Need more gas - now what?
- Thread starter Dan_P
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@Dan_P no, the CCR is not the best tool for those dives. Many of them require true sidemount passages, and the dives really aren't that long, or that deep. 100ft for 3 hours is not a dive that warrants a CCR. It adds complexity that isn't needed for those dives as well as restricts where you can go with most of them. A sidemount rebreather is possible, but again you still have to carry quite a bit of stuff with that and it isn't without its own issues.
@Gareth J I mostly share your opinion regarding the gas availability, but on CCR you are still keeping your fingers crossed that the unit stays breathable and it may not always be, so you always end up as an OC diver in the end. The risk:reward for me has led to CCR, but it's taken many years for my diving to get to a point where the risk:reward tipped in the favor of the CCR.
Back to Dan.
Even without the stages, a dive on a pair of 121's would require 4x al80's for the same gas. The 121's are more efficient in the water so you get farther in the cave than carrying 4 tanks. AL80's aren't used in most training scenarios either because the weight
buoyancy doesn't work out. An AL80 for most people in the Florida caves is good for about 500-600ft of penetration. That gives 1000ft penetration since we have to carry 2. If we go to 4x al80's, you will not make it to 2000ft because of the extra drag associated with the extra cylinders. If you go to something like HP100's, then you will be able to actually go about 30% farther and will likely make it to 30% farther, and the rig is no heavier than the al80's. Why would you choose al80's vs. the 100's? Same weight, 30% more gas, that's a no brainer.
You do NOT swap primary sidemount tanks in the water. No one actually teaches that as a practical means of sharing air. Again, come and actually cave dive and try it, you share air on the long hose. One required, not two, no primary bottles move. Stages move, but that's it.
Most CCR divers in cave country use steels as bailouts, for all of the reasons mentioned above. CCR on back, big steels sidemounted, stages as needed.
@Gareth J I mostly share your opinion regarding the gas availability, but on CCR you are still keeping your fingers crossed that the unit stays breathable and it may not always be, so you always end up as an OC diver in the end. The risk:reward for me has led to CCR, but it's taken many years for my diving to get to a point where the risk:reward tipped in the favor of the CCR.
Back to Dan.
Even without the stages, a dive on a pair of 121's would require 4x al80's for the same gas. The 121's are more efficient in the water so you get farther in the cave than carrying 4 tanks. AL80's aren't used in most training scenarios either because the weight
buoyancy doesn't work out. An AL80 for most people in the Florida caves is good for about 500-600ft of penetration. That gives 1000ft penetration since we have to carry 2. If we go to 4x al80's, you will not make it to 2000ft because of the extra drag associated with the extra cylinders. If you go to something like HP100's, then you will be able to actually go about 30% farther and will likely make it to 30% farther, and the rig is no heavier than the al80's. Why would you choose al80's vs. the 100's? Same weight, 30% more gas, that's a no brainer.You do NOT swap primary sidemount tanks in the water. No one actually teaches that as a practical means of sharing air. Again, come and actually cave dive and try it, you share air on the long hose. One required, not two, no primary bottles move. Stages move, but that's it.
Most CCR divers in cave country use steels as bailouts, for all of the reasons mentioned above. CCR on back, big steels sidemounted, stages as needed.
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Love the "mexico is so shallow" part. Keep the good stuff coming
do you disagree? there are obvious exceptions, just as there are shallow caves in Florida, but in general, they're quite shallow. When they get deep, they get REAL deep, and then you are in CCR hypoxic trimix land. Doesn't change the fact that Florida caves in general are at twice the pressure so you consume twice the gas....
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Dan_P
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So, from the mouth some 50% of the time.
Yes, CCR adds complexity and I'd say tinkering with the gasshare protocol definitely qualifies for that category.
I'm not willing to carry over this one to O/C.
I know that you do not swap tanks in a gasshare, from one of our previous discussions. I'm saying that's one of the gasshare solutions I too am not attracted to, handing off tanks - so are two longhoses and inconsistent gasshare protocol in O/C.
As for static primary tanks: I do dive a rig where all bottom tanks and stages are identical and interchangeable. I see some benefits, but:
In terms of which tanks to choose on that rig, it's clear that there's a drawback in the lesser volume if choosing AL80s, as you say. I don't disagree with that.
I just find them easy to handle, which is something that I also care about because they're not static on my rig.
If you're keeping your primaries static, sure, why not go with the biggest you can find if you're not moving them anyway, the bouyancy works out and you're comfortable.
Fair enough. But mine aren't static is all.
If I need different tanks for a specific dive in a specific place, I'd rent different tanks.
Like I said, if I find myself in Florida, rest assured I won't have checked 4 AL80s in my luggage
Conversely, you won't find any LP121s with a dive shop I've ever seen in the region I live in.
15L 232 bar steel, sure, but... nah.
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