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

[Blue Sparkle]

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

 

[caveandreas]

There is no magic. +10 is just +10 no matter what you start with.

 

[lowtech]

I think the other factor is the size of the tank, since buoyancy is a factor of water displacement.
AL80 7.25" diameter and 26" tall weights about 31lbs empty
HP100 7.25" diameter and 24" tall weights about 33lbs empty

As you can see the smaller steel tank weights more and displaces less water, therefore making it negatively buoyant. Say about -2lbs vs the +4lbs for the AL tank.
So we have about a 6lbs delta. About 2lbs is seen in the weight, and with a bit of math,
you'll find that there is about a 3lbs difference in water displacement.

These numbers aren't that exact, but I think you can get the idea.

 

[Scott L]

Your rig must be weighted to be neutrally buoyant with 500psi at your safety stop. It's as simple as that...

 

[Kern]

As I'm talking drivel, thought I'd remove it so as not to confuse to many.

 

[Blue Sparkle]

If a tank goes from very negative to much less negative, it's still negative, it will never float. Therefore you will never need to add lead because of the buoyancy change.

If a tank goes from negative to positive it will float. Now lead needs to be added to counter the buoyancy change.

Okay, so if I get this right:

1) For a tank that stays negative even when empty, you add weight just to counter the lost weight of the consumed gas.

2) For a tank that goes positive as it empties, you add weight - as above - to counter the consumed gas, but you also add additional weight to counter the positive buoyancy of the tank? If so, this is the part I was missing. On the other hand, Scott and sptz seem to indicate that #1 is always true -- if I'm understanding them (?)

So far I have taken notes and experimented as I've dived various combinations, so that I think I am diving properly weighted (for the various tanks/wetsuits I've worn so far), But even if I'm doing something "right," it drives me nuts if I don't understand it and can't figure it out mentally

A tanks external dimensions, material its made from & therefore internal dimensions, where you position it on your back etc., will all affect how it trims out, & therefore how it affects your trim.

I have noticed the trim part, very obviously. But the the always-negative vs. negative-to-positive tank difference was eluding me!

PS. Foolish people remain ignorant, because they don't seek answers or try to increase their understanding. You don't look foolish.

Thanks

Blue Sparkle

 

[dumpsterDiver]

I think that people notice the diference much more with an aluminum tank. When a tank changes from negative to positive, it "feels" more different, than if it simply becomes less negative.

However, with regard to lead requirements, it makes no difference. If you consume 6 lbs of air in the tank, then you need to be 6 lbs heavy at the start of the dive. That 6 lbs can come from the weight of the tank or lead or a backplate. You need to think of your body, your tank and suit and ballast as a system; it does NOT matter that the tank goes from negative to positive.

 

[Kern]

Its been a long day, see dD post above.

 

[TMHeimer]

Blue Sparkle, This is indeed a difficult concept. I have kind of asked the same question before. I pretty much understand the explanations above. But having dived steel only my first 4 years or so, then adding aluminums to my collection, they just feel different when near empty. My buoyancy is not quite as good near the surface as with my steel. I'm properly weighted in both cases. My buoyancy should be the same one would think. Maybe it IS just mental? The steel tank never becomes positive, but the AL one does, so it "wants" to ascend. But you would thus at any given time have to have less air in the BC to compensate....Then you'd think that would balance out and your buoyancy would be as good as with a steel, wouldn't you?

 

[lowviz]

.............PS. Foolish people remain ignorant, because they don't seek answers or try to increase their understanding. You don't look foolish.

Big message above. (edit: -looks like I snagged this before Kern's edit, too bad he removed it.)


Both steel and aluminum are heavier than water so you can construct a tank that will sink out of either material.

<<When you are trying to grasp a concept, try looking at the extremes.>>

First extreme: Imagine a very small tank that will hold your 50 cuft of gas. Stupidly high pressure, but no matter. It will sink whether it is full of gas or empty. The ten pound swing won't get it buoyant as the walls have to be thick and it doesn't displace much water.

Other extreme: Imagine a tank that is the size of a bathtub. Not much pressure is needed to fill it so that it will deliver 50 cuft and it doesn't have to be very strong. It will float whether it is full or empty.


*** So somewhere in the middle, you can design a tank that will be neutrally buoyant in water. You can design the tank to be neutral either full or empty or anywhere in between, you just play with the tank's wall thickness (weight and strength) and internal volume. ***

(If you want it to be neutral when it is half full, it will swing negative-to-positive as it emptys.)


The weight (specific gravity) and strength of steel and aluminum are quite different. There is a 'best compromise' in size/strength for both materials, so generally speaking, we see steel tanks as being more negative than aluminum.

Hope this helps...