OP
FIXXERVI6
Contributor
isn't it kind of bad not to have an iso ?
why not doubles on AL?
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Don Burke
Contributor
If you are treating the doubles as a big single, the isolator isn't an issue.
If you are not in a situation that calls for full redundancy in regulators, the use of just one first stage is not a problem.
Much of my diving with double 72s is so I can stay with the guys diving 131 singles. They manage to make do with one first stage.
Once I'm in an overhead environment (real or virtual), all the pieces and parts become necessary. It's easier for me to have all of my doubles set up to support overhead environments. Three sets is all I really want to maintain.
Small doubles with isolation manifolds also keep me out of the Y-valve and H-valve business, so I think it's cheaper in the long run.
If you are not in a situation that calls for full redundancy in regulators, the use of just one first stage is not a problem.
Much of my diving with double 72s is so I can stay with the guys diving 131 singles. They manage to make do with one first stage.
Once I'm in an overhead environment (real or virtual), all the pieces and parts become necessary. It's easier for me to have all of my doubles set up to support overhead environments. Three sets is all I really want to maintain.
Small doubles with isolation manifolds also keep me out of the Y-valve and H-valve business, so I think it's cheaper in the long run.
ew1usnr
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I've been cave diving for about the last 15-years with twin aluminum 100's. The GUE DIR people will say that this is wrong because the aluminum tanks get bouyant, but I've never had a problem. I dive in fresh water with a wet suit and sink like a rock when the tanks are full. The bouyancy doesn't become an issue untill the psi drops to below 500. But by the rule of thirds, I always exit with about 1000 psi. I like the aluminum tanks because, from my experience, they are much more maintenance free then steel tanks. The dive shops seemed to want me to tumble my steel tanks (when I had steel tanks) every time they needed a VIP. I've never been asked to tumble an aluminum tank.
Don Burke
Contributor
Actually, the GUE people I have talked to are of the opinion that aluminum is the best way to go with doubles and a wetsuit.
My personal experience is that a safe application of steel doubles with a wetsuit would be the exception rather than the rule.
My personal experience is that a safe application of steel doubles with a wetsuit would be the exception rather than the rule.
ew1usnr
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Don! I stand corrected. I based my remark on something I read on a GUE site that said that (paraphrased) "a diver should have enough weight to still stay under even with his tanks empty". To me, that does not necessarily make sense on several fronts. For a long time steel tanks (104's) were regarded as the standard for cave divers because they held a larger air volume and were not as bouyant when empty. My brother and I were the exception with our aluminum 100's. Maybe as time has passed some people (though not all) have realized that the bouyancy issue was not so much a problem, and that a little less negativity might be a bit of an asset. Some of those steel tanks were so heavy that if a diver tore his BC he would literally have to walk out of a cave. That was when they started using backup BC's. Even if the BC doesn't blow, it has to be kept partially inflated to offset the weight of the steel tanks, thus creating extra frontal area and drag. The aluminum tanks are roughly neutral for much of the dive so the BC is not such a critical safety issue. Maybe my brother and I were "ahead of the curve" on this one.
OP
FIXXERVI6
Contributor
ok, so double neutrals are a bad idea then because of the bouancy issue? I mean, if I have no wet suit on, I 'd have 0 ditchable weight.
Don Burke
Contributor
You need enough ditchable weight to solve the following problems:
1. You roll off the boat and snag your inflator hose on something, ripping it to shreds. How much weight do you need to ditch to stay on the surface?
2. You get problem one straightened out and try the dive again. Upon reaching the bottom, you snag your inflator hose on something, ripping it to shreds. How much weight do you need to ditch to get to the surface?
If the number is zero, you don't need any ditchable weight. That can be the case with a single tank and no neoprene.
With the gas in my 80s and wetsuit compression, I need about 9 pounds for problem 1 and about 20 pounds for problem 2, so 20 pounds it is. I prefer not to rely on being able to swim any weight up.
With my wetsuit, standard 80s and a steel backplate, I need about 24 pounds of weight to be neutral, so I have enough.
If I went to neutral 80s, I would have to take 8 pounds off the belt, leaving 16. That means I would be four pounds short. I can swim it up, or switch to an aluminum backplate to put five pounds onto the belt. I don't need an aluminum plate for anything else.
Since I don't need neutral 80s for anything else, I stay with the standard 80s. If I decide to give up wetsuit diving, the standard 80s make better stage/deco bottles.
1. You roll off the boat and snag your inflator hose on something, ripping it to shreds. How much weight do you need to ditch to stay on the surface?
2. You get problem one straightened out and try the dive again. Upon reaching the bottom, you snag your inflator hose on something, ripping it to shreds. How much weight do you need to ditch to get to the surface?
If the number is zero, you don't need any ditchable weight. That can be the case with a single tank and no neoprene.
With the gas in my 80s and wetsuit compression, I need about 9 pounds for problem 1 and about 20 pounds for problem 2, so 20 pounds it is. I prefer not to rely on being able to swim any weight up.
With my wetsuit, standard 80s and a steel backplate, I need about 24 pounds of weight to be neutral, so I have enough.
If I went to neutral 80s, I would have to take 8 pounds off the belt, leaving 16. That means I would be four pounds short. I can swim it up, or switch to an aluminum backplate to put five pounds onto the belt. I don't need an aluminum plate for anything else.
Since I don't need neutral 80s for anything else, I stay with the standard 80s. If I decide to give up wetsuit diving, the standard 80s make better stage/deco bottles.
Don Burke
Contributor
Those 80s hold about 12 pounds of air, about 10 of that is extra weight at the begiinning of the dive.
If you can deal with a wing failure with no ditchable weight, you don't need any ditchable weight.
If you can deal with a wing failure with no ditchable weight, you don't need any ditchable weight.
No!
A common misconception. Air weighs about 1 pound for every 13 CF. It doesn't matter if the cylinder is steel, AL or wood, if you stuff, say, 80 CF of air in it, it'll be 6 pounds more negative when full than when empty.
I really hate the phrase "neutral 80" -- it's typical marketing department BS to make you think the wrong thing. A normal AL80 will swing about 6 pounds from full to empty, as will a Steel 80, as will a "Neutral" 80.
It's just that the neutral 80 is more negative when empty than a normal AL80 (like 0 versus +3), which can be accomplished far more cheaply by strapping a three pound weight on your cam band...
I hear this from dive shops all the time: An AL doesnt change buoyancy as much as a steel I dont know how physics works where they come from, but in this universe a cylinder will become 1 pound more negative for every 13 CF of air you pump in it, no matter what.
Roak
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