Figuring thirds....

[Cave Diver]

If you look at it a bit closer, you will probably come to the same conclusion that I have -- which is that where you put the heavy breather makes no difference in the worst case, but in many failure scenarios having the heavy breather on the big tank gives you additional margin. So putting the heavy breather on the small tank make calculations easier, but increases risk.

Or to put it another way --- if thirds (properly calculated based on smallest tank) are first reached by someone on a tank other than the smallest, then you will have more gas left if the big tank fails.

I always leave the hoover at home. Vaccuums belong in the closet, not the dive trip.

 

[Meng_Tze]

Same volume for penetration; (1/3) of smallest tank(set) is used up quicker by the heavy breather, thus turning sooner. It makes no difference where that penetration volume sits, either in large tanks or small tanks.

The heavy breather uses the same volume, but more quickly if he sticks to the gas plan....

 

[rjack321]

It makes no difference where that penetration volume sits, either in large tanks or small tanks.

My thoughts exactly.

 

[Spectre]

Same volume for penetration; (1/3) of smallest tank(set) is used up quicker by the heavy breather, thus turning sooner. It makes no difference where that penetration volume sits, either in large tanks or small tanks.

The heavy breather uses the same volume, but more quickly if he sticks to the gas plan....

Actually this is ideally incorrect if the gas plan is calculated accurately. The volume is based on 1/3 of the smallest tank(s) and that volume is passed through the team. So, for example, if you've got a set of 80s pumped to 3300 (for a practical example), you're looking at 56.76cuft for 1100 psi. Passing that volume to a set of 130s, it's still 56.76cuft.

Ideally, it takes the same time to breathe 56.76 cuft.

Now, taking it to 'technically'... it's a -tiny- bit longer, since on the 80s he gets 56.76 cuft. but when you work the math with rounding on a set of 130s, that 1100 equates to 750 cuft, or 56.65 cuft. So technically the dive gets extended by a couple seconds.

Now if you want to talk practically; the smaller tanks get you more time, since smaller tanks equate to a smaller profile and thus less drag. less drag should equate to a lower SAC rate. Of course, that only applies if the hoover is used to diving those tanks; if he's used to the other tanks and the smaller tanks make for more task loading due to differences in trim... (fighting the tanks), then that throws it out the window.

But since we're talking theory and ideal situations here.... the statement is wrong. with a constant SAC rate, turn gets hit at the same time. The only difference, as people state, is that if you should lose all the gas in the larger tanks, you'll have less available gas if the hoover was on the smaller tanks.

[113.52cuft w/ equal sac rates, 127.79 w/ 25% lower sac rate diver, 141.96 w/ 50% lower sac rate diver]

Either way, the actual answer is ditch the hoover, or ditch the weenie tanks.

 

[limeyx]

Actually this is ideally incorrect if the gas plan is calculated accurately. The volume is based on 1/3 of the smallest tank(s) and that volume is passed through the team. So, for example, if you've got a set of 80s pumped to 3300 (for a practical example), you're looking at 56.76cuft for 1100 psi. Passing that volume to a set of 130s, it's still 56.76cuft.

Ideally, it takes the same time to breathe 56.76 cuft.

Now, taking it to 'technically'... it's a -tiny- bit longer, since on the 80s he gets 56.76 cuft. but when you work the math with rounding on a set of 130s, that 1100 equates to 750 cuft, or 56.65 cuft. So technically the dive gets extended by a couple seconds.

...
Either way, the actual answer is ditch the hoover, or ditch the weenie tanks.

First off, if your plan is really taking into account fractions of a cft of gas, it's not a good(safe) plan -- you need way more leeway than that.

Second, a lot of people tout 1/3's as a safe way to dive "Oh, I was fine as we were on 1/3's" in reality, 1/3's is the *least* conservative safe(ish) gas plan for overhead diving.

Plenty of things can screw up 1/3's
- Current/flow going against you
- Stress -- a buddy who has a similar SAC to yourself when not stressed but increases far more in proportion when stressed
- entanglement
- losing line/buddy

If you dive 1/3's in a team of 2, I think you are asking for trouble -- you need to be more conservative.

For a team of 3, depending on what failures you plan for, you have a "third 1/3" handy on the exit (providing you have a good enough team not to end up losing anyone)

I agree, leaving the "hoover" home is one option, but some people are just going to have higher SAC's due to their body-type/physiology. Doesn't mean you canot safely dive together, as long as the plan is a good one

 

[Meng_Tze]

Actually this is ideally incorrect if the gas plan is calculated accurately. The volume is based on 1/3 of the smallest tank(s) and that volume is passed through the team. So, for example, if you've got a set of 80s pumped to 3300 (for a practical example), you're looking at 56.76cuft for 1100 psi. Passing that volume to a set of 130s, it's still 56.76cuft.

Ideally, it takes the same time to breathe 56.76 cuft.

With the same breathing rate, yes.
[/QUOTE]

Now if you want to talk practically; the smaller tanks get you more time, since smaller tanks equate to a smaller profile and thus less drag. less drag should equate to a lower SAC rate. Of course, that only applies if the hoover is used to diving those tanks; if he's used to the other tanks and the smaller tanks make for more task loading due to differences in trim... (fighting the tanks), then that throws it out the window.

Too many 'if's' and not relevant to the problem at hand in the strictest sense. I agree it is realistic

But since we're talking theory and ideal situations here.... the statement is wrong. with a constant SAC rate, turn gets hit at the same time.

You agree that we have the same usable volume, say 56.76cft. This was 1/3 that the team needed to reserve.

Diver uses 1cft/min, Diver B uses 0.7cft/min

Diver A uses this gas in 56 min (give or take)
Diver B uses that in 81min (give or take)

Diver A reaches turn pressure sooner than B.

Not sure if your comment on constant sac relates to constant or the same/different

The only difference, as people state, is that if you should lose all the gas in the larger tanks, you'll have less available gas if the hoover was on the smaller tanks.

You will have the same amount of gas as other wise, but again the hoover is going to cost you. My statement did not cover where the hoover should sit, merely that the place of the volume is irrelevant to its usage. Maybe that was not clear enough

 

[Spectre]

You agree that we have the same usable volume, say 56.76cft. This was 1/3 that the team needed to reserve.

Diver uses 1cft/min, Diver B uses 0.7cft/min

Diver A uses this gas in 56 min (give or take)
Diver B uses that in 81min (give or take)

Diver A reaches turn pressure sooner than B.

Not sure if your comment on constant sac relates to constant or the same/different

You will have the same amount of gas as other wise, but again the hoover is going to cost you. My statement did not cover where the hoover should sit, merely that the place of the volume is irrelevant to its usage. Maybe that was not clear enough

I think we're pretty much saying the same thing, but the statement is intrinsically obvious. The diver that uses gas faster (be it breathing harder, leaky inflator, higher rate of inflation gas, whatever) will hit their turn pressure first.

I inadvertently assumed you were making a statement in regards to which tanks the "hoover" wears. I thought you were stating that if that diver wore the smaller tanks, the dive would be shorter than if that diver wore the larger tanks (which confused me BTW, 'cause I know you know this stuff).

But I think I was just mistaken in your statement, and you're just saying that the diver who uses gas faster will be the one to turn the dive (which should be obvious to everyone unless you completely mess up the concept.... which it seems lots have).

(BTW: What I meant by "constant SAC rate" was that the diver with the higher sac rate uses the same rate regardless of what tanks he's wearing....)

I think what people are getting thrown by is what is truly meant by 1/3s. While we work in 'pressure', 1/3s has nothing to actually do with pressure, it has to do with volume. The dive is turned when one member has consumed a volume equal to 1/3 of the smallest volume started with(*).

So if the smallest volume on a team-member's back is 90 cuft, then the team gets turned as soon as someone consumes 30 cuft. If the smallest volume is 450 cuft, then the team is turned as soon as anyone has consumed 150. (*)

Since imperial gauges read in PSI, those that use those gauges must convert that cuft to PSI so the individual knows when to turn.

The final result is that 1/3s is You call the turn when your tanks hit 2/3 of their starting PSI only if you were the person with the smallest starting volume.. If you're not the one that initially defined the usable volume, then you'll be calling the dive when your pressure reaches a number that will be greater than 2/3 of your starting pressure (**)

In other words... by design of the system; the guy that calls the dive is the guy that had the highest consumption for that dive. No math is needed to determine who has to buy the first round of deco beverages. (***)

Caveats:

(*) - this has a ton of caveats that all come down to "diving 1/3 of your available gas is appropriate for the dive at hand"... so we don't start confusing the issue with syphons, stages, scooters, rebreathers, etc.
(**) - assuming, of course, you started with a multiple of 300 and there was no rounding down.
(***) - assuming they turned because they consumed the defined amount of penetration gas, not for another reason. But if you wimp out and turn the dive early, you gotta buy the first round anyway.

 

[Meng_Tze]

...
I think what people are getting thrown by is what is truly meant by 1/3s. While we work in 'pressure', 1/3s has nothing to actually do with pressure, it has to do with volume. The dive is turned when one member has consumed a volume equal to 1/3 of the smallest volume started with(*).

This is the core of the entire principle and confusion.


PS. I knew you and I could not be confusing things..... either one of us would have been left in a cave by now...

 

[Spectre]

PS. I knew you and I could not be confusing things..... either one of us would have been left in a cave by now...

You've not been left yet? Damn, your buddies must like you more

 

[Spectre]

If you dive 1/3's in a team of 2, I think you are asking for trouble -- you need to be more conservative.

I agree with everything you've written, with the exception of this. While I agree with this as a blanket general statement when speaking to an unknown audience, I don't agree in a realistic sense.

We all went through timed line drills in cavern. We all know that it took us far longer than 2x the time to follow the course with our eyes closed and in touch contact. We all were shown that 2/3 is not necessarily a guarantee.

However, let's look at a similar, but completely unrelated situation. When I walk through my house on my way to bed it takes a certain amount of time. Had I come home with some woman, I bet I can lead her to the bed faster... even in a lights out 'air sharing' situation then my leisurely going to bed pace.

My point? Familiarity with the location and the experience play a huge role. With the right buddy, I don't believe that diving a double staged scooter dive in Jackson Blue to 1/3s is 'asking for trouble'. (granted the plan actually gives me about 7 cuft more than 2/3 reserve). But that confidence absolutely has to do with personal knowledge of the cave, understanding of -actual- transit times, and distance limitations.