.
And I still don't see a point of making up some Min Gas number and then applying Rule of Thirds on top of that. At that point, you're making it way more complicated than just "Rule of Thirds". Why would you not just do a formal gas plan? Min Gas + Rule of Thirds and still no allowance for an elevated SAC from the turn point on? Seems like a complete waste of time, to me.
Do it right or don't do it at all. Definitely don't fudge something together that results in an unmerited sense of confidence.
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.
A simple way to associate useable gas and RB calcs is this;
First, useable gas is tank gas minus RB
All usable = navigation doesn't matter (open ocean)
1/2 useable = navigation isn't critical (dive objective)
1/3 useable = navigation is critical (cave/wreck/ice)
RB gives you a concrete air allowance for a 2 diver ascent (including stops) and useable gets you to the location where your ascent begins.
First, useable gas is tank gas minus RB
All usable = navigation doesn't matter (open ocean)
1/2 useable = navigation isn't critical (dive objective)
1/3 useable = navigation is critical (cave/wreck/ice)
RB gives you a concrete air allowance for a 2 diver ascent (including stops) and useable gets you to the location where your ascent begins.
ps. You can muddy the waters by bringing Rule of Sixths into it, but the basic premise still remains.
Imagine you're diving Rule of Sixths and your SAC is 0.8. You happen to be diving today with a buddy that is a total water nymph. Her SAC is 0.35 and she is diving with double AL53s to your HP100s. If you both hit your 1/6 TP at the same time, will your buddy have enough gas to get you both out? How do you know? I think you can only know by doing the math. And if you do the math, what was the point of guessing, by using the Rule of Sixths?
Imagine you're diving Rule of Sixths and your SAC is 0.8. You happen to be diving today with a buddy that is a total water nymph. Her SAC is 0.35 and she is diving with double AL53s to your HP100s. If you both hit your 1/6 TP at the same time, will your buddy have enough gas to get you both out? How do you know? I think you can only know by doing the math. And if you do the math, what was the point of guessing, by using the Rule of Sixths?
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