Stage Diving: What are you doing?

[Chris Laughrey]

The 1/2 plus 200 approach to stage diving does not violate the rule of thirds. The rule of thirds applies to the total gas supply used for penetration. This means 1/3 of your total gas supply (back gas and stage or stages) is available for penetration, 1/3 or less for exiting, and 1/3 for contingencies. If you use 1/3 of a stage and then do the same with back gas you are adhering to the rule of 3rds and keeping the math simple. The rule of thirds, however, is subject to diminishing returns at maximum penetration. If it hits the fan at that point in a dive, you may or may not have enough gas to solve the problem.

Using the 1/2 plus 200 rule provides you with more of (actually all of) your emergency third at maximum penetration if you need it. If you use the 1/2 plus 200 rule for a stage, then you must transfer your stage's third to your back gas before calculating 3rds for your back gas supply. For example, with my double 95's and an 80 stage, I'll plan to use 1300 psi (26 ft3) from the stage swimming into the cave. I must therefore subtract 26 ft3 (361 pisg) from my double 95 starting pressure BEFORE I calculate 3rds for my back gas. So if I start this dive with 3500 psi in my back gas, then I first subtract (put in reserve) 361 psi and figure my back gas 3rds from 3139 psi. That's 1000 psi I can use from my back for penetration. If I have a problem at maximum penetration, then I have the 1000 psi emergency 3rd available AND the additional 361 psi or 26 ft3 to deal with the problem. If it was a lost line, lost buddy, or gas sharing you have something like 17 to 20% more gas to deal with the problem. You are still operating within the rule of 1/3's, and you have ALL of the gas remaining in your doubles and stage when you return to it for exiting the cave. Other advantages include the fact that you can return to the parked stage sahring gas like you trained for it, move quicker, etc.

I place the stage in a smart and convenient place, not necessarily where I hit the 1/2 plus 200. Many of you have noted the advantage of parking it further in, i.e., closer to you on exit. That may not be advantageous, however, relative to restrictions, passage configuration, line placement, mud and silt etc. Where I leave a stage always, "depends".

A final observation - divers using the 1/2 plus 200 ruke usually penetrate about the same or slightly less distance than those using 1/3's. he advantage is simply the fact that you have your entire contingency third with you at maximum penetration in case a problem arises.

Cheers,

Chris

[atedeschi]

Andrew's Response from the other thread, incase others were not following both:

"Always carry excessive gas at the deepest point of penetration. if you're going to go to the edge, do it on stages nearer the exit.

On a catastrophic gas loss at the deepest point of penetration, you need the gast THERE, NOT near the entrance. However, you now carry that surplus back with you, so it's always available everywhere else.

So if you want to do 1/2 + 200, ONLY do it on a multi stage dive on cylinders nearest the exit, and DO NOT DO IT on the last few cylinders that you use. Be more conservative at the deepest point of penetration. Note that on a single stage dive, that means your ONLY stage should not be dived to 1/2 + 200 as it IS at your deepest point of penetration. I personally go to thirds in high flow, and reserve more than 2/3 in no flow on backgas and my last 2 stages.

Scooters add to the complication. Do as above BUT additionally calculate reserves based on your SWIMMING rate of exit, such that you have at least double the gas needed to exit on your backgas and last couple of stages. I like to drop full stages along the way for a swimming exit. This often needs a setup/setdown dive in addition to the normal dive.

Here's an example. A single stage at 1/2 + 200 may get you to the furthest point you're allowed to scooter in Ginnie... but good luck swimming out on 1700! It may be doable but it'll be close, and you won't have any reserves. So if you swim from the Henkel on thirds, and have a catastrophic loss at thirds, you'll just get back to your stage... find your scooter flooded/stolen/dead/drifted off... and be on the ragged edge the whole way out. Not pretty. Alternatively if you'd dropped a full stage there or at Stage Rock, you'd be fine.

And the nice thing is, that stage at Stage Rock can sit there for days while you play around, ready for an emergency. No need to redump it every dive. In essence you're using a rebreather approach to reserve for your scooter.

(Now I'm going to get yelled at for teaching people to dive on the internet. Like half the people at Little River even have a DPV card! Isn't it better to teach them on the internet than let them go in and dive it to thirds on their shiny new scooter with no further thought?)"

[Jason B.]

but thats not what jason said he does. Jason said do 1/3 then take off 200, which is like reserving 600 in BG ?

unless i misunderstood

You understood me.

I think rjack was pointing out another method which looks to gives similar results?

[limeyx]

Using the 1/2 plus 200 rule provides you with more of (actually all of) your emergency third at maximum penetration if you need it. If you use the 1/2 plus 200 rule for a stage, then you must transfer your stage's third to your back gas before calculating 3rds for your back gas supply.

Chris

What do you do for additional stages ? Surely the *minimum* you have to reserve "extra" in your backgas is just the amount you have "violated" 1/3's by.

In this case, going in on the stage, you violate 1/3's by 300 psi in an 80, which is 7cft of gas.


Therefore, the min you need is to subtract out 7.5 cft from your start volume in doubles before calculating your 1/3's right ?

I was trained to reserve the entire 1300 psi from first stage into BG (which I make to be 32.5 cft) and then just the amount which you exceed 1/3s from additional stages

Which is why I asked Jason as he seemed to be reserving a lot of gas for additional stages (and actually quite a bit on 1st stage unless I dont get it) not a problem at all, but I was wondering

[limeyx]

You understood me.

I think rjack was pointing out another method which looks to gives similar results?

I *think* richard says

(Start Pressure - #of stages * 200) then third it which reserves 200 psi for each stage in BG

you said
Start pressure, 1/3 it then take out 200 * # of stages which (I think) is 600 psi per stage ? which is way more.

but again, I could be mis understanding

[Gary]

Someone please point out the recklessness. why is this less safe than doubles to thirds?

Gas failure is only one way to have problems. Simply getting chilled, or getting a cramp can impede one's exit. Possibly enough to render that into a fatal accident without needing a gear failure.

Diving to 1/2+200 on the stage and 3rds on backgas. If someone had a gas failure, shared air and made it back to their stage. They quite likely would find themselves light and floating (as in several accident examples including Sheck's) and not utilizing gas as efficiently. In fact there could be dozens of reasons to be less efficient on a return after an air share back to the stages.

----

Sheck's air share in "Caverns Measureless to Man" (p145+) would have probably been a double fatality had they been diving 1/2+200 on the stages.

They weren't even to 3rds on backgas and Sheck's buddy wasn't even completely out of gas. They didn't have a loss of vis and they still ran out of gas before reaching their stages.

They dove to 3rds on the stages. Once back to the stages they should have had twice the gas they needed to exit. Instead they were almost out and one of them had to abandon deco temporarily to surface to yell for help.

If they'd dived 1/2+200 on the stages they wouldn't have gotten back at all.

[rjack]

You understood me.

I think rjack was pointing out another method which looks to gives similar results?

no I botched it.

1300psi in an AL80 = 400psi in 104/130s.
13*2.5 = 32.5cf
4*8 = 32cf
close enough, manufacturing tolerances aren't this close.

plan:
2 stages to 1/2+200 and "thirds" of 104s

3600 in 104s - 800psi for the 2 stages = 2800 available
2800/3 = 900psi penetration

drop stage 1 at 1700
drop stage 2 at 1700
turn on backgas at 2700

I have enough gas to cover loss of both my stages and a OOA buddy (who still has stages) at maximum penetration.

The advantage is simply the fact that you have your entire contingency third with you at maximum penetration in case a problem arises.

Yes I agree, I want that reserve 3rd with me and not subject to possible loss via officer Gibson or otherwise. Despite this, I have almost never dived thirds. Mostly because I have done most of my cave diving with only 1 buddy and we reserve more gas in these cases.

[limeyx]

plan:
2 stages to 1/2+200 and "thirds" of 104s

3600 in 104s - 800psi for the 2 stages = 2800 available
2800/3 = 900psi penetration

drop stage 1 at 1700
drop stage 2 at 1700
turn on backgas at 2700

I have enough gas to cover loss of both my stages and a OOA buddy (who still has stages) at maximum penetration.

Yes. And if you were open to the additional risk, you can reserve "what you exceed thirds" from stages after the first, and still be within 1/3's (or similar for whatever gas plan you chose)

so in those cases, you take off 400 psi for 1 stage, and just 7.5cft (300 psi in stage and whatever that is in BG) for subsequent stages, if you are OK assuming just one will fail

[Jason B.]

I *think* richard says

(Start Pressure - #of stages * 200) then third it which reserves 200 psi for each stage in BG

you said
Start pressure, 1/3 it then take out 200 * # of stages which (I think) is 600 psi per stage ? which is way more.

but again, I could be mis understanding

Yes, I got that he's doing it different than me, but our end results are similar.
One difference is Rjack was talking 104's and I was talking 95's. Other than that (for one stage) we are tit for tat the same end result in gas use. For additional stages I'd deduct 100psi.

I could (and sometimes do) subtract only 100psi for the single stage but that is only a 300 psi reserve of about 24cu/ft of gas vs the 32.5 I need if the stage is missing. (Ainslie's Flow Law at work ).

It''s just an easy method. Take thirds, then subtract from thirds for various items, such as stages and scooters.

[Jason B.]

1300psi in an AL80 = 400psi in 104/130s.
13*2.5 = 32.5cf
4*8 = 32cf
close enough, manufacturing tolerances aren't this close.

plan:
2 stages to 1/2+200 and "thirds" of 104s

3600 in 104s - 800psi for the 2 stages = 2800 available
2800/3 = 900psi penetration

drop stage 1 at 1700
drop stage 2 at 1700
turn on backgas at 2700

I have enough gas to cover loss of both my stages and a OOA buddy (who still has stages) at maximum penetration.

1300psi in an AL80 = 460ish-psi in 95s.
13*2,5 = 32.5cf
6*7 = 42cf
(a little extra gas!)

plan:
2 stages to 1/2+200 and "thirds" of 95s

3600 in 95s
3600/3 = 1200psi penetration
1200psi-200 for 1st stage = 1000psi
1000psi-100psi for 2nd stage = 900psi

3600-900 = 2700psi turn pressure.

drop stage 1 at 1700
drop stage 2 at 1700
turn on backgas at 2700

I have enough gas to cover loss of both my stages and a OOA buddy (who still has stages) at maximum penetration.