Which short steel tank??? help!

[jonnythan]

In this case its "Which would float, a ton lead or a ton of feathers?" Weight is what complicates it for many people.

If you packed them into the exact same rigid container, they would both do the same thing.

Same thing with tanks. The density of gas inside the tank makes 0 difference.

 

[Scubakevdm]

If you packed them into the exact same rigid container, they would both do the same thing.

Same thing with tanks. The density of gas inside the tank makes 0 difference.

I tend to agree with this statement, and cannot account for the difference in swing in PST's table. I figured there must be something else that I might not be considering in the equation. I always wondered about it. Maybe I'll call them up tomorrow.

Edit: I agree that the feathers and lead having equal mass placed into containers having equal volume will be bouyantly identical.
I do not agree that having identical gasses of differing density inside identical rigid vessels will do the same thing.
I do agree that volume is constant for each tank (although different) in the case of the LP vs HP swing, and I do agree that an equal mass is removed from each tank throughout the course of the dive. One would think that would result in identical swings.

 

[Don Burke]

I tend to agree with this statement, and cannot account for the difference in swing in PST's table. I figured there must be something else that I might not be considering in the equasion. I always wondered about it. Maybe I'll call them up tomorrow.

Those lists are loaded with typos and many of the buoyancy figures are computations of dubious accuracy.

 

[derwoodwithasherwood]

There seems to be some confusion caused by the word "volume". In one sense, "volume" of air can refer to the volume the contents would expand to if the pressure was reduced to 1 atm. Perhaps a better phrase for that would be "potential volume", because the air inside a full 80cu ft scuba cylinder doesn't actually occupy 80 cu ft. while it's in there. The air in has been compressed down into a much smaller space. It's "potential volume" or mass* of course is unchanged, no matter how small the space may be.

A high pressure cylinder has a smaller internal space than a low pressure cylinder. In order to get 80 cubic feet of air into that smaller HP cylinder, it must be squeezed more. Now, think about what happens when a scuba diver squeezes air. Take a neoprene wet suit for example. As one descends, the air bubbles are squeezed smaller by the increased pressure. The more the air is squeezed, the less buoyant the wetsuit is. And the diver begins sinking faster and faster unless (s)he compensates. Buoyancy is inversly proportional to pressure, and pressure is directly proportional to density.

The same thing happens inside the cylinder. The more the air is squeezed, the more pressure is exerted against the container walls and the less buoyant it is. So the air within a high pressure cylinder, when full, will be less buoyant than a larger, lower pressure cylinder containing the exact same mass of air.

Note that this refers to the air within, not the cylinder itself. There may well be an additional difference because of the respective wall thickness, circumferences and height of the cylinders themselves.

So, consider the case when the cylinders are empty and the pressure in each cylinder is 0psig (1atm). The density of the air within each will be the same. The mass of air contained within each will be different, since the LP cylinder's displacement is larger and can it contain a greater mass of air at the same pressure as the smaller HP cylinder. Therefore the air within the LP cylinder will still be slightly more buoyant than the air within the HP cylinder, just like a large lift bag is more buoyant than a small lift bag.

That holds true at any time both the HP and LP cylinders are filled to equal pressure. As the pressure in each cylinder is increased by squeezing more air in, they will become less buoyant but the LP, having a larger displacement, will hold a greater mass of air and be more buoyant than the HP. The rate of change in buoyancy will be equal so long as the pressures are equal.

But, if each cylinder is filled with the same mass or “potential volume” of air, the rate of change is different. If the same mass of air is squeezed into each respective cylinder, the HP cylinder pressure rises faster than the LP cylinder pressure and so the HP cylinder loses buoyancy at a greater rate than the LP cylinder.

Therefore, it seems quite reasonable to assume that PST is correct that the HP cylinder has a greater “swing” than the LP cylinder. On the other hand, since the difference in “swing” is only ½ a pound, is it really worth worrying about?

*Don’t confuse mass with weight, they are not equivalent terms. The earth has a mass of 6.0 x 10^21 metric tonnes, but is completely weightless.

 

[DA Aquamaster]

Hmm... I thought I smelled smoke somewhere. Some of you guys are thinking way too hard.

Assuming tank expansion is minimal at high pressures (which it is if your tank passed its last hydro test) the amount of water displaced by the tank is not going to change whether the tank is ful or empty. The only difference then is the volume of air that is lost during the dive. Since the mass of this air will be acted up on by a gravitational force of approx 1 G at or near the surface of the earth, the air has weight. When the air leaves the tank, its mass and weight go with it and by the end of the dive, an 80 cu ft tank will be lighter by the weight of that 80 cu ft of air whether it is a LP tank or an HP tank.

The internal tank volume may differ and the LP tank, having more internal volume, may hold slightly more air at ambient pressure than the HP tank, but the difference in mass and weight of that difference in air will be miniscule.

A difference in swing weight between two tanks of identical advertised capacity is far more apt to differ due to the difference between advertised capacity and actual capacity in each tank or due to errors in measuring the bouyancy of the tanks in question.

 

[Sideband]

When Thread Hijacks Go Bad! Next on FOX!

Joe

 

[AmyJ]

Actually, I'm still here... just always been a bit slow with physics, but I find it fascinating.... keep on... I don't mind! Hopefully I'll feel smarter once I've figured out what everyone is talking about! hee hee

 

[Scubakevdm]

There seems to be some confusion caused by the word "volume".

Edited for tone and content.
Yes, there does. The volume of an 80 cu ft. tank is not 80 cu ft. 80 cu ft. is the volume of the air inside the tank at whatever the working pressure of the tank is if it were at atmospheric pressure. The tank itself has volume, probably about 1.5 cu ft. The volume of the tank can be measured by submerging the tank in water and measuring how much water is displaced. Volume is how much space something takes up.
When we pack more air into a tank we are adding mass by increasing the density of the air, but the volume of the tank remains the same. This does cause the tank to effectively become more dense. We all seem to agree that full tanks are more negatively buoyant than empty ones. In theory it is true that if the change in density between two rigid containers having different volumes will be the same as long as the same amout of mass is removed from each.
I do not dispute this.
Mass divided by Volume equals Density and density relative to whatever fluid something is surrounded by will determine buoyancy in that fluid. It works every time, I promise.
I have a call in to PST concerning their data and will post their response.

 

[Dan Gibson]

Can you please delete all posts after #35 as it is correct and there really is no need to go any deeper into this?

Hmm... I thought I smelled smoke somewhere. Some of you guys are thinking way too hard.

Assuming tank expansion is minimal at high pressures (which it is if your tank passed its last hydro test) the amount of water displaced by the tank is not going to change whether the tank is ful or empty. The only difference then is the volume of air that is lost during the dive. Since the mass of this air will be acted up on by a gravitational force of approx 1 G at or near the surface of the earth, the air has weight. When the air leaves the tank, its mass and weight go with it and by the end of the dive, an 80 cu ft tank will be lighter by the weight of that 80 cu ft of air whether it is a LP tank or an HP tank.

The internal tank volume may differ and the LP tank, having more internal volume, may hold slightly more air at ambient pressure than the HP tank, but the difference in mass and weight of that difference in air will be miniscule.

A difference in swing weight between two tanks of identical advertised capacity is far more apt to differ due to the difference between advertised capacity and actual capacity in each tank or due to errors in measuring the bouyancy of the tanks in question.

 

[geraldp]

Actually I have a related question about HP tanks vs. LP tanks that I've been too embarassed to ask, but it seems like we have lots of shingle-board physicists on here so I'll ask away. I own HP steel 100s (3450psi), but last weekend I was diving with a buddy who rented LP steel 100s (2400psi). So I know for my tank exactly what PSI to turn around with on a given dive. How do I correlate a turn-around pressure of, say 1500psi on my HP tank with that of my buddies LP tank of the same volume? Is it simply 1500/3450 * 2400 = 1045 psi?

If so can I assume that a SAC rate on an LP100 will be about 70% of that for an HP100 (for a given diver on a given dive, etc.)?

Thanks,

Jerry

p.s. AmyJ: I was on a charter once with a guy who wore double AL40's. He was an "older" guy who was short and wiry. He absolutely loved the feel of the double 40's. He claimed that the longer single 80's hurt his back and hit him in the back of the head. He also said he paid a premium (dollar wise) for diving doubles, though.