Would like to get my mind around a tank/buoyancy concept

[dumpsterDiver]

Also reference this web post:

http://www.thedivingblog.com/aluminum-steel-scuba-cylinders/

"If you’re curious, a standard steel cylinder weights about 30 lbs, while an aluminum tank weighs about 35 lbs. If aluminum tanks are heavier, then how can they be more buoyant? For the same reason they’re not as strong as steel: aluminum is less dense than steel, and thus has a lower specific weight."

I'm out. did my best. The OP understands why floating tanks tend to be annoying and the buoyancy swing is so noticable.

Also steel 72's are great tanks, better than an aluminum 63 for similar applications, BUT some local Florida shops will NOT fill them, regardless of condition/hydro. My wife used to use them and we were perfectly matched when she had one of those and I had my 100 cu-ft tank. Seems like the newer HP 80 is a decent (although expensive) alternative.


BTW a zillion ton ship does NOT displace one ounce more of water than it weighs...

 

[Blue Sparkle]

Also steel 72's are great tanks, better than an aluminum 63 for similar applications, BUT some local Florida shops will NOT fill them, regardless of condition/hydro.

Hi DD,

Thanks for your comments on the 72 - just from looking at the specs it looked like a nice choice for the type of Florida diving I have been doing most, but specs on a chart don't carry the same weight as hearing about it from someone with your experience.

Can you tell me why some shops won't fill them even if they are current/in good shape... do you know their reasoning?

 

[TSandM]

To explain this whole thing very simply:

When I go to the dive site with my steel HP 80, I wear 31 pounds of ballast that I have to carry into the water.

If I go to the same dive site with an Al80, I have to wear 37 pounds of ballast. That's six more pounds that I have to find room for on my rig, and six more pounds that I have to carry to the water, and back UP from the water. And the reason for that is that the aluminum tank, when empty, instead of forming part of my ballast, actually requires weight to sink it.

Any time you can ensure that gear you require for the dive works as part of your ballast, you are ahead of the game . . . unless you are diving with so little exposure protection, and with so little requirement for weight, that you don't WANT any negative gear. Being as cold-blooded as a lizard, I have yet to encounter such diving conditions

Trim is a secondary matter -- ballast is the big thing.

 

[dumpsterDiver]

Hi DD,
Can you tell me why some shops won't fill them even if they are current/in good shape... do you know their reasoning?

"too old and unsafe".... that's what they tell me. It is funny because those old steels, seemed to have much better galvanization than the newer expensive OMS tanks I have now.


I've owned several of them and dove them as doubles many years ago. They are just the right size, not too bouyant, not too heavy in the water, narrower than an aluminum (so they probably swim better), less pressure so they are probably easier on the first stage. Great tanks if you don't need the volume.

My big single tanks are MUCH easier to deal with than the old double 70's.

 

[Blue Sparkle]

I see what you are saying, TSandM, thanks for contributing. I was able to take advantage of that on the trip I just came back from, when I used an LP95. However so far my diving has been in situations where I couldn't "spare" the extra weight for something that starts out 13# negative, like an HP80 (at least not with my current rig).

But.... I'm still drooling over them And I can see where they would be a great "deal" when you are able to substitute them for lead you would otherwise have to wear.

 

[Blue Sparkle]

"too old and unsafe".... that's what they tell me. It is funny because those old steels, seemed to have much better galvanization than the newer expensive OMS tanks I have now

.

Hmmph, that seems a bit annoying as a blanket reason if they are current and in good shape. I mean, if there is a real reason, sure; but the "just because they are old" for various things drives me nuts (not just in SCUBA gear). They do seem like they would be really nice for me for Keys diving. Of course I'm sure there are others that will work too. But still...

"
My big single tanks are MUCH easier to deal with than the old double 70's.

As I continue to hijack my own thread...

Actually that was in the back of my mind, since I had started to make a few deeper dives on this trip. Meaning.... would I want to use double tanks at some point in time for the redundancy/minimum gas (at which point., theoretically.. oh, lookie, I have two of these 72s I have been using for shallower reef diving...). How do you handle any redundancy desire with the large singles? What was hassle-ish with the other setup --- just the fact of doubles at all (which I can see would be more complicated/heavy/etc).

Mind you, this is not exactly a pressing issue for me now; I just tend to think/plot ahead sometimes - especially when I'm freshly back home and still have major dive fever

 

[aquaregia]

I got a pair of twinned 72s because the price was right and I dive a wetsuit. The problem is that I pay $12 for my 130cf of gas, whereas filling an HP130 only costs $8. Plus double hydro/O2 costs. Also they're still 80lbs and the wing is harder to vent than a singles wing. And I have to service another first stage.

I like diving my single 72s, I just wish they were bigger. I don't really like 95s but they have a lot of gas. I like diving 85s and 108s, but I can't get them very often.

 

[Blue Sparkle]

Good points on the cost/hassle factors of the double 72's vs. a large single. I'm probably a looong way from using doubles, if ever. But, for the typical one-hour boat dive on a shallow reef that you get in the Keys, a single 72 would be plenty for me.

What had turned my mind to more/redundancy was a few deeper dives I made while I was there, but that's a different set of parameters. Seems like either a larger tank + pony or doubles would be the way to go for that. But that's not my main focus right now.

Now, if I had shore diving handy..... ah but that sounds luxurious, when you can go on your own schedule, and spend long enough in the water to use more gas than 50 minutes takes (And compared to boat diving, $12 for air sounds gaspingly cheap! But I see what you mean.)

At any rate, the max-one-hour shallow reef dives of the Keys, plus the size (shortish) and buoyancy characteristics of the 72 intrigued me. And, it would be nice to have a tank to put my regs on while they soak

Blue Sparkle

 

[dumpsterDiver]

.

Hmmph, that seems a bit annoying as a blanket reason if they are current and in good shape. I mean, if there is a real reason, sure; but the "just because they are old" for various things drives me nuts (not just in SCUBA gear). They do seem like they would be really nice for me for Keys diving. Of course I'm sure there are others that will work too. But still...



As I continue to hijack my own thread...

Actually that was in the back of my mind, since I had started to make a few deeper dives on this trip. Meaning.... would I want to use double tanks at some point in time for the redundancy/minimum gas (at which point., theoretically.. oh, lookie, I have two of these 72s I have been using for shallower reef diving...). How do you handle any redundancy desire with the large singles? What was hassle-ish with the other setup --- just the fact of doubles at all (which I can see would be more complicated/heavy/etc).

Mind you, this is not exactly a pressing issue for me now; I just tend to think/plot ahead sometimes - especially when I'm freshly back home and still have major dive fever

this would be a major hi-jack... As for what I do.. I usually dive solo, especially when deep. I TRY to keep my deco short enough that I could ascend and complete it on my pony bottle and also possibly a stage bottle of oxygen, if I have it. That is my redundancy with a single big tank.

I have a single 149 cu-ft tank that is heavy but manageable for me but I am 220 lbs.

The steel 71.2 cu-ft tanks hold 10% less volume than 71.2 unless they are over-filled by 10%. (Rated pressure is 2250 but you need a fill to like 2640 or something to really get the 71.2 and some shops may not overfill a 30 yr old tank). Thus your double steels may only contain around 134 cu-ft if when filled to their stamped pressure.

You can carry around 130 cu-ft with less weight in a single big tank, but you will need a pony or stage bottle to gain redundancy. Also, shops tend to allow overfilling of new steel tanks, more often than tanks that are older than the customer and the dive shop employee.

In my expereince, they are nice for singles, but just not worth the weight when doubled. Others will argue the balance of doubles underwater is better, but you will need to buy another wing for doubles as well. Also some (most)?? charter boats are equipped to handle single tanks, but doubles are more of a hassle in some situations.

If you just stick with diving small singles and large singles, you need to own only one BC, which may or may not be an issue as well. Lastly, my shop charges me only for one fill when I get a single tank filled, if it were doubles it would cost double for every dive. If you dive a bunch and use nitrox, these costs may be relevant. Also each year you have to break down the manifold and pay for TWO VIP's.

If you are "good on air" and can do "normal" recreational dives on a 63 cu-ft tank, it is very likley that a larger tank (maybe a 112 cu-ft) will provide the volume to dive deep to the limits of recreational, no-deco diving.. this is not that heavy of a tank

If you are going to pursue formal "technical dive" training, then you will eventually get into doubles.

It comes down to personal preference.

 

[Eric Sedletzky]

The whole entire thing revolves around the idea that you want to be completely neutral at 15' at the end of a dive with an almost empty tank and no air in your BC, and if you're wearing a drysuit then only enough air in the suit to hold loft and avoid being shrink wrapped.
Whatever the amount of weight you need to have on the achieve this is what it is. 7 lbs of weight in air consumption will happen equally with an aluminum or a steel. The nice thing about steels is you get to use less lead and the tank continues to lay nice on your back even when it empties. Aluminum is not as nice to use because you have to use more lead to make up for the added lightness and when the tank empties out the butt end of the tank want's to rise up while your heavier weightbelt want's to pull you down. Even though if weighted properly you can hold a perfect stop at 15' and control it with breathing with both combos, the steel/lighter weightbelt is a better combo AFAIK.