Sudden and uncontrolled acsent.

[Blackwood]

Right, we're talking about the buoyancy characteristics of an object submerged in water...you're talking about the buoyant force itself, which is kind of irrelevant to our discussion.

It's not irrelevant. I brought it up because I thought it would help clear things up for Andy if he looked at the forces acting upon a diver individually rather than trying to think of a sum of two forces, one that changes and one that doesn't.

Apparently I was wrong

 

[Soggy]

It's not irrelevant. I brought it up because I thought it would help clear things up for Andy... let him look at the forces acting upon a diver individually.

Apparently I was wrong

Yeah, you engineers are always overcomplicating matters.

What term would you use to describe the changing buoyancy characteristics of an object of variable weight? To me, the concept of buoyancy of an object implies the amount that object 'weighs' when submerged (I know, we can apply it to gasses in the atmosphere, too) and involves the weight of the object. This is something measured in lbs/kgs/etc

What you are referring to is the buoyant *force* (Archimedes principle) which is how much force the liquid is exerting on the displaced area. This is something measured in Newtons.

(I know that you know this, but I'm trying to clarify this to the forum)

 

[pants!]

buoy·an·cy Audio pronunciation of "buoyancy" ( P ) Pronunciation Key (boin-s, byn-)
n.
1. The tendency or capacity to remain afloat in a liquid or rise in air or gas.
~~~~~~~~~~~~~~~~~~~

A full tank is less buoyant than an empty one. Buoyant force != buoyancy for purposes of diving. If you have two people of the same volume, and one of them sinks but another one of them floats, no one will say that they are equally buoyant. If you have a 1-ft diameter ball of lead and a 1-f diameter ball of wood, they will both experience the same buoyant force from the water, but no one on earth will tell you that they are equally buoyant. At least, no one I know, and I've never seen buoyancy referred to that way in any discussion or textbook.

 

[Lightning Fish]

Reference "Fluid Mechanics" by Streeter and Wylie. (You could also reference "Fluid Mechanics" by White).

Bouyant force is equal to the volume of displaced fluid times the specific weight of the fluid. In other words, what Blackwood posted above since the specific weight is equal to density times the acceleration of gravity.

Fb = Vol x rho x g

Which, for a tank when submerged, will be a constant. Now consider a free body diagram of the tank, which includes the bouyant force (Fb) and the weight of the tank and contents (W). The resultant force (Fr) acting on the tank is:

Fr = W - V x rho x g

A positive result and the tank will sink, a negative result and the bouyant force of the tank is greater than the total weight and so it will rise. So the weight of a full tank (AL80) is greater than the bouyant force of the tank and sinks while the weight of an empty tank (AL80) is less than the bouyant force and rises.



Bill.

 

[pants!]

Notice the continued use of the term "buoyant force" and not "buoyancy" or "overall buoyancy" in that excerpt

 

[Blackwood]

Yeah, you engineers are always overcomplicating matters.

haha... I thought I was undercomplicating it. To me, it's always more simple to look at things independently rather than to group things together, especially when they are driven by different inputs.

What term would you use to describe the changing buoyancy characteristics of an object of variable weight?

You got it! Variable weight.

No, I like the terms neutral, positive and negative buoyancy. I do use those terms. I only broke them up because there seemed to be some misunderstanding.

To me, the concept of buoyancy of an object implies the amount that object 'weighs' when submerged

And by that you mean that, for example, a neutrally buoyant object is weightless?

See, THAT, I think, is how people get confused. If an object 'weights' something different while submerged than it does while it's not submerged, then what is weight?

To me, the concept of buoyancy of an object is the relationship between the gravitational and buoyant forces. Neutral buoyancy implies weight=buoyant. Negative implies W>B, and positive implies W<B. However, due to misunderstandings, I chose to break that relationship up. Bad choice, I see

This is something measured in Newtons.

It can be, or any other unit of force (like lbs).

 

[Blackwood]

buoy·an·cy Audio pronunciation of "buoyancy" ( P ) Pronunciation Key (boin-s, byn-)
n.
1. The tendency or capacity to remain afloat in a liquid or rise in air or gas.

You edited that

buoy·an·cy Audio pronunciation of "buoyancy" ( P ) Pronunciation Key (boin-s, byn-)
n.

1.
a. The tendency or capacity to remain afloat in a liquid or rise in air or gas.
b. The upward force that a fluid exerts on an object less dense than itself.

Buoyant force != buoyancy for purposes of diving

I know, but I thought it might lend clarity to the concept to discuss the piece parts.

 

[The Kraken]

Well, the way I look at is like this . . .
. . . if I've emptied my B/C and my butt is sticking up out of the water, then I'm positively buoyant

. . . if I've completely inflated my B/C and I just planted my butt in the silt and can't see a thing, then I'm negatively buoyant.

. . . I like to keep things simple.

the K

 

[MikeFerrara]

Thanks for the reply everyone.

Just to clear a few things.
While at 55 feet, we weren't at 500 psi. We had about 5-7 minutes worth of air before getting to 500 psi. Our air planing this time and every other time is to be on the shore or the boat with 500 psi left. My mistake for bad wording at the original post.

Why 500 psi?

BTW, while doing flips I think you just weren't paying attention and got behind on your buoyancy control.

 

[CIBDiving]

Of course.

The buoyancy of the tank = the volume of the tank * the density of the fluid in which it is submerged * the acceleration of gravity.

there's you're mistake -
The formula you've quoted is the DISPACEMENT of the tank!!! that for the most part is a constant, there will be a little change as pressure changes because of 'stretch'.


The buoyancy = Displacement - weight of the tank( and it's contents). It's that simple!

Unless you change the geometry of the tank or alter the density of the water or change the acceleration of gravity (none of which will happen to any noteworthy amount), the buoyancy of the tank will remain the same (V&#961;g=V&#961;g). You could fill it with lead, sand, pizza sauce, helium, etc., and its buoyancy won't be affected. Its weight will.

Buoyancy of the tank = (the volume of the tank * the density of the fluid in which it is submerged * the acceleration of gravity) - ( density of the tank * the acceleration of gravity).

so if the tank displaces 5 gals of water and water is 8 lbs/gal that's 40 lbs displacement, right? now an emptyl tank weighs 39 lbs, it floats, right?
If you fill that same tank with 4 lbs of air it now weighs 43 lbs, eh? what's it do?

If you said it floats, you need to try this -

fill an AL80, take it out to a lake and see if it floats ( just the tank and valve nothing else), now let (or do a dive and breath) it down to 100 psi, retest it in the same lake. Guess what it'll float!