Stage Diving: What are you doing?

[limeyx]

Here's the scenario. With thirds, you have JUST enough to get out of the cave. With thirds and 1/2 + 200, you have just enough to get back to your stage... where the emergency now continues. In essence you've now massively increased the period during which resources are critical. There's the old argument that SAC's tend to be high during emergenceis - I've definitely found that to be true. So having a critical resource at that point for the continuation really sucks. You want wiggle room on that tank.

Maybe we all mean different things by "1/2 + 200" when I say it, I mean

At a *minimum* I am reserving out of myh doubles, "the amount I exceed thirds by" on the stage. In this case, if I breathe 1300 PSI out of the stage, have "violated" 1/3's by 300 psi in an 80 and must subtract this from the BG before dividing that up (of course on cft not psi

For the first stage, I reserve in BG the entire amount I am breathing out of the stage (so if I assume Double 80s on BG for ease of calc that means I breathe 1300 out of the stage, so I reserve 650 (so 700) out of my doubles befor calculating my 1/3 or 1/4s etc

For each stage after the first, i would just reserve "the amount I exceed 1/3" or 300 PSI in an 80

This means
- I can suffer OOA at max penetration *and* lose a stage on way back (we did this in class easily)
- my reserve gas is on my back, so I can ditch stages morew quickly in an emergency and dont end up with 3 or 4 80s each with a 1/3 in them

potential downsides ?
- maybe i end up donating a long hose more often (I may not have a stage to pass off) so 2 divers are joined by a hose
- at some point, you "run out" of BG to back up your stages (this would be a different kind of dive

This assumes no scootering (out of my area of experience)

just doing 1/2+200 with no other changes does not seem to make any sense.

I never run 1/3s in team of 2, no matter what, but I would in a no-flow cave with a team of 3

Siphons are another matter. I dive "gentle" siphons in Mx, but a fullon siphon is another beast

[phillip1]

Now I guess you guys do way more stage diving than me, but I do a fair amount of stage diving involving up to 5 al80's, sometimes more.
I take my lowest tank pressure and use that to calculate third for all my other tanks and I have always breathed my stages to thirds plus 20bar (for bonus air) and dropped them a bit past that point to overlap. It may be overkill but I feel way safer having way more than enough air for me or my buddy should an ooa emergency arise.
Having only enough air to get to my next stages and then to the the next set of stages etc.. till you get out is too close for comfort for me.

[Jason B.]

We dive our stages (AL80) to 1/2 + 200
Take thirds of backgas (dbl95's) 3600/3 = 1200
Deduct 200psi from thirds to allow for the stage 1200 - 200 = 1000psi usable out of backgas.
If we come back and the stage is gone or empty, we have half a stage worth of gas in reserve in backgas to make it out on.

[Marc Bryan]

We dive our stages (AL80) to 1/2 + 200
Take thirds of backgas (dbl95's) 3600/3 = 1200
Deduct 200psi from thirds to allow for the stage 1200 - 200 = 1000psi usable out of backgas.
If we come back and the stage is gone or empty, we have half a stage worth of gas in reserve in backgas to make it out on.

I do not see how you are arriving at half a stage of reserve in your BG. 200psi of 95's is only 14.4 cuft. Half a stage would be 39 cuft. Even if you double it for the other buddy, you still only have 28.8 cuft.

Not saying anything is wrong with the approach, just the math. What am I missing?

edit: Are you including your original 1200 reserve in the calculation? If so, you have way more than a stage, but I do not see how that fits into the planning.

[Jason B.]

I do not see how you are arriving at half a stage of reserve in your BG. 200psi of 95's is only 14.4 cuft. Half a stage would be 39 cuft. Even if you double it for the other buddy, you still only have 28.8 cuft.

Not saying anything is wrong with the approach, just the math. What am I missing?

I'm subtracting the 200psi from my thirds. That's a net savings of 600psi or 7.2 x 6 = 42ish cu/ft of gas.

[Marc Bryan]

Whats wrong with breathing it down a 1/3 so that it is neutral, not positive, so that it rides properly. Then leave it, and and still reserve an extra 100psi (if diving 104's, and staging an al72, or al80) in your back gas to allow for getting back to it in a catastrophic gas loss (assuming diving with a buddy. solo is an entirely different discussion) at max pen, silt out, etc.... What is out there that that extra 100 psi is so important?

Or just carry it with you as long as possible.

[Marc Bryan]

I'm subtracting the 200psi from my thirds. That's a net savings of 600psi or 7.2 x 6 = 42ish cu/ft of gas.

I edited that in while you were responding. Thanks.

[limeyx]

We dive our stages (AL80) to 1/2 + 200
Take thirds of backgas (dbl95's) 3600/3 = 1200
Deduct 200psi from thirds to allow for the stage 1200 - 200 = 1000psi usable out of backgas.
If we come back and the stage is gone or empty, we have half a stage worth of gas in reserve in backgas to make it out on.

Yes, this is basically same as I said above, but just for the 1st stage right ? If you had a 2nd or 3rd, you'd reserve less?

[limeyx]

Whats wrong with breathing it down a 1/3 so that it is neutral, not positive, so that it rides properly. Then leave it, and and still reserve an extra 100psi (if diving 104's, and staging an al72, or al80) in your back gas to allow for getting back to it in a catastrophic gas loss (assuming diving with a buddy. solo is an entirely different discussion) at max pen, silt out, etc.... What is out there that that extra 100 psi is so important?

Or just carry it with you as long as possible.

You dont carry it as long as possible (which I interpret to mean "until empty" as you might in the ocean) as you are then swimming a long way with a stage, which is extra drag and bulk especially if you have multiple or are in small caves

If you breathe to 1/3 then, the more stages you have, the more stages with 1/3 of gas you have in them on the way home, so you end up needing to carry them further out of the cave on the exit in an emergency.

If you dive 1/2+200 and reserve what you "break 1/3's" from in your BG, you can ditch empty stages earlier in an emergency and so you aren't dragging them out of the cave as far (you can swim faster and with less bulk)

[Marc Bryan]

You dont carry it as long as possible (which I interpret to mean "until empty" as you might in the ocean) as you are then swimming a long way with a stage, which is extra drag and bulk especially if you have multiple or are in small caves

If you breathe to 1/3 then, the more stages you have, the more stages with 1/3 of gas you have in them on the way home, so you end up needing to carry them further out of the cave on the exit in an emergency.

If you dive 1/2+200 and reserve what you "break 1/3's" from in your BG, you can ditch empty stages earlier in an emergency and so you aren't dragging them out of the cave as far (you can swim faster and with less bulk)

LOL - I certainly did not mean breathe it as long as possible till empty (who would even think that?).....I meant when you come off of it, carry it with you until such a time as you are entering tight sidemount passages/smaller cave on BM and then leave it. I was referring to one stage, obviously you would not carry five the entire dive.