Modifying rule of thirds

People often ask me why my isolator points down. My answer is that I expect my chances of isolating during a dive are approximately zero. In the first place, we dive the rule of thirds. If you have a gas failure, you share air. Period.

In the second place, in about 99.9% of all the times that people have isolated during a dive, isolating was the wrong thing to do.

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Originally Posted by addexdiver

Given BY FAR the most common gas loss failure mode, an isolable leak - say something on one of the regs fails. In all cases, you isolate

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My point in a nutshell. If you have a reg failure, you turn off that post. Don't isolate!

There are two reasons to isolate during a dive: burst disk failure and cylinder neck o-ring failure. Otherwise, don't close the isolator.

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Originally Posted by addexdiver

One can (and has) argued that if SM, you can feather or swap regs underwater - true, but you will lose some gas, adds to task loading and ADDS to exit time. For BMI this is difficult to out of the question because it is behind you (lets agree it is not very easy...say, a hard possibility).

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Actually I have had this happen and had no trouble feathering the valve (which I only needed for buoyancy off that reg anyway since I switched to the good reg for exit). But that still gave me adequate gas to easily leave the cave on the one tank - plus the entire other tank at some minor inconvenience - and a buddy with full reserves...

Yes the manifold provides the "best" solution in the "most likely" failure of an isolable leak in that you have access to both tanks once isolated. On the other hand it also gives the possibility of loosing all gas from both tanks if isolation is delayed or impossible.

Either independent tank configuration gives the best result in a "limiting" air accident - meaning you don't typically need your full reserve (both tanks - manifold) to safely exit but you dang sure want at least half that reserve (independents). Gas isolation between the tanks is automatic. In an isolated leak the gas is still available but probably unnecessary - in an unisolable leak you breath it down if possible and then have the full minimum reserve still remaining. Sidemount is slightly better in that it does provide an "improved" ability to swap regs between tanks and better access to the hoses - but as you said this is generally not needed anyway and could be done in backmount if needed (which it shouldn't be).

Also sidemount gives the option to swap tanks between divers but this also may not always be advisable - but is a nicer option to have then having to share air between divers (especially since many sidemount diver's ability to share air between them is done only by swapping tanks since they don't generally carry a long hose on either tank). In BMI not only does the diver have the ability to independently exit with adequate reserves - plus less convenient reserves in an potentially isolated leak tank - but still may have a buddy with sufficient reserves for them both to exit in tandem.

I know many people who find traveling in sidemount to be more efficient. I have only 50-100 dives in sidemount and, for me, sidemount is clumsy and inefficient. I'd much rather deal with any problem (other then a bottle flooded with water) in BMI. :)

[And for swapping regs: bringing an extra reg along (shrink-rapped or on a 3rd bottle) would simplify swapping regs between tanks.]

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Originally Posted by skip

I'm not sure why I feel solo sidemount is ok without a stage, but solo backmount requires a stage....I must have some unconscious lack of trust in the manifold? or in my ability to actually shut it done in sufficient time?

-skip

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I only carry a buddy bottle solo when I'm scootering. Otherwise, if it's a swim dive, the buddy bottle isn't needed. I have 2 completely independent air sources. I plan even more conservative air management. If I lose one bottle completely, I will still have plenty to get me out. If I lose both (not likely), then it's just my time. With backmount you don't have completely independent air sources. Sure having a failure that loses all your gas isn't likely, but it's still possible.

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Originally Posted by Gary

[And for swapping regs: bringing an extra reg along (shrink-rapped or on a 3rd bottle) would simplify swapping regs between tanks.]

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Some folks have a H valve on one of the tank with just a small reg for that exact purpose.

When solo, I'm usually using a RB, so thirds isn't the issue. It's "can I get out, if multiple things go wrong". I'll carry an extra primary, and "80" for primary bailout, and if I'm scootering or doing a long swim, I'll drop a "40" somewhere along the way.

One day I just "went for a swim" in devil's. Lovely dive, right up until I realized I was at the Hinkel w/o enough bailout to exit if the 'breather took a vacation. Talk about a paranoid exit...

Sorry for the confusion. When I said isolate, it was isolation of the leak using the post isolator. English is so imprecise.

If you aim the x-over valve down and don't want to use it, why don't you get rid of it use a x-over no valve configuration to take advantage of the reliability gains through fewer failure points? I understand it is protect the valve thing.

I agree on not going to the x-over valve first. Shut the post - it is much more likely to solve the problem, and save on loss of gas. So restating your conclusion - "My point in a nutshell. If you have a reg failure", you isolate that post. Don't shut the x-over valve.

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Originally Posted by Slüdge

People often ask me why my isolator points down. My answer is that I expect my chances of isolating during a dive are approximately zero. In the first place, we dive the rule of thirds. If you have a gas failure, you share air. Period.

In the second place, in about 99.9% of all the times that people have isolated during a dive, isolating was the wrong thing to do.



My point in a nutshell. If you have a reg failure, you turn off that post. Don't isolate!

There are two reasons to isolate during a dive: burst disk failure and cylinder neck o-ring failure. Otherwise, don't close the isolator.

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Originally Posted by Gary

Actually I have had this happen and had no trouble feathering the valve (which I only needed for buoyancy off that reg anyway since I switched to the good reg for exit). But that still gave me adequate gas to easily leave the cave on the one tank - plus the entire other tank at some minor inconvenience - and a buddy with full reserves...

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BMI or SM?

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Originally Posted by Gary

Either independent tank configuration gives the best result in a "limiting" air accident - meaning you don't typically need your full reserve (both tanks - manifold) to safely exit but you dang sure want at least half that reserve (independents). Gas isolation between the tanks is automatic. In an isolated leak the gas is still available but probably unnecessary - in an unisolable leak you breath it down if possible and then have the full minimum reserve still remaining. Sidemount is slightly better in that it does provide an "improved" ability to swap regs between tanks and better access to the hoses - but as you said this is generally not needed anyway and could be done in backmount if needed (which it shouldn't be).

Also sidemount gives the option to swap tanks between divers but this also may not always be advisable - but is a nicer option to have then having to share air between divers (especially since many sidemount diver's ability to share air between them is done only by swapping tanks since they don't generally carry a long hose on either tank). In BMI not only does the diver have the ability to independently exit with adequate reserves - plus less convenient reserves in an potentially isolated leak tank - but still may have a buddy with sufficient reserves for them both to exit in tandem.

[And for swapping regs: bringing an extra reg along (shrink-rapped or on a 3rd bottle) would simplify swapping regs between tanks.]

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Points all well taken. But in a "limiting" air accident, which I take to mean one which was isolable "using a post valve", you will lose a roughly given amount of gas which does not depend much on whether you are independent or connected, so the total volume remaining after problem isolation (post valve is shut) in any configuration whether independent (SM, BMI) or manifolded (BM) is roughly the same. I do agree it is somewhat available in SM(requiring concentration and added task loading) and with more difficulty in BMI, but not near as easily as in manifolded (nothing to do other than breathe)...thus manifold wins due to ease of gas access. Independents only win (as I pointed out) when the leak cannot be isolated using a post valve.

Pausing to swap tanks or regs is slowing your exit - NOT what you ideally want to do. I do agree swapping tanks (if an option) makes sense at a certain point when total gas loss on the person of the stricken diver is near. Here, the tank swap saves time ultimately because the diver can swim faster on the remaining egress and is not slowed by a buddy air share. Swapping regs on the other hand, makes no sense to gain access to the "bad" side (except as a last resort when both you and your buddy are otherwise done) and only reinforces my point on benfit of the x-over. It will definitely delay your exit. In this case, I certainly agree the buddy provides additional available reserves that now make sense to use rather than adding time to exit.

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Originally Posted by addexdiver

BMI or SM?

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Come to think of it - it was in Mexico so it was manifolded AL80 doubles (eek!) but was still pretty easy. (So the reg failed free-flow but I had full gas plus a stage and feather the valve for the power inflator.)

Being able to reach valves easily should be something every backmount diver shoud be able to do. In BMI my tanks are turned inward slightly so the knobs are even more accessable (and out of the way).

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Originally Posted by rchrds

Andrew, I had a huge message with examples and stuff all typed out, but I lost interest in making the point.

Anyway, thanks for the thread, as this will quite likely get people thinking a little more about how they calculate their gas consumption, and perhaps think a little more about what they are doing before they commit themselves to a large dive.

Let me narrow my whole previous paragraph down to this-

You are not letting Murphy have a large enough portion of your gas supply. You can (and should) calculate gas consumption for large dives, but always add a fat portion for stuff you will never see coming, and by all rights should not happen. I've had enough completely stupid stuff happen that almost killed me, that I never could have planned for, and a fair number of "completely astronomical odds" events.

Anyway, something to think about, it used to be, (and its been a few years since I've dove with any) that the WKPP dove 1/2+2 on their stages- leaving backgas untouched. Which at first, didn't make sense, until someone clued me in that they also had SAFETY bottles, in addition to, (and actually in replacement of, in the case of emergencies) their regular gas supplies. Then it clicked- they didn't calculate that gas, but if needed, it could replace or supplement a threatened gas supply (and add additional regulators and bottles) so, in fact, they dove with large safety margins.

Anyway, already too long. Lets have more arguments like this.

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Hey Jason,

Thanks for the feedback. I THINK I agree with you... but why exactly do you want more Murphy gas?? For me it sort of depends on the dive. Big dives I am way over conservative. But what that spreadsheet helps with is knowing the exact level of conservativeness ofr a specific situation. Rules of thumb result in varialble levels of risk.

Jason is heading for Afganistan, so he may not answer very soon.

Essentially he is saying the *minimum* safe reserve is 1/3 of your total gas supply, on a short dive. IE 1/3 of your backgas or sidegas if SM.

On a complex, multi stage dive, save *all* of your back/side gas, and only breathe off the stage bottles. Also plant extra *safety* bottles along the way.

One of the few things I ever agreed with GI3 is having lots of spare gas around. It doesn't have to be 1/3 of your total gas supply on a big dive, but should never fall below 1/3 of back/sidegas.

I have had too many friends cut corners on reserve gas, and pay for it with their lives :(

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Originally Posted by aainslie

Hey Jason,

Thanks for the feedback. I THINK I agree with you... but why exactly do you want more Murphy gas?? For me it sort of depends on the dive. Big dives I am way over conservative. But what that spreadsheet helps with is knowing the exact level of conservativeness ofr a specific situation. Rules of thumb result in varialble levels of risk.

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