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

[Bubbletrubble]

@BlueSparkle: Let's forget about trim considerations for a second. Recall that a properly weighted diver wears the minimum amount of lead to be neutrally buoyant at safety stop depth with a fully deflated wing and an empty tank. Now focus on what it means to be properly weighted -- in particular, the amount of lead a diver needs to wear and the buoyancy characteristics of the tank being used.

In your original example, the only "buoyancy advantage" of using the steel tank over the aluminum tank is that it contains 7 more lbs. of inherent ballast.

Empty Steel tank buoyancy: -2 lbs.
Empty Aluminum tank buoyancy: +5 lbs.
Empty Aluminum tank buoyancy - Empty Steel tank buoyancy = buoyancy difference
(+5) - (-2) = 7 lbs.

As you can see, the 7 lb. difference was calculated from the respective empty tank buoyancy specs. Empty tank numbers are used since the tank should be empty (or near empty) when doing a weight check.
All other gear being the same, the diver in question will have to wear 7 lbs. of extra lead with the aluminum tank (vs. weighting with the steel tank).

Read the second paragraph of dumpsterDiver's post a few times and mull it over a little while.

Completely separate issues are "dry carry" weight and how a tank trims out.

 

[Aaron528]

There is a tank/buoyancy concept that I think I "know," but that..... when I try to understand it, I can't quite get my mind around it.

This is the difference in effect of diving with a tank that stays in the negative even when empty, vs. a tank that goes from negative to positive as it empties. Now, I'm clear on (what I believe is) the fact that a tank changes in "weight" (buoyancy) by the weight of the gas consumed, and that, say, 50 cubic feet of gas weighs the same in any tank, and thus subtracts the same amount no matter what tank it is consumed from.

My confusion comes in with aluminum tanks that go from negative to positive. It seems that that somehow "counts" more but I can't exactly understand why. Here is an example with made-up tanks and numbers (just to make the part I don't understand more obvious). Let's say both hold the same amount of gas.

Diver #1 uses Steel "605" tank that is -12# when full, and -2# when empty.

Diver #2 uses Alum. "605" tank that is -5# when full and +5# when empty.

As I understand it, both divers need to account for a potential of around a 10# change in buoyancy from beginning to end of dive (adding enough weight so that they can hold a safety stop). But I feel like I've read that Diver#2 somehow is in a different category by virtue of the fact that the aluminum tank actually becomes positively buoyant, even though both the divers experience a potential 10# change in buoyancy due to the breathed gas.

Is there actually something different about the Diver #2 scenario? If so, can you help me to understand it?

I have not asked this up until now as I have a feeling the answer is going to be something totally obvious and that I will look a bit foolish :blush:

Oh, I have noticed a difference between using, say, an AL80 and an LP95. Over and above carrying the right amount of weight to account for breathed gas, with the AL80 I have to have some weight way back (down) on my body or else I get very head heavy (or leg light) towards the end of the dive. I guess that is the difference between having a floaty tank on vs. a non-floaty one.... is that all there is to the difference? Or is there something I'm totally missing?

Thanks,
Blue Sparkle

I'm no physics whiz, but it seems like you are trying to take into account two factors affecting the buoyancy that are acting independently. The change in gas weight is constant; 10lbs of air lost is 10 lbs regardless of what tank it's loaded into. The aluminum tank is more positively buoyant when empty because of the lower specific weight of the aluminum material (2.64) relative to steel (7.85). The specific weight of water is 1.00...the closer any substance is to a value of 1.00, the more relative buoyancy it has when immersed in water. Any substance with a specific weight less than "one" will float in water. If you flooded an aluminum tank and a steel tank inside they would both sink, but if you used a lift bag to raise them to the surface, it would require much more air to lift the steel than the aluminum tank.

 

[redacted]

Here is a good reference for tank specifications: Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

One great comparison is an HP100 (E7- 3442) versus an Al80. About the only thing an Al80 has over that HP100 option is cost (and that rust thing). Smaller, lighter tank with less weight on your belt (or in your pocket) and about the same weight out of the water when full. the numbers don't lie.

 

[dumpsterDiver]

I'm no physics whiz, but it seems like you are trying to take into account two factors affecting the buoyancy that are acting independently. The change in gas weight is constant; 10lbs of air lost is 10 lbs regardless of what tank it's loaded into. The aluminum tank is more positively buoyant when empty because of the lower specific weight of the aluminum material (2.64) relative to steel (7.85). The specific weight of water is 1.00...the closer any substance is to a value of 1.00, the more relative buoyancy it has when immersed in water. Any substance with a specific weight less than "one" will float in water. If you flooded an aluminum tank and a steel tank inside they would both sink, but if you used a lift bag to raise them to the surface, it would require much more air to lift the steel than the aluminum tank.

The specific gravity of steel versus aluminum has NOTHING to do with this! Otherwise steel ships would not float. This is like 6th grade science class.


repeat after me: any object wholly or partially immersed in a liquid is buoyed up by a force equal to the weight of the liquid displaced.


It really is that simple

 

[Peter Guy]

If the tank never becomes positive there is nothing to counter.

FALSE!!! You ALWAYS need to counter the weight of the gas that is lost. IF you are "properly weighted" when you start your dive, you are ONLY negative the amount of the gas you will use (plus probably a little bit more but THAT is another discussion).

The issue with Aluminum tanks, in my opinion, isn't that they are positive when empty (or almost empty), but that they are "butt light" when almost empty (one reason they are great for stages). A steel tank, being negative when empty, won't "attempt" to float the bottom of the tank (the bottom of your BC or your butt) but an empty AL 80 WILL start to float, but NOT evenly. Because of the construction (and probably the valve and 1st stage), the positive nature of the empty AL80 means the neck (top) stays negative while the butt tries to lift you.

This can cause significant issues relating to trim IF you have not put your weight in the proper spot (one reason I use a tail weight when diving double 80s -- I want the weight very low on the tanks).

I think this is the primary issue with the difference between steel and aluminum tanks.

 

[Blue Sparkle]

Thanks everyone for the input Kern, I see that evidence of your deleted post remains in my reply to it, but..... would you mind if I left it, since it might be useful for other readers in following the discussion?

I really appreciate everyone's responses. I like to calculate my weighting (in addition to actually trying it, of course), and this one thing was nagging at me every time I thought about it.

Peter, I think you've hit on something I found only through some "what the...?" trial and error. My first dives were all with AL 80s, and I was really getting frustrated with how head heavy I was. I just could not get trimmed out! After a while I started strapping some weight to the bottom of the tank and that did the trick. Before that I was starting to wonder if I was some "un-trimmable" freak Strangely, no-one mentioned anything like "Oh, well the AL80s will do that." Would have saved me some frustration.

Since then I have found that for the typical, shallowish Florida reef dives I've been making I can use an AL63, and that has much less of the "butt bubble" effect (less positive and also shorter), although it still has some (which I have figured out how to manage). So far they have almost always been available to rent.

I made a couple of dives with an LP95 in December, and that was a dream in comparison. Wow! No need to put any of my weight "way out back" and yet I still was able to trim nicely. The HP80 that a dive shop friend uses caught my eye, size-wise, but I think that would not be doable for me with my current gear/locale setup (Florida diving; too negative - I think the LP 95 was about all I could reasonably use at about 8# negative when full). Still..... the HP80 sure looks like a nice tank.

Not to hijack my own thread, but I've been thinking it would be nice to have a tank or two of my own, and looking over the specs (that Huron Scuba chart is very handy; I printed a copy off for my dive log book), I have been wondering if maybe a steel 72 would be a nice tank for Florida reef diving. Not that the AL63's don't work fine, but then if I'm buying there is no reason to "automatically" buy that size without thinking over the options, I figure. That's probably another thread though!

I'll keep reading, so please don't take this as an "okay, thread's done"

Blue Sparkle

 

[Aaron528]

The specific gravity of steel versus aluminum has NOTHING to do with this! Otherwise steel ships would not float. This is like 6th grade science class.


repeat after me: any object wholly or partially immersed in a liquid is buoyed up by a force equal to the weight of the liquid displaced.


It really is that simple

Please correct me if I'm wrong because I admit I am no science major, and I'm definitely not an instructor like you are. But it doesn't make sense to me that specific gravity has "nothing" to do with the buoyancy of a tank. Steel ships float because they are containers that displace a certain tonnage of water. Like you wrote, it's the displacement volume that allows the ship to float; the volume of water displaced by the ship weighs more than the weight of the ship itself. If a steel ship gets a hole in it and water replaces the space that was once occupied by air, it doesn't float any more.

But if what you wrote is true and dependent only on displacement volume, then an aluminum and steel tank of equal size should be equally buoyant because they displace an equal weight of water. But they are not equally buoyant.

 

[lowviz]

.......... then an aluminum and steel tank of equal size should be equally buoyant because they displace an equal weight of water. .....................

Correct, but only the buoyant force is the same.

Now subtract (from the buoyant force) the weight of the tank, valve, and gas. This is our tank's "buoyancy". And as Peter noted, it isn't evenly distributed.

 

[Aaron528]

The specific gravity of steel versus aluminum has NOTHING to do with this! Otherwise steel ships would not float. This is like 6th grade science class.


repeat after me: any object wholly or partially immersed in a liquid is buoyed up by a force equal to the weight of the liquid displaced.


It really is that simple

It seems like you contradicted yourself. Specific gravity has "everything" to with why lots of substances will float on water, like wood. The specific gravity of wood is less than one, so the volume of water the wood object displaces always weighs more than the object itself. So it makes sense that a metal object with a lower specific gravity would be relatively more buoyant, because of its weight relative to water.

 

[Aaron528]

The specific gravity of steel versus aluminum has NOTHING to do with this! Otherwise steel ships would not float. This is like 6th grade science class.


repeat after me: any object wholly or partially immersed in a liquid is buoyed up by a force equal to the weight of the liquid displaced.


It really is that simple

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."