Stage planning in caves

[Jason B]

So when you say "take out 100 psi" you mean calc 1/3 and then reduce the "penetration" (1/3) by 100

That's what we do, yes.

 

[rjack321]

You also mentioned earlier that you never go back on your stage on exit. I'd suggest getting on back gas as soon as you get to the stage and reserve the back gas. Now if the **** hits the fan, you can ditch the stages and make a more expedient exit if needed.

I think you meant OFF backgas not on chief

 

[Jason B]

I think you meant OFF backgas not on chief

Duh! My bad.

Fixed.

 

[limeyx]

That's what we do, yes.

Can you explain more about where the 30cft per stage comes from? Seems like it should be that you reserve "the amount of gas that diving 1/2+200 exceeds 1/3'ds"

but I am not sure how that comes to 30 cft ...

 

[Jason B]

I probably should have left the 30cu/ft out of my original statement as the number was just an estimate. I was trying to point out that if diving stages to 1/2 plus 200 is still diving thirds, just carrying the extra third from the stage in back gas (or part of it). 1/2 + 200 on an Al80 stage gives a usable gas of 1300psi (about 32cu/ft). Since we us 32cu/ft in and out, the reserve third would be 32 as well (some of which is in the stage - a very small amount, the majority is in the back gas - something close to 30 - more like 20-25 I guess).

 

[DIR-Atlanta]

Twice now I've heard things like, "I only use my stages", "1/2+200 isn't safe" and "Thirds on stages is stupid."

The reason why "Thirds on stages is stupid" is because that last 1/3 in reserve in your stage bottle does you absolutely no good if you have an issue or delay further back in the cave and can't make it out to the stage drop. Also, the 1/2+200 rule allows the team to penetrate further into the cave on stages than the 1/3s rule, which means that if they are sharing gas on the way out, they will more quickly reach a point where they can switch back to their own gas sources and become independent divers again. This might save some time (and gas) in an emergency situation.

What a lot of people fail to understand about the "1/2+200" rule is that there is an implicit assumption that the back gas turn pressure will be adjusted up by a corresponding amount, so that the "thirds in" rule is not violated (at least in terms of the total gas supply). Here's a simple example to illustrate:

An AL80 is the preferred stage bottle, and double 104s are the preferred back tanks for cave diving. In terms of cubic feet, 100 PSI in a set of double 104s equates to about 300 PSI in an AL80 stage bottle. So the rule is actually:

• Breathe the stage to 1/2+200 on the penetration and switch to back gas
• Breath the back gas to 1/3+100, and turn the dive

So a diver who plans a dive based on the "1/2+200" rule will consume about 1300 PSI out of an AL80 on the penetration, as compared to 1000 PSI for a diver using the more traditional "thirds" method. However, the "1/2+200" diver will reserve an additional 100 PSI out of the back gas as compared to the "thirds" diver, which is equal to the volume of extra gas that was used from the stage. Therefore the total gas consumed on the penetration is the same for both methods, and the thirds rule is not violated. Obviously you will need to make a different adjustment on the back gas for tanks other than 104s, but that's just math.

Also with the 1/2+200 rule, you only leave enough gas in your stage to get back to the next stage or deco bottle, and try to preserve as much back gas as possible. What that means is that a stage is mostly consumed by the time you get to the next stage during the exit, and in a true emergency, you just leave the spent stage, pick up the new one, and keep moving. If you somehow consume an entire stage during the exit, then you leave it and go to an air sharing configuration, and then go back on the next stage when you get to the next drop. This helps to streamline the exit, since you are never carrying more than one stage bottle at a time.

Finally, once you convert your diving to "stages only", you will find it to be a much more economical use of the gas, particularly if you are diving mix. I almost never touch my back gas on a dive any more, and use stages for everything. That means there is very little gas wasted when I go to get fills (~200 PSI in each of the stages), and I almost never have to pay for a doubles fill any more. I sometimes go for months without ever filling a set of doubles, even though I am actively diving with them. In fact, I recently used a set of 104s with a trimix fill (so I could take them up for hydro), which had been sitting "unused" for almost two years. I was actually actively diving with the tanks during that time, but the gas never got consumed because I used the stage bottles for everything, and then just refilled those after each dive (and yes, I analyze the back gas on every dive, even if I did not use the tanks on the last dive).

This has totally changed the way I cave dive, and for the better. You Cave 1 folks just don't know what you're missing.

 

[Jason B]

Looking at it another way.

I dive dbl 95's

Let's fill them to 3600psi. That gives us 7 x 36 or a total of 252cu/ft in back gas.
Our stage is filled to 3000 so that gives us 2.5 x 30 or a total of 75cu/ft in the stage.
This gives us a total of 327cu/ft.

We can use 1/3 of that or 109cu/ft.

Since the stage is dived to 1/2 + 200 (or 1300psi) that means we used 32.5cu/ft of the usable 109cu/ft of gas.

That means we can use (109 - 32.5 ) 76.5cu/ft out of our back gas which is (76.5/7) 1092psi. Close enough to 1100psi.

So our original thirds on back gas (3600psi) was 1200psi, but with the stage we have to deduct 100psi from our original thirds which gives us (1200 - 100) 1100psi to use out of back gas.

 

[rjack321]

So somewhere around 4 stages it gets to the point where you might as well not even bother using backgas (from AL80 doubles). Maybe 5 stages for bigger backgas tanks.

So I think I understand the general progression from dives where using stages + "1/3" of backgas is plausible/logical (a couple stages) to the point where backgas is all reserve (alot of stages).

Thanks fellas

 

[Data]

for simple math I use the following fill numbers and assume to be in a shallow <100ft cave.

1XAL80 to 3200psi = 80cf (TF=2.5)
2X104's to 3600psi = 280cf (TF=8.0)

total of 360cf

1/3rd for penetration = 120cf

breath whatever tank where ever you want first, depending on your teams decision.

you could:
start with half of your AL80 = 40cf or 1600psi
leave stage behind and dive 80cf or 1000psi on BG for further penetration.
pick up stage on the way out and breath of it or not.

or you could:
breath AL80 empty, leave AL80 behind and use 500psi of your BG for penetration,
pickup empty AL80 on the way out.

or you could
start with BG, use 1500psi on the way in turn around and never touch you stage AL80
in this case your AL80 would be like a bail out bottle.

many options, all depends on your dive, your goal, conditions and team unity.

 

[rjack321]

This has totally changed the way I cave dive, and for the better. You Cave 1 folks just don't know what you're missing.

Dude, Cave2 is on the schedule! Why do you think I asked this? :d