Question Question for Experienced O2ptima CM Divers
- Thread starter rx7diver
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OP
Well, 150 fsw was just a number chosen for this "thought exercise." I imagine that if this "thought exercise" ever manifested into reality, my first dives would be in the 150 fsw range. But, I likely would be diving a "bit" deeper, eventually.
I know that you know that I was doing much deeper "Advanced Deep Air" extended range dives once upon a time. So, this "thought exercise", realized, would take me back to those depths. And deeper.
Now that I have a reasonable (?) plan, I am going to run some numbers to see the max run times that are possible if I am carrying two Luxfer Al 40's of EAN50, planning to deco out breathing from only one cylinder of EAN50.
Thanks,
rx7diver
If you're set on carrying 2 deco cyl, why not make one O2? Odds are pretty high that you will have both, so you might as well have the more efficient deco (assuming you did have to bail out).
Being solo, I can see going further down the failure tree, but you should be specific about what you mean by "loss". If you're carrying both, "loss" probably means the reg dies somehow, but you have several others in a pinch, so it's not really "lost".
(A similar argument can be made to carry one deco gas rather than two.)
Being solo, I can see going further down the failure tree, but you should be specific about what you mean by "loss". If you're carrying both, "loss" probably means the reg dies somehow, but you have several others in a pinch, so it's not really "lost".
(A similar argument can be made to carry one deco gas rather than two.)
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Please find a competent instructor on this this. The “thought exercise” here is flawed in multiple ways. There are at least 2 CM instructors already offering opinions on this and I will be the 3rd.
It is not the same as OC diving and gas planning for BO is not the same as OC gas planning. Lots to be learned here on a unit and it may be a step back in your diving before you can move forward safely.
It is not the same as OC diving and gas planning for BO is not the same as OC gas planning. Lots to be learned here on a unit and it may be a step back in your diving before you can move forward safely.
Like I said, too much to fix. He needs to start at the fundamentals of gas planning, deco planning and ccr. He’s a long way off.
OP
UPDATE: I was hoping to compute a couple of profiles by hand. However, it turns out that I don't have any printed tri-mix tables here (that I could find). So, I fired up an old laptop running an old deco software program just to get some idea of OC gas consumption and run times for a Tri-Mix (comprised of 30% He and 70% EAN32) dive to 165 fsw, for a bottom time of 30 min, decoing out on EAN50 from 70 fsw to surface. Descent rate 60 fpm. Ascent rate 30 fpm.
Turns out (my first surprise) that my baby doubles (LP50's) pumped to 3,000 psig + Cho2ptima won't work for me for this dive!!
I assumed:
(a) At the point in time that I reached my turn time (RT ~15 min), my Cho2ptima would die, and I would have to bail out to OC and immediately begin my ascent (retracing my descent route), and ...
(b) ... shortly after I began breathing OC, I would need to isolate my doubles (with the gas in one of the doubles cylinders becoming lost/inaccessible thereafter)--so that I would need to breathe OC from only one of my doubles cylinders, essentially almost immediately, until my gas switch to EAN50 at 70 fsw.
(c) The EAN50 deco gas I would breathe will come from only one of the two Luxfer Al40's I would be slinging.
Okay. Turns out that there is more than enough EAN50 in a single Al40 for this dive. However, there is NOT nearly enough Tri-Mix diluent in a single LP50 to use as bail out gas during my return to my gas switch depth (retracing my descent route).
Upshot: This solo dive requires (for me) larger doubles for diluent. My LP50's won't do!! And since I would prefer at least a 45 min BT (rather than only a 30 min BT), my LP50's really, really won't do (for me) for a solo dive to 165 fsw!
NOTE: Years ago I could easily do this dive (30 min BT), as a buddy dive, using air (bottom gas) and EAN50, using my manifolded HP120's.
rx7diver
Turns out (my first surprise) that my baby doubles (LP50's) pumped to 3,000 psig + Cho2ptima won't work for me for this dive!!
I assumed:
(a) At the point in time that I reached my turn time (RT ~15 min), my Cho2ptima would die, and I would have to bail out to OC and immediately begin my ascent (retracing my descent route), and ...
(b) ... shortly after I began breathing OC, I would need to isolate my doubles (with the gas in one of the doubles cylinders becoming lost/inaccessible thereafter)--so that I would need to breathe OC from only one of my doubles cylinders, essentially almost immediately, until my gas switch to EAN50 at 70 fsw.
(c) The EAN50 deco gas I would breathe will come from only one of the two Luxfer Al40's I would be slinging.
Okay. Turns out that there is more than enough EAN50 in a single Al40 for this dive. However, there is NOT nearly enough Tri-Mix diluent in a single LP50 to use as bail out gas during my return to my gas switch depth (retracing my descent route).
Upshot: This solo dive requires (for me) larger doubles for diluent. My LP50's won't do!! And since I would prefer at least a 45 min BT (rather than only a 30 min BT), my LP50's really, really won't do (for me) for a solo dive to 165 fsw!
NOTE: Years ago I could easily do this dive (30 min BT), as a buddy dive, using air (bottom gas) and EAN50, using my manifolded HP120's.
rx7diver
tmassey
Contributor
Just to highlight what some other people have said: you are creating dive plans that are expecting to handle multiple major failures. We never do that, even with open circuit, recreational or technical. We assume one major failure.
On technical OC, we assume you lose half your gas at the worst possible point and plan accordingly: dive thirds. We also assume that you might lose a deco gas: make sure you have enough backgas or other alternative to complete your deco. But when planning a dive, we don’t assume both will happen.
You’re planning for multiple major failures: a complete failure of your CCR in the first place, *then* a major failure of your bail out gas, and *then* a loss of deco gas. There’s no doubt you’re having a hard time closing that loop: it’s a crazy amount of failures, and it would take a crazy amount of resources to address it.
Like @Tracy said: it’s always possible to pile more failures on until eventually you don’t have the resources. Eventually, the only decision you can make is not to dive. It would be no different than trying to protect yourself while driving a car from a sinkhole magically appearing. Does it happen? Yes! Do we design our driving plan to address it? No: it’s both impractical and improbable.
And like others have said, the assumptions that you are making do not align with basic rebreather strategies and philosophies. I’m not a rebreather diver, so I don‘t want to comment on the exact specifics — I’m not qualified. But the people who are experts on rebreather diving are also saying this as well.
Maybe a different way for you to approach this would be to create a specific dive plan and then ask someone how they as a CCR diver would approach that. (Even better would be to do some searching for one of the dozens of posts that already do this… ) @Tracy above gave you some general ideas of this with a dive he is planning. Given that he is a CCR instructor, and stated the requirements in a public forum, I would probably assume that those aren’t cutting corners. Then, see if you can do the analysis in reverse, and understand why what he wrote is sufficient.
Until you can do that, it’s a fairly safe assumption that there’s some fundamental aspect of CCR diving that you are missing. And it might be a little unrealistic to ask people to teach you all of the details of CCR diving one post at a time on ScubaBoard…
These type of threads come up fairly routinely: open circuit divers trying to plan CCR dives. There should be numerous examples for you to examine to be able to understand the philosophy such divers use in planning those dives.
On technical OC, we assume you lose half your gas at the worst possible point and plan accordingly: dive thirds. We also assume that you might lose a deco gas: make sure you have enough backgas or other alternative to complete your deco. But when planning a dive, we don’t assume both will happen.
You’re planning for multiple major failures: a complete failure of your CCR in the first place, *then* a major failure of your bail out gas, and *then* a loss of deco gas. There’s no doubt you’re having a hard time closing that loop: it’s a crazy amount of failures, and it would take a crazy amount of resources to address it.
Like @Tracy said: it’s always possible to pile more failures on until eventually you don’t have the resources. Eventually, the only decision you can make is not to dive. It would be no different than trying to protect yourself while driving a car from a sinkhole magically appearing. Does it happen? Yes! Do we design our driving plan to address it? No: it’s both impractical and improbable.
And like others have said, the assumptions that you are making do not align with basic rebreather strategies and philosophies. I’m not a rebreather diver, so I don‘t want to comment on the exact specifics — I’m not qualified. But the people who are experts on rebreather diving are also saying this as well.
Maybe a different way for you to approach this would be to create a specific dive plan and then ask someone how they as a CCR diver would approach that. (Even better would be to do some searching for one of the dozens of posts that already do this…
) @Tracy above gave you some general ideas of this with a dive he is planning. Given that he is a CCR instructor, and stated the requirements in a public forum, I would probably assume that those aren’t cutting corners. Then, see if you can do the analysis in reverse, and understand why what he wrote is sufficient.Until you can do that, it’s a fairly safe assumption that there’s some fundamental aspect of CCR diving that you are missing. And it might be a little unrealistic to ask people to teach you all of the details of CCR diving one post at a time on ScubaBoard…
These type of threads come up fairly routinely: open circuit divers trying to plan CCR dives. There should be numerous examples for you to examine to be able to understand the philosophy such divers use in planning those dives.
broncobowsher
Contributor
Some of the more solid advise here on Scubaboard.
As stated, a bit of an unreasonable expectation for failures. If you had half the failures you are trying to plan for all at once, that would be one of the worst rebreather dives that ever happened in the history of diving.
OP
It's curious that, by my calculations, I can do the above dive (165 fsw, 30 min BT) as an open circuit air + EAN50 solo dive using my manifolded "cave-filled" HP120's.
But, looks like the smallest manifolded doubles I have access to that I can use with the CM for this solo dive (165 fsw, 30 min BT; Tri-Mix + EAN50) are old-school 72's.
This has been an enlightening "thought exercise" (for me)!
Thank you, everyone who contributed.
rx7diver
But, looks like the smallest manifolded doubles I have access to that I can use with the CM for this solo dive (165 fsw, 30 min BT; Tri-Mix + EAN50) are old-school 72's.
This has been an enlightening "thought exercise" (for me)!
Thank you, everyone who contributed.
rx7diver
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