Stage planning in caves

[Mike Edmonston]

What is interesting to me is you start to see when RB80s start to come into play, RTodd, where is that in terms of amount of stages? I imagine gas planning is the same, but the RB80 effectively reduces consumption rate . . . but my head is spinning a bit.

Good thread.

If this thread keeps expanding with RB-80's and deep deep team procedures, I'm going to have to go back to school and get an advanced math PHD just to keep up.

Cheers

 

[RTodd]

What is interesting to me is you start to see when RB80s start to come into play, RTodd, where is that in terms of amount of stages? I imagine gas planning is the same, but the RB80 effectively reduces consumption rate . . . but my head is spinning a bit.

Good thread.

Not a rebreather guy so I can't speak to it precisely. Yes, it cuts down on number of stages needed for an exploration, but you still plan open circuit bailout which would include safeties.

 

[RTodd]

by extension, for scooters and stages, would you never want to be violating 1/6ths on total gas?

diving two stages (to 1/2+200) and reserving all of backgas would consume 33.4 cu ft out of 2 stages and would then mean you need to carry in 400 cu ft of gas which is (just) satisfied by 2 stages + double-130s in backgas (414 cu ft total, not much left over for deco).

diving three stages (to 1/2+200), reserving all of backgas (2x130) violates 1/6ths and puts you closer to 1/5ths of total gas at your turn.

Sixths is what I would recommend for new scooter divers, particularly since they are usually not towing a backup and aren't as cognizant of the distances they can go. But, after that it is dependnt on a whole bunch of other factors. Backup and safety scooters, distance, depth, other exits and their viability, etc. I would say that personally I never go below fourths anymore but everything else is situation dependent. For instance, if the dive is a combination of swimming and scootering, the effective gas used for the scootering portion might be around 6ths or better while the swimming portion might be closer to thirds. In mexico, with a three man team and plenty of backup scooters plus alternate exists, it would get closer to fourths for the whole dive. For a pure swim dive with the right buddies and a single stage, in ideal conditions thirds with a slight shade for conservatism. A swim dive where we are in tight clay filled passages looking for leads, back to something much higher.

 

[RTodd]

by extension, for scooters and stages, would you never want to be violating 1/6ths on total gas?

diving two stages (to 1/2+200) and reserving all of backgas would consume 33.4 cu ft out of 2 stages and would then mean you need to carry in 400 cu ft of gas which is (just) satisfied by 2 stages + double-130s in backgas (414 cu ft total, not much left over for deco).

diving three stages (to 1/2+200), reserving all of backgas (2x130) violates 1/6ths and puts you closer to 1/5ths of total gas at your turn.

Let me add, back gas deco in a cave is usually insignificant and rarely really factors into my gas planning. If it is more than a few minutes, you drop and O2 bottle. If is is deep cave, you start dropping 70, 120, 190 bottles as the time at those depths goes beyond fairly short stops.

Also, three stages means scooters to me and the distances covered on three stages and scooters means multiple long bodies per team member which starts become part of your bailout.

 

[DIR-Atlanta]

I agree with everyone that this is a great discussion so far. I did want to clarify one thing that I wrote last night that I did not catch until I reread it this morning:

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.

When I wrote that, I was thinking specifically of a situation where one of the divers had a complete gas failure on the back gas (busted isolator, etc). Strictly speaking, if you completely consume a stage, then you ditch it and go to back gas if you have any. If not, then your next best option in that situation is to take one of your buddy's stages and have them go on their back gas. If the second bottle is also consumed, then you go to air share (the idea being to keep the team swimming as independent divers for as long as possible to allow for maximum travel speed).

On the swim/scooter thing, I agree with RTodd that swimming two stages is a real bear, and I personally don't do it that often (mostly in caves where scootering is not allowed, or where it would be ill-advised due to the nature of the cave system). I also agree on using fourths for longer swim dives - that would basically be a double stage dive using the 1/2+200 rule, except that the back gas would not be touched. Sixths is 1/2+200 on a single stage, without back gas being touched (actually that is closer to "sevenths", but I am not one to quibble ).

In any case, the back gas is always reserved for emergencies only - that's pretty much how I plan all my cave dives these days.

 

[limeyx]

I do seem to remember a really good article on exactly this discussions (perhaps minus the scooter parts) on an Australian DIR site, but cannot locate the site or the article.

Does anyone remember that one ?

 

[Mike Edmonston]

swimming two stages is a real bear, and I personally don't do it that often (mostly in caves where scootering is not allowed, or where it would be ill-advised due to the nature of the cave system). I also agree on using fourths for longer swim dives - that would basically be a double stage dive using the 1/2+200 rule, except that the back gas would not be touched.

Let's say that you were doing a dive in Madison Blue (no scooters). Would you double stage it, and go to 1/3's on backgas, or would you only stage the 80's and not touch the backgas?

Assuming that you are staging 80's, and diving 1/2+200, you would use approx 70cf of gas. Now a set of 120's run at thirds would give you 80cf of gas, without the drag of the stages.

I guess my question is, on a double stage dive if you're not going to touch your backgas, would it not be more efficient to dive the doubles to 1/3's instead of staging?

Thanks

 

[lamont]

Sixths is what I would recommend for new scooter divers, particularly since they are usually not towing a backup and aren't as cognizant of the distances they can go. But, after that it is dependnt on a whole bunch of other factors. Backup and safety scooters, distance, depth, other exits and their viability, etc. I would say that personally I never go below fourths anymore but everything else is situation dependent. For instance, if the dive is a combination of swimming and scootering, the effective gas used for the scootering portion might be around 6ths or better while the swimming portion might be closer to thirds. In mexico, with a three man team and plenty of backup scooters plus alternate exists, it would get closer to fourths for the whole dive. For a pure swim dive with the right buddies and a single stage, in ideal conditions thirds with a slight shade for conservatism. A swim dive where we are in tight clay filled passages looking for leads, back to something much higher.

Okay, I guess the larger point I was trying to get at, though, is that you can't be dropping stages all day at 1/2s and not be thinking about how much backgas reserve you've actually got? At some point all of backgas isn't enough and you need the safety bottles and backup scooters and alternative exits need to be considered -- and that point approaches faster as you introduce scooters (or siphons?).

Also, on a slight tangent, at some point as you go deeper don't you need the rebreather just to extend the distance between stage drops? I mean at 10 ATA and 0.50 SAC you'll be doing stage drops every 7 minutes (And a 2 minute stage drop burns 400 psi so you'll want to reserve that, so 5 minutes between stage drops). Even if you can get your SAC a bit better than that, it doesn't seem worth it and the margin for error seems a little slim and the distance you could explore would become limited, even on a scooter, and your trigger-time-to-stage-fiddling ratio would be fairly poor...

 

[timle]

Let's say that you were doing a dive in Madison Blue (no scooters). Would you double stage it, and go to 1/3's on backgas, or would you only stage the 80's and not touch the backgas?

Assuming that you are staging 80's, and diving 1/2+200, you would use approx 70cf of gas. Now a set of 120's run at thirds would give you 80cf of gas, without the drag of the stages.

I guess my question is, on a double stage dive if you're not going to touch your backgas, would it not be more efficient to dive the doubles to 1/3's instead of staging?

Thanks

I'm interested in the logic behind this also.

2 person dive team - 2 x 80@3000 each + double 120's or so each.

1300PSI used in each stage ~= 35 cf

max penetration = 70 cf

So assuming a highly unlikely situation of losing ALL stages + one backgas at max penetration. You still have 100cf more than needed to exit in one set of doubles. Seems like a lot of gas just to tow around.

 

[RTodd]

Okay, I guess the larger point I was trying to get at, though, is that you can't be dropping stages all day at 1/2s and not be thinking about how much backgas reserve you've actually got? At some point all of backgas isn't enough and you need the safety bottles and backup scooters and alternative exits need to be considered -- and that point approaches faster as you introduce scooters (or siphons?).

Also, on a slight tangent, at some point as you go deeper don't you need the rebreather just to extend the distance between stage drops? I mean at 10 ATA and 0.50 SAC you'll be doing stage drops every 7 minutes (And a 2 minute stage drop burns 400 psi so you'll want to reserve that, so 5 minutes between stage drops). Even if you can get your SAC a bit better than that, it doesn't seem worth it and the margin for error seems a little slim and the distance you could explore would become limited, even on a scooter, and your trigger-time-to-stage-fiddling ratio would be fairly poor...

Yes, back gas becomes not enough but very, very few people ever do stage dives that involve 3+ stages. At some point on deep stuff the logistics do start requiring rebreathers. I don't personally do anything remotely on this scale, so can't really comment definatevely on where the line is. They did use open circuit out past 10k feet in Wakulla. Not many caves of this magnitude and depth. Also, keep in mind that a .5 SAC rate even completely stressed is unlikely for divers at this level and that few caves have an actual average depth of 10ATA. So, you are looking at more like 10+ minutes of trigger time and some stage changes are done mostly on the fly. Having dived with some of these guys when they were doing this open circuit and had the get it done fast attitude, I can tell you that causing the team to spend 2 minutes on a stage drop would cause you to catch an enormous amount of grief. I used to wait ready to go at drops with Danny and Chris thinking what is taking these guys so long. Now that I have the take your time and be careful attitude of stage drops more appropriate to Mexico, they are back to waiting on me since my ability to do the drops carefully is slower than theirs.