Tank Weight (vs Buoyancy) In Water Question

[guruboy]

So I was checking my tank (AL67) in the pool at various PSI to see when it would start floating. This is just the tank with valve installed and submerged in water. I noticed the following-

1) At 1500 psi and above, if I place the tank horizontal on the bottom, the tank will remain horizontal and sink. If it is vertical, it sinks.

2) At around 1000 psi, if I place the tank horizontal on the bottom, the neck portion will want to rise while the bottom will try to sink. If it is vertical, it sinks.

Note - I did not try the inverted tank orientation to see if it would try and right itself.

Based on math, the tank is negative, even in situation #2.
https://www.subaqua.co.uk/cylinder-...volume=67&pressure=3000&weight=32&water=fresh

Even if it is considered negative, what does this mean in terms of buoyancy? If I had this on my back and was trying to stay horizontal, it seems to me the tank would affect my trim.

Would a better model be a split tank where the top half is positive but the bottom half is negative (with combined weight negative)?

 

[WarrenZ]

my experience with tanks under water is all qith a regulator attached so I usually notice the tanks getting floaty and the regulator pulling the tanks into a valve down position however this isnt enough to affect my trim once I pick up the tank and clip it back on.

 

[Bob DBF]

The tank is negative, but it sounds like the top of the tank is more buoyant than than the bottom, that's why tanks are different to trim.


Bob

 

[NAM001]

I would have thought the tank would go valve down and bottom up because of the amount of material in the neck. 1" thick neck walls and the butt 1/2 inch thick. Same amount of material rolled into the rounded neck and that portion having less displacement per # of material. I guess the bottom 4" of tank has more material than the top 4" of the tank.

 

[Bob DBF]

I would have thought the tank would go valve down and bottom up because of the amount of material in the neck. 1" thick neck walls and the butt 1/2 inch thick. Same amount of material rolled into the rounded neck and that portion having less displacement per # of material. I guess the bottom 4" of tank has more material than the top 4" of the tank.

It may have more to do with the shape, all the weight is on the same plane on the bottom, but the weight at the top is spread over half a foot or so vertically around an airspace that is buoyant. When the tank is full, the weight of the air just sinks the tank but as it approaches neutral buoyancy the shape may make a difference.

I have not seen a tank that reacts that way, however I've not spent a lot of time trying.


Bob

 

[guruboy]

my experience with tanks under water is all qith a regulator attached so I usually notice the tanks getting floaty and the regulator pulling the tanks into a valve down position however this isnt enough to affect my trim once I pick up the tank and clip it back on.

I'll probably see the same when I put a reg on it. The Center of Mass probably doesn't change much.

Would be interesting to measure how much buoyant force is actually being generated in the situation I described.

 

[Leadking]

When I ran my company distributing tanks, I did not trust any data on the web and used the following to develop buoyancy data; I filled a 55 gallon drum with fresh water and placed each variation of steel tank with valve we were distributing into the tank attached to a digital scale to get a base line on each tank. From that point we could calculate full/empty buoyancy by the weight, the the cubic weight of air, and the specific gravity of fresh vs. salt water. I have found much of the data on the web to be inaccurate, even those from a major manufacturer. I called them on it but they claimed it was accurate, now the data is not even there