Rule of 1/3s and different tank sizes

[AlexL]

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"

i was agreeing completely. op said 'thirds', and there's two kinds of 'thirds', which is what i was pointing out initially as to why you wouldn’t just reserve the bottom third of the tank. if it was posted in basic scuba, then i would have assumed 'recreational thirds'. subsequent posts were about where the assumptions about 'recreational thirds' breaks down compared to computed minimum gas reserves. this was touched on during my basic open water, and was one of the reasons given for the BOW/AOW split in depth limits.

min gas was not 'made up', and was ran with the mentioned SAC, depth, and assumed 1 min fix time and 10fpm ascents, which is extremely conservative to some for recreational open water, so i could see how it could be taken as a WAG. you could shave it closer with faster ascents and argue 'rec thirds' works a bit deeper then 60ft, but it's not what i would personally use.

if you're in an overhead, then 'rec thirds' does not apply in any way, shape, or form. that calls for a full gas plan based on volumes and SAC histories.

 

[stuartv]

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.

And the 1/3 usable still doesn't allow for elevated SACs or different tank sizes and SACs. If my buddy is an air hog with big tanks and we get to our turn point, at the furthest point in our wreck dive and he loses all his gas, just as I am at 2/3 of my usable gas left, we're still not going to have enough to get us both back to the anchor line.

 

[stuartv]

i was agreeing completely. op said 'thirds', and there's two kinds of 'thirds', which is what i was pointing out initially as to why you wouldn’t just reserve the bottom third of the tank. if it was posted in basic scuba, then i would have assumed 'recreational thirds'. subsequent posts were about where the assumptions about 'recreational thirds' breaks down compared to computed minimum gas reserves. this was touched on during my basic open water, and was one of the reasons given for the BOW/AOW split in depth limits.

min gas was not 'made up', and was ran with the mentioned SAC, depth, and assumed 1 min fix time and 10fpm ascents, which is extremely conservative to some for recreational open water, so i could see how it could be taken as a WAG. you could shave it closer with faster ascents and argue 'rec thirds' works a bit deeper then 60ft, but it's not what i would personally use.

if you're in an overhead, then 'rec thirds' does not apply in any way, shape, or form. that calls for a full gas plan based on volumes and SAC histories.

When I said "made up", I didn't mean you made up 900psi out of thin air. I meant that you calculated it from.... numbers you made up. You're guessing what your and your buddy's SACs would be in that situation. You're making up a 10fpm ascent, versus 30. You're guessing that your problem can be resolved in 1 minute versus 2 or even 5. So, even though the last step is a calculation, 900 psi is still made up. You could use aboslutely the same process and come up with a wildly different number, if you wanted, that was just as legitimate.

Anyway, the OP specifically says "Rule of Thirds". This is the Adv Scuba forum and he did not say "rec thirds". And, any way you slice it, I don't see a place for Rule of Thirds (of any kind) outside of the simplest, single tank, recreational dive planning. Just my opinion.

I am looking forward to taking Fundies at some point. And if they teach some Min Gas + Thirds thing, I will learn their rules and perform the required exercises to pass the class. But, I'll be dipped if I'll actually plan a real dive that way versus actually doing a real gas plan.

 

[decompression]

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

 

[Ayisha]

Am I overcomplicating it?

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

And the 1/3 usable still doesn't allow for elevated SACs or different tank sizes and SACs.

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.

I am looking forward to taking Fundies at some point. And if they teach some Min Gas + Thirds thing, I will learn their rules and perform the required exercises to pass the class. But, I'll be dipped if I'll actually plan a real dive that way versus actually doing a real gas plan.

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.

 

[stuartv]

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. AlexL gave an example where he came up with 900psi.

If you're diving tanks with 3000 psi, then that leaves you 2100 psi usable.

If your diving environment dictates that you use the 1/3 Usable rule, then that means you can use 700 psi of that. So, your turn pressure is 2300 psi.

Is that correct? If not, please explain how. You don't have to give me the full course. Just point me in the direction of how it's wrong.

If that IS correct, then how does it allow for an elevated SAC? Meaning an elevated SAC from the OOA at the furthest point of the dive to when you get back to the anchor line.

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]

In other words, I think we are now debating semantics. Specifically, the semantics of the phrase "1/3 usable". The is not the DIR or GUE or whatever subforum. 1/3 usable means I can use 1/3 of my tank. The other 2/3 are reserved for me and my buddy to get back where we came from. That is how I was trained in my non-DIR, non-GUE tech courses.

If you are "right" because you define "1/3 usable" in some other way, then that's fine. But, please keep in mind that in this forum, you are likely to be addressing an audience of people some of whom don't know how you are redefining "1/3 usable". Like me.

 

[decompression]

The simplest answer to an elevated SAC and how it's accounted for is the buffers that you are giving. Emergency ascent is calculated with SAC of 1.0 cuft, if your teammate has a higher SAC then that.......use that instead and practice more. The plan allows for one minute to solve issue, so share gas and begin route to Ascent, rarely takes a full minute. SAC may still be one but probably a bit lower. The exit to the ascent will be considerably faster than first third. Begin Ascent. So, buffers of high SAC in RB, faster exit time and the extra third of gas should more than cover a normal ascent with two divers. Does that make sense?

 

[decompression]

Stuart, check your email.

 

[tbone1004]

so the rule of thirds is typically only used for penetration, and not often incorporated with rock bottom in a cave diving sense, but it doesn't matter, just means you are essentially recalculating thirds. It can get fairly complicated since you have your fill pressure, have to deduct your rock bottom from that, then have to deduct what you actually used on descent, THEN you get to cut thirds. This is a lot of math if you have dissimilar tanks, so I would just as soon make sure you have the same size tanks or use solo gas management if you're doing dives like that

The key to the rule of thirds is the buddy with the higher sac rate doesn't get the larger tanks. Said rule actually requires that the person with the higher sac rate has the smaller tanks, and is something that has been brought up here many many times over the years and apparently has just cropped up again. You have obviously come to the same conclusion on your own, but haven't read or been taught that that is actually a really important part of buddy gas management.
If the diver with the higher SAC has the larger tanks, then the rule of thirds can't be applied. You have to do some more complicated math to get that to work out and it gets really strange. Typically involves you figuring out how many cubic feet a third from the larger set of tanks is, and then deducating that from your available gas. I.e. if diver 1 has a higher sac rate and HP120's, and you have a lower sac rate and AL80's, a third of his is 40cf which is half of yours. You then get half of your tank and it's half in, half out, so you are diving to quarters not thirds.
It's a nonissue IMO if it's within about 10% for SAC, and the tanks are similar sizes. I.e. if someone has HP117's and a SAC of 0.6, and another as HP120's and a SAC of 0.65, it's not worth worrying about it in my opinion, but others may disagree. You obviously would ideally swap the tanks, but personally If I were the 117/0.6 diver, I would go to thirds plus 100-200 and call it good enough. Alternatively make the other diver run the line or something else that will cause them to eat up a bit more gas before you start penetrating.