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

[TSandM]

Well, if you're looking for bigger tanks, you can always get an Al100. They're heavy, at 40 lbs, but have similar buoyancy characteristics to the Al80.

It is challenging to put together a balanced rig with big tanks and no exposure protection. That's one setting where I would be very sure to carry redundant buoyancy (mostly for the surface -- I can swim up the contents of most single tanks, but staying on the surface without being able to ditch anything would be a challenge).

I do believe that redundancy, for a 50 minute dive in warm water with good visibility, in recreational depths, is adequately provided by an attentive and well-practiced buddy

 

[redacted]

Also reference this web post:

Aluminum vs. 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 believe dD's point is that the specific gravity of steel vs aluminum need not be considered if you already know the buoyancy characteristics of a tank. If you do not know the buoyancy characteristics of a tank but do know the masses (volume) and specific gravities of the materials (air, metals), you could calculate the specific gravity (and mass) of the tank and from that calculate buoyancy. So, specific gravity (and mass) is of no consequence if you already know buoyancy characteristics.

 

[Blue Sparkle]

Even thought (I've caused) the thread to drift a bit from the original question, I really appreciate everyone's input. I've been rolling various thoughts around in my mind, making calculations, etc. -- but input from Scubaboard denizens is priceless. A person can figure and calculate all they want, but the voice of experience is a whole 'nother level.

Although I've got it figured out now, thanks to SB, it sure would have been helpful if I had really grasped what was going on with the AL80 in terms of buoying up the "aft" end as it emptied. Figuring that out really helped my trim, but even in my PPB AOW dive, when I was nose-diving like mad, it wasn't really mentioned specifically. Water under the bridge now, but perhaps another new diver will read this later and benefit.

DD, thanks for your thoughts on the larger single vs. doubles. These considerations just really came up on my most recent dive trip, as the result of a combination of learning gas planning (minimum gas/rock bottom), and making some deeper dives (Keys wrecks). While an AL80 will easily cover me for the actual dive, it does not really/barely allow for minimum gas, so my thoughts turned to how to carry more. I did carry a pony on some dives, but as it was rented I'm still thinking about how to go for future/my own gear.

TSandM, you make a good point about my buddy providing redundancy/gas for shallow reef dives, and I have been completely comfortable with that. I really have two separate considerations (beyond my original question, which I think I have a handle on now!). One is buying my own tank(s) for the dives I'm doing now in AL63s (either buy those or something else, or continue to rent). The other is tank(s) for deeper Keys dives (wrecks). It is for the latter that the idea of more/redundant gas/regs came into question. For both my buddy and me. I don't know that I'm planning to do a lot more of those particular dives... we'll see.

And it was just on this last trip that I finally got to experience diving with a steel tank (the LP95, which I don't think I could support in summer with no exposure protection, unless I change some things about my rig, which I may do). This is when my thoughts turned to the steel 72 as a possibility (sub for AL63 on shallow dives). I think it would work with my current rig (but maybe not without exposure protection.... I feel a lighter plate coming on). The LP95 was nice too, although not needed for shallower dives of only one hour (and I don't think I could get two dives out of it, but... maybe).

I like the shortness factor of the AL63, and the steel 72 has that as well.

It's been really nice to get some feedback on my thoughts, in addition to hearing what you all have to say. Thank you! I don't really have a local dive community, so SB is it (and has been great) (Tho, I did actually meet a ScubaBoarder on the dive boat last month - that was fun.)

Blue Sparkle

 

[CT Sean]

Also reference this web post:

Aluminum vs. 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."

Not that it matters but pretty much everything about that statement is wrong - well everything but the fact that AL is less dense than steel. Steel is stronger than AL, but not due to specific weight. If that were the case than titanium would be much weaker that steel and lead and gold would be much stronger.

The reason that a "standard" steel tank (not that there really is such a thing) that weighs 30 lbs is less buoyant than a standard AL80 that weighs 35 lbs is because that 30 lb steel tank is SMALLER, therefore displaces less water, therefore is "heavier" underwater.

If that 30lb steel tank were the same size as an AL 80 it would be MORE buoyant since it would displace the same amount of water but be 5 lbs lighter.

 

[Blue Sparkle]

Just to summarize the basic ideas I've taken away, since I proceeded to drag the thread off topic somewhat:

1) There is no absolute difference in the amount of weight required for a tank that becomes positive as it empties vs. one that stays negative. In other words, in both cases you compensate for the weight of gas consumed as the tank empties.

2) There can be, however, a change in "characteristics of handling" with a tank that goes positive. Its buoyancy can shift or "pull" on you in different ways than an always-negative tank and can therefore affect your trim.

Thanks again all.

Blue Sparkle

 

[TMHeimer]

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.

Yes, that explains the floopy buancy near the surface I talk about. My 35 year old steel 72 got me spoiled.

 

[dumpsterDiver]

Well, if you're looking for bigger tanks, you can always get an Al100. They're heavy, at 40 lbs, but have similar buoyancy characteristics to the Al80.

It is challenging to put together a balanced rig with big tanks and no exposure protection. That's one setting where I would be very sure to carry redundant buoyancy (mostly for the surface -- I can swim up the contents of most single tanks, but staying on the surface without being able to ditch anything would be a challenge).

I do believe that redundancy, for a 50 minute dive in warm water with good visibility, in recreational depths, is adequately provided by an attentive and well-practiced buddy

I've owned 3 of those and still have one. The 100 cu-ft 3300 psi tank is a pig, It is big fat, heavy AND with that volume of gas you really notice the change in buoyancy.
25yrs ago. I thought they were wonderful for beach dives when we only really had 80 aluminums,

However, they are typically considerably more expensive than an 80 aluminum. When you remove a considerable portion of the cost advantage that aluminum tanks provide (as in this case), it tips the scales heavily toward comparably sized steel tanks that may weigh less and have better bouyancy characteristics for all situations except tropical diving without any exposure protection.

anybody want to buy a used Al 100 (which needs a hydro)

 

[Hepcat62]

1) There is no absolute difference in the amount of weight required for a tank that becomes positive as it empties vs. one that stays negative. In other words, in both cases you compensate for the weight of gas consumed as the tank empties.

Hmm? I think you might be combining two issues into one here, and getting a bit confused.

First, let's deal with the weight of the gas. For all tanks, if you do a proper weight check in a given tank while the tank is full, you will need to add enough weight to the result to compensate for the weight of the gas inside. Otherwise, you would be too light when the tank is nearly empty. For an 80 cuft tank, the weight of the gas is approximately 6 lbs. So if I do a weight check with a full AL 80, I will add 6 lbs to whatever amount of lead I find works. If I do a weight check with a full steel 80, I will add 6 lbs to whatever amount of lead I find works. However, if I do a weight check with a full 100 cuft tank, I will need to add about 8 lbs. So this is a function of volume when full, not the tank material.

Now for the absolute difference issue. There is absolutely an absolute difference in weight required when diving a tank that stays negative vs. one that goes positive, just like there's a difference between a tank that stays negative and a tank that stays MORE negative. Your total amount of ballast across your entire rig - tanks, weights, other equipment - should be the same, but the amount of ballast you will need to carry as additional lead (on a weight belt, or in weight pockets, etc.) can change significantly.

The numbers that matter are the buoyancies of each tank when empty (or nearly empty). If an AL80 is +5 when empty, and a given steel 80 is -2 when empty, then you will require 7 lbs less additional lead with the steel tank. In both cases (as described above), if you do a weight check while the tank is full you will need to add 6 lbs to compensate for the gas in the tank.

Hypothetical Example

I do a weight check with a full aluminum 80 (+5 when empty), and find that I require 20 lbs of lead. I add 6 lbs to compensate for the gas I'll breath, and discover that I need 26 lbs of lead to be properly weighted. Preparing for my next dive, I find out that I'll be using a friend's steel HP80 (-2 when empty). Approximately how much lead should I use on this dive?

The buoyancy of each tank when empty is different, so we'll need to compensate. The steel tank, being 7 lbs heavier when empty, will require 7 lbs less lead, so assuming no other changes in buoyancy (due to comfort, breathing rate, etc.) I should only need to take 19-20 lbs of lead on this dive.

 

[knowone]

So stay away from boats and harbours and aluminium
unless you need some excess excess shaved off your butt.

But how does the pressure get in, to cut your gas volume in half at 33ft.

 

[The Chairman]

FALSE!!! You ALWAYS need to counter the weight of the gas that is lost.

I disagree with "ALWAYS".

If you don't need additional weight to stay down after your tank is at/near empty, they you don't have to do anything else additional. I can dive my steel 130s with absolutely no weight if I am not wearing exposure protection in salt water, and never in fresh water (even in a dry suit).

As for her feet becoming floaty at the end of the dive, they are probably floaty at the beginning of the dive too. You're just able to compensate with the extra weight. Slide your tank down an inch and make another dive. It's important to make your adjustments in small increments so you can discern how they are really affecting you. Mark the tank with a Sharpie to help out.