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Scuba Forum / UK Scuba / November 2005Ali or steel stage tanks question - perhaps a bit boring, but I'm confused.
I'm a bit flumoxed on the Ali versus steel question for stage tanks, perhaps someone can help, thought perhaps at the risk of stiring up a big debate. The DIRistas (from whom I've learned a lot, it must be said), and the (excellent) www.bitz.fsnet.co.uk are all pretty adament that Ali's the thing, due to more desirable bouyancy characteristics. However, I'm still unconvinced and don't want to lug an extra 2 or 3 kilos up a ladder unless there's really a very good reason, and they're less robust. Perhaps Yankee tanks aren't the same weight, which would explain my confusion with the "official" line. So here's the figures from go-dive's website (): Luxfer ali tank 7l 9.8kg, (buoyancy stated as +1.87kg empty) Faber steel 7l - 8kg (presumably empty?) but Ali tank "should" be perhaps 1.1 kg light empty, and 0.7kg heavy full though if Ali was actually 8.8, then 1.8 bouyancy empty and 0 full sounds plausible and steel "should" be 0.3 light empty, and 1.5 heavy full. Either way, 7l air @ 232 bar = 1.9kg, so I'd need an extra block of lead with the Ali cylinder, + the extra 0.8 (or 1.8kg) of the cylinder itself, so I'm best part of 3kg heavier climbing the ladder. Whichever way you look at it, it's going to vary by 1.8kg full to empty, but otherwise isn't it just more or less lead somewhere else. I guess the Ali is nearer neutral when full at least, so I imagine the idea is it's less of a nuisance in the water, at least when it's full. As it stands I shall probably buy a steel one, which all my diving gang use. Will I be going wrong, or at least, is it badly wrong? Or am I doing the the classic beginner thing of worrying about stuff that makes little difference? Thanks Hywel >The DIRistas are all pretty adament that Ali's the thing, due to more >desirable bouyancy characteristics. Yep. > However, I'm still unconvinced and don't want to lug an extra 2 or 3 > kilos up a ladder unless there's really a very good reason . . . If buoyancy characteristics don't convince you, then don't. There's no better reason. >and they're less robust. Not in my opinion, they aren't. BTW, aluminum tanks don't rust. > So here's the figures from go-dive's website (): > Luxfer ali tank 7l 9.8kg, (buoyancy stated as +1.87kg empty) > Faber steel 7l - 8kg (presumably empty?) Buoyancy is more of an issue when the tanks are full than when they are empty. In order to safely dive a steel tank, at least by DIR standards, you need sufficient buoyancy to swim it up when full and redundant buoyancy to do the same if your primary device fails. > Either way, 7l air @ 232 bar = 1.9kg, so I'd need an extra block of > lead with the Ali cylinder . . . Lead is not the issue, lift is. Lee > Either way, 7l air @ 232 bar = 1.9kg, so I'd need an extra block of > lead with the Ali cylinder, + the extra 0.8 (or 1.8kg) of the cylinder > itself, so I'm best part of 3kg heavier climbing the ladder. Whichever Don't climb the ladder with your stages on. If you don't have a diver lift, unclip the stages on the surface and hook them onto a DSMB, get someone on the boat to recover them, or pick them up once you're back on board. Luxfer redesigned the Al 7 a couple of years ago and screwed up the buoyancy characteristics, so there is now very little difference between a steel and a new Al. What you really want is an older Al 7, or a US Al-80 if you can get them filled. -- Pete > Luxfer redesigned the Al 7 a couple of years ago and screwed up the > buoyancy characteristics, so there is now very little difference > between a steel and a new Al. What you really want is an older > Al 7, or a US Al-80 if you can get them filled. I presume this means that Luxfer only makes what we could call their neutral buoyant tank in a UK 7 liter model. That's interesting since they make two 80 cubic foot tanks for the US market, one neutral, one buoyant when empty. FWIW, I use neutrally buoyant (when empty) Catalina aluminum 80's for most of my warm water single tank diving. Between the less buoyant tank and my stainless back plate, I'm near perfectly weighted with no lead at all. When you're that near neutral during the entire dive, you really don't need ditchable weight. Lee >> Either way, 7l air @ 232 bar = 1.9kg, so I'd need an extra block of >> lead with the Ali cylinder, + the extra 0.8 (or 1.8kg) of the cylinder [quoted text clipped - 9 lines] > between a steel and a new Al. What you really want is an older > Al 7, or a US Al-80 if you can get them filled. ...or you could just buy one of the new CE-approved AL80s or AL40s which are available through most dive shops these days. They're fully CE-approved with M25x2 valves so you'll have absolutely no problem at all getting them filled or retested! I've been using them myself for about a year now (I used to buy mine direct from Germany before they caught on over here) and find them a lot more comfortable than the UK-spec AL7. Although the AL80 is bulkier, it's actually lighter in the water than the AL7. Thanks everyone. This has helped my (confirm) my understanding, and particularly that there may be something in the "all ali tanks not equal" theory. I'm not so sure that leaving a sling tank floating on a blob while the skipper has to go round again is going to be that popular, but I dare say may have a place if you've got lots of tanks (some way off for me). And also in reply to Lee, I don't think overall boyuancy is affected (is it?) since I'll need enough lead to be (supposedly) neutral with everything empty - I'm using dry suit, not wetsuit, so it's not a question of no lead at all I guess it's still down to a choice, but if ali then I should get one of the "good" ones - though given these seem to be US 80cuft, re-stamped CE or whatever the story is so I'd have a few misgivings on getting quite the right thing with my level of knowledge. I understand correctly (??) the "bad" ali tanks might be the worst of both worlds. Anyway, it's helped my thinking at least. It may be that I can borrow one for my forthcoming course, so can defer decision till I've at least had a go. Cheers Hywel > And also in reply to Lee, I don't think overall boyuancy is affected > (is it?) since I'll need enough lead to be (supposedly) neutral with > everything empty - I'm using dry suit, not wetsuit, so it's not a > question of no lead at all I've forgotten the context of my comments, but you're surely correct that, in your case, it's not a matter of no lead at all. I didn't quite plan it that way for my kit. I originally went to a neutral buoyant tank because, unlike the standard 80, they don't get floaty at the end of the dive. It was good luck that my plate and the less buoyant tanks met my needs so precisely. Do watch out which tank you chose. Luxfer, Walter Kidde and some other tanks made prior to 1988, were made of an inferior aluminum alloy. Nobody knew, or at least won't admit to having known, that it was inferior at the time they were made, but they are an occasional problem. Not only do they seem to be failing more often than other tanks, they fail explosively. All Catalinas and post 1988 Luxfers do not share the problem. One more thing worth considering. Steel tanks and aluminum tanks gain their strength differently. Steel tanks expand and contract without a significant loss of strength. Aluminum tanks don't. They are not made to expand significantly. While both aluminum and steel tanks are rated for considerably less than their actual safe pressure capacity, overfilling aluminum tanks weakens them realatively quickly and can lead to premature failure. If you're going to be pumping your tanks to significantly more than their rated working pressure, chose steel tanks. Lee > I'm not so sure that leaving a sling tank floating on a blob while the > skipper has to go round again is going to be that popular, but I dare > say may have a place if you've got lots of tanks (some way off for me). Time to find another skipper that doesn't have a problem with it. Ask him if he'd rather you had a DCI brought on by trying to climb a ladder with too much weight. > And also in reply to Lee, I don't think overall boyuancy is affected > (is it?) since I'll need enough lead to be (supposedly) neutral with > everything empty - I'm using dry suit, not wetsuit, so it's not a > question of no lead at all You want to be neutral with your back gas and no stages. Don't rely on having weight in steel stages to balance you, because you may one day find yourself in a situation where you don't have your stages (perhaps you have given one to a buddy?) > Anyway, it's helped my thinking at least. It may be that I can borrow > one for my forthcoming course, so can defer decision till I've at least > had a go. If you still need convincing, borrow a couple of steel 7s and go for a dive. One dive should be enough - if you survive it, you'll be convinced that the buoyancy characteristics are not ideal ;-) -- Pete > Time to find another skipper that doesn't have a problem with it. Ask him > if he'd rather you had a DCI brought on by trying to climb a ladder with > too much weight. Is this much of an issue these days? I can't think of a skipper I'd want to do trimix diving with that doesn't have a boat with a lift. Jason  SignatureSee http://www.scuba-addict.co.uk/ for UK diving reports and the UK Underwater Visibility Database. View the database or add your own report > Is this much of an issue these days? I can't think of a skipper I'd want > to do trimix diving with that doesn't have a boat with a lift. Most of the south coast day boats seem to have lifts, but I don't think it's particularly common anywhere else. It's very difficult to see how you could fit out something like the Loyal Watcher with a lift. -- Pete > Most of the south coast day boats seem to have lifts, but I don't think > it's particularly common anywhere else. It's very difficult to see how you > could fit out something like the Loyal Watcher with a lift. Seems to have made it down to the south west now too. Once one boat gets one, they all seem to. I think the boat UKRS usually use in Pembrokeshire's got one too, so they are spreading slowly out from Weymouth. Personally I just used to climb out the ladder with everything on. Jason SignatureSee http://www.scuba-addict.co.uk/ for Caribbean trip reports to Aruba, Cuba, Grand Bahama, Barbados, St Lucia and Mexico > Thanks everyone. This has helped my (confirm) my understanding, and > particularly that there may be something in the "all ali tanks not [quoted text clipped - 6 lines] > everything empty - I'm using dry suit, not wetsuit, so it's not a > question of no lead at all If everything is empty you can ditch the stages (if they are empty they are doing nothing for you). I often send Bottom gas stages up a DSMB line so they are floating on the surface waiting for me. HTH John Well, I've ordered an Ali one, so that's that. Thanks everyone. Hywel > So here's the figures from go-dive's website (): Luxfer ali tank 7l > 9.8kg, (buoyancy stated as +1.87kg empty) Faber steel 7l - 8kg > (presumably empty?) Those figures probably don't include the valve. Then there's your first stage, which for something like a TX40 is a big lump of negatively bouyant metal. I like the ali tanks because the bouyant bottoms mean they tuck up nicely under your arms, but I've got the old style 7l with the better characteristics as Pete mentioned. Jason Signaturehttp://www.scuba-addict.co.uk/ for Aussie diving reports including Stradbroke Island, Terrigal, Jervis Bay and Portsea |
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