DPV Gas Planning

[Anders Knudsen]

Hi CDF,


I was reading through some old threads regarding gas planning for CCR bailout, and was thinking a bit about my own gas planning for OC scooter cave dives and though I would start a discussion on how all of you are are calculating gas reserves for cave scooter dives.
1. With stages and 1 scooter
2. With stages and a backup scooter
3. Backgas and 1 scooter

In all scenarios I am assuming diving with either a buddy or in a team of three.
I am not a RB diver, but I don?t think the approach would change a lot, besides the fact that all your cylinders would be full and the chance of 2 divers having to bail out at max penetration is probably very limited.


Sorry about my units, as I am a European cave diver with the majority of my cave dives in the US and Mexico I still use metric units, besides for distance where I use feet.

I am in the following assuming a swimming SAC rate of 20l/min (0.71 cf/min) and a swim speed of 50 ft/min. In general my swimming SAC is significantly below that and I tend to be a fast swimmer, but as we are talking gas reserves better be conservative.

lets take a dive in a random cave, for the sake of math lets make it average depth 30m (100ft) we are diving a single scooter, 2 stages (11.1l/80cf) and 2x16l back/side gas (108cf?) (plus oxygen deco)

I have never been really happy with the rule of 1/4th, 1/5th, 1/6th or whatever when scootering despite those rules probably allowing us to increase distance a bit depending on the dive. I would much prefer to have a drop pressure/distance where I know I have to drop the cylinder. This also allows me to poke around other leads when going back, as long as I don?t break my: no less than XX bar at XX distance.

Assuming that we have to swim out in case of a scooter failure (with a buddy that is unlikely, but possible) a drop pressure of 150 bar (not saying this is the correct pressure, but it is a starting point) a 80cf with the assumptions above would allow me to swim 1041ft. Meaning that I could drop the stage at 1000ft or 150bar whichever comes first and still be able to swim out/to the next stage.
Assuming the same for the 2x16L that would allow me to turn at 150 bar or 3000ft after I dropped my last stage.
Under this assumption at 30m depth and with 2x 80cf stages and 2x16 it would allow me to swim back from 5000 ft if I dropped all my cylinders at 150 bar and turned at 150bar of backgas (5081ft to be exact).

This assumes that everything besides the scooter failure goes perfectly and I would for sure not be comfortable with that, I can then either add a reserve or go with the following. The only portion of the dive where we would potentially need to share gas would be the backgas portion. Assuming OOG at the worst time possible we move the max penetration since last stage to 1500ft instead of 3000ft. This would allow 2 divers to lose their DPV?s and one of them to be OOG at max penetration. This would also allow that if we don?t have an OOG but instead one of the stages are not working then I can still swim out on the remaining reserve of the D16?s instead of using the stage - but does not take a further increased SAC into account. (besides the increase from normal rate to 20l/min(0.71cf/min))

Also in this is the case it would be possible to just add a 80cf for every additional 1000ft of penetration you are planning. (assuming 60bar to scooter 1000ft and 150bar reserve) this would mean that I could always swim to the next stage, and given a functioning scooter I would have plenty of gas.


Do you guys plan for an OOG and not having the ability to tow out? This would be very unlikely, but gives a decent reserve?

How would you guys approach that if you added a backup scooter? This would basically eliminate the chance of a swimming exit.?


with a backup scooter you could also argue: it would take ~2800l of gas to scooter out of the cave from 3500ft (assuming 120ft/min,20l/min sac, 2 min for each gas switch and 2 min to solve a potential problem) lets just double that and be done with it.( that would not be enough to swim out from max penetration, but close)



Hopefully this makes sense, if not then I apologize - hopefully it can spark some conversation.

[kensuf]

I wrote an article on this for UWS recently, but it appears that UWS may be no more. So I posted it on my site.

http://www.divegainesville.org/cave-dpv-dive-planning/

[Jack Hammer]

Nice article, Ken. I really like that you stress conservatism in numbers.

[Anders Knudsen]

Great article Ken! Well written and well really supports your thoughts behind adding the backup scooter.

My general rules of thumb has been very similar, I tend to bring a backup scooter at 4000ft for dives shallower than 100ft and use thirds on scooter burn time (so 66% rather than 70%, but that is fairly close) but these are minor differences and I may start adding the backup scooter a bit earlier ? why not.

If you don?t mind I will follow up on the issue of gas planning. In your article you bring up that bringing double the gas needed to swim out from 10000ft in Manatee or from 2200ft in Eagles Nest is not practical, and that?s why you add the backup DPV. But in that case, when you add the backup DPV what do you do with gas planning? Assume twice the gas it would take to scooter out?
From 10000 ft back in Manatee (25m average depth) assuming a scooter speed of 150fps and SAC of 0.71 that will be ~9300l of gas (I think that is around 330 cf?) and 2200ft at 300ft will be ~5900l (208cf).

Also curious, how about before you add the backup DPV, so lets say 3000ft in at 100ft.

Thank you for this! Great stuff!

[bamafan]

One of the first things I was taught was plan for any 2 failures. I use this as a bare minimum and I am notorious for carrying extra gas. You can plan yourself out of any dive.

Oh, that is a great article by the way.

[Napalm]

but it appears that UWS may be no more.

Could you say more about this? I haven't received an issue of UWS in going on two years and no one has been able to figure it out.

[Kelly Jessop]

Could you say more about this? I haven't received an issue of UWS in going on two years and no one has been able to figure it out.

Worth another thread versus derailing this one,but an answer is deserved. I recall something about not having the software to do the journal.

[FW]

UWS is back. I got a digital version in email today.

[MORGAN]

UWS is back. I got a digital version in email today.

Yup, me too. And the paper version is supposed to be in the mail in a week or so.