stage diving

[Dan Gibson]

Who do you think was loading the 4-wheeler this past week?. It wasn't Edd's helper.

I think Edd needs to install an incline.

That's what the cart and 4-wheeler are for!

 

[Dive-aholic]

To change the subject for just a minute, who hear actually uses this method (i.e. actually breath 1/3 of the stage)?

I breath the stage to the 1/2+2 and reduce backgas available prior to applying 1/3rds.

Example: Assume buddies have same volume of gas
stage 3000 psi
104s 3600 psi
breath stage to 1/2+200 = 1700 psi
Turn backgas at 2400+200=2600 psi

I like the fact that one person uses the stage and reserves his buddies gas in the doubles where it is far safer to donate from. I can always drop an empty stage and reduce drag when necessary when something goes south.

Good point. I haven't done many stage dives yet and am just being overly conservative. In those few dives, the 2nd 1/3 usually doesn't even get used and not much of the 2nd 1/3 of the back gas (the nature of high flow systems ). As I get more comfortable with not having all my gas with me, I'll change to the more traditional method.

 

[do it easy]

I don't stage dive in caves- just on wrecks to get two dives out of a single set of doubles- first dive on the stage and the second dive on the doubles. This is due to lack of space, lack of a compressor on the boat, or just being too plain lazy to lug 2 sets of doubles onto a boat.

Anyhow, my question about the "rules" for how much gas to keep with you in a cave. Using a single stage, I think it would be possible to keep enough gas so that you didn't have to rely on the half+200 stage to exit. What about with multiple stages where you might stage past where your backgas could get you out of? Are there limits on how many stage failures that you plan for?

Just curious...

 

[Dive-aholic]

My limited experience, and the reason I do what I do, gives me this:

3000psi stage, 3600psi back gas.
Breathe stage to 2000, switch to back gas, breathe back gas to 2400, turn dive. Even if my breathing is equal on the way out I will hit my deco bottle with at least 700 left in my back gas and not having to touch my stage. That's never the case. Usually I hit my deco bottle before I even hit 1200 in my back gas.

half+200 method
Breathe stage to 1700, switch to back gas, breathe to 2600, turn dive. Breathing at the same rate will require...1650 leaving me with 950 in back gas if I don't touch my stage.

Okay, I just learned something! I should have done these calculations a while ago!! :doh:

 

[do it easy]

I think the difference has to do with the difference in conservatism of the two scenarios. In your 3000/3600 psi scenario, you use less gas from the stage, but more (volume) from the back gas, so it seems logical that you would end up with less in the back gas than the half+200 scenario. Am I missing something? Or maybe the point is to dive more conservatively?

 

[Dan Gibson]

I believe keeping more gas on you back gives you a better chance if something goes wrong. You can always dump a stage and move faster. It is generally better when you need to donate gas. I never really liked the idea of donating a stage to an already OOA diver because that person could be put in another low on air or OOA shortly. That's not to say its never done. If you leave more gas in a stage a loose that stage, that is more gas that is unavailable. I'm sure there are many more reasons that I have not thought out or haven't realized just yet, or maybe it's just too early in the morning to think. I'm sure some exploration type divers could add a lot more.

I think the difference has to do with the difference in conservatism of the two scenarios. In your 3000/3600 psi scenario, you use less gas from the stage, but more (volume) from the back gas, so it seems logical that you would end up with less in the back gas than the half+200 scenario. Am I missing something? Or maybe the point is to dive more conservatively?

 

[Johnny Richards]

To change the subject for just a minute, who hear actually uses this method (i.e. actually breath 1/3 of the stage)?

For cave diving- which is all I do- I always dive thirds and no more from my stage and always pressurize the IP hose with the valve off when not in use.

Jeez, the exchange between the two sounds more suited to The Deco Stop.

Johnny

 

[Dan Gibson]

Why do you choose this method over 1/2+? Besides the math being slighty easier, are there cases where 1/3 from any source would be better. I chose the half plus based on preserving as much in the backgas as possible.

For cave diving- which is all I do- I always dive thirds and no more from my stage and always pressurize the IP hose with the valve off when not in use.

Jeez, the exchange between the two sounds more suited to The Deco Stop.

Johnny

 

[MikeFerrara]

Maybe some aren't seeing how the half+ 2 thing works.
Dan didn't do a great job of presenting the math. Don't get mad at me Dan.

If we call an al80...80 cu ft (it's close), we use half (save a little because guages don't accurately read zero) and half on the way back. Now the stage is empty but did we violate the rule of thirds? No because we reserved an extra 40 cu ft of our back gas. Essentally we subtract that 40 cu ft before calculating what a third is.

If the reserve is on your back, in theory, you have some redundancy there and you are not going to ditch it or fail to pick it up. the idea is that on your back is a more reliable place to keep your reserve gas.

My comment about Dans math is just related to the fact that it might not have been obvious what he was doing. 40 cu ft is about 500 PSI in lp 104's (about 8 cu ft/100 PSI). subtract 500 psi, calculate 1/3 on that useable supply and subtract that 3rd from the starting pressure to get turn pressure rounding to the more conservative side to the nearest 100 psi. If using more than one stage, you just reserve 40 cu ft (5-- PSI in lp 104's) for each stage. Obviously if the doubles aren't lp 104's the math changes accordingly. My wife dives hp 100's so she is dealing with about 6 cu ft/100 PSI so she reserves 700 psi for each al80 stage.

I too have used a stage bottle to get two wreck dives out of one set of doubles. Here I do it different. I drain the stage on the first dive so I don't have to carry it on the second dive because we usually don't drop tanks in OW like we do in a cave and why carry a stage and two decompression gasses on both dives if you don't have to? This is a bit different than a cave dive where the stage is to add distance or time. With 20 - 30 minute bottom times just about the whole first dive can be done on the stage reserving almost the total volume in the back gas...maybe you use a little on the deep part of the ascent before the first gas switch. We still never come close to pushing "the rule of thirds".

 

[ianr33]

I see a couple of advantages to breathing a stage to 1/2 +200 (and reserving extra in backgas) as opposed to breathing it to thirds:

1)Stage will be deeper into the cave so closer to you in the event of problems.
2)You have 2 regulators on your backgas,so both divers can access that gas.Suppose you drop stages at 1/3rds,continue on backgas to 1/3rds at which point one diver has a total gas loss.They share air and make it back to the stages with empty tanks.If for whatever reason either of those stages is not available their only option would be to buddy breathe the stage to get out.Not something I ever want to do!
The above contains 2 failures (backgas and stage) so is pretty unlikely but I think it is worth considering.

The advantage of breathing stages to 1/3 rds is that you do not have to drag them as far.

Having said all that the differences are not great.Breathing an AL 80 to 1/3 rds it will be dropped at 2000 psi. Dropping at 1/2 +200 it will be at 1700.

I am merely a lowly Intro diver so take the above for what it is worth.