Negative or positive buoancy

[Iguana Don]

Rainreg or others who can answer,

Would like to find out how - or + buoancy is determined, regarding a piece of equipment. For example: A regulator, how much does it weigh underwater and how is that determined.

Do they use a scale? How are tanks determined?

I understand the formula for figuring how to use a lift bag, but that is not what I am looking for.

ID

 

[hover]

Found a little technical info on this. I hope it will help you understand a little more about bouyancy and how it can be measured. Not that I really understand myself! I am probably clueless, but this is the best my pea sized brain can come up with! The law of buoyancy states that any material or object immersed in a fluid will tend to rise through the fluid if the fluid density is greater than the material density. The force associated with buoyancy is the difference between the weight of the displaced fluid and the weight of the immersed material. Bouyant force
Measuring cylinder of metal in air and in water
Suspended from the spring scale is an empty, hollow copper cyliner, and underneath a solid iron cylinder of the same volume as the copper cylinder. These two cylinders weigh a certain amount you can read off the scale.

Place a beaker of water under the weights, and immerse the iron cylinder completely into the water. The weight shown on the scale will decrease.

Fill the copper cyliner with water and you will see the weight measured by the scale returns to its original value, demonstrating that the bouyant force experienced by the iron under the water is equal to the weight of the water.

Running through this in advance will give you a good idea how much (or little) water to have in the beaker you place under the iron. Alternatively, you could place the iron in an empty beaker and pour water into it until the iron is completely immersed.

Archimedes principle
As a bonus, you can easily illustrate Archimedes principle as well. Mark or note where the water level is on the beaker when the iron is immersed. Remove the iron at the end of the above experiment (catching the drips in the beaker). Then add the water from the copper cylinder, and the beaker should fill up back to the mark it was at originally, showing that the volume displaced by the iron was equal to the volume of water required to compensate for the buoyant force.

 

[Iguana Don]

Hover,

That is just what I was looking for, thanks for the quick response and the geat information.

ID

 

[Tom]

Don- A regulator weighs the same thing under water that it weighs in your suitcase.

If you are standing on the dock holding a five pound regulator, you have to exert a 5 pound force to keep it from falling. This is because the weight of an equal volume of air is almost nothing.

5 pounds - almost nothing = almost 5 pounds

When you are in the water, you have to exert a one pound force to keep it from falling. This is because the weight of an equal volume of water is 4 pounds.

5 pounds - 4 pounds = 1 pound
(these numbers are examples only - I haven't weighed one)


Lets look at it from the other side.

Take a chunk of water that ways 50 pounds. Now take an item that is the same size as that chunk of water. If the item weighs less than 50 pounds, it floats. If the item weighs more than 50 pounds, it sinks. If the item weighs exactly 50 pounds, it is neutrally bouyant and stays exactly where you put it.

A diver adjusts bouyancy two ways.

Stick a couple of pounds of lead in your pocket. You have made youself heavier without making youself bigger. You sink.

Inflate your BC. You have made youself bigger without making youself heavier. You float.

To determine the bouyancy of an item, you need to know it's volume and weight. The weight is easy. Use a scale. Determining the volume is more difficult. Hover's post gave a complicated and technical description. To visualize it easier, fill your stock tank right up to the top. Toss in the item. Measure the amount of water that overflows the tank. That is equal to the volume of the item.

Quick example water weighs about 62 pounds per cubic foot
You have an item that weighs 60 pounds. Toss it in the tank. You spill one cubic foot of water (about 7 1/2 gallons). This item will float. Same size, less weight. Add 2 pounds without increasing size and it is neutral. Same size, same weight. Add more than 2 pounds, it sinks.

Back to the weight of the regulator in the water. If the regulator weighs one pound more than an equal volume of water, you have to supply one pound of lift. (the item is negative by one pound) As far as you are concerned, the regulator weighs one pound. The regulator still weighs five pounds. The density of the water supports the other four pounds.

This is simplified. Water density varies with temperature and contaminants. Salt water is an example of contaminated water. Hence the different weight needed by a diver. White water going over the rapids is contaminated with air. It is less dense. (a gallon weighs less). An item that is neutral in your stock tank will sink in white water.
Also I'm not sure if gold lemay is heavier or lighter than leopard print.

I hope I understood your question (as much as I hope you understood my answer)

Tom

 

[Tom]

It took me an hour to type all that and Don figures it out while I'm gone.

Tom

 

[Warhammer]

Pull his tail!!! LOL

 

[Iguana Don]

Tom,
Thanks for the reply, I,m working on an apparatus now to determine these findings.

BTW---Gold lamae is more buoyant than leopard due to the reflective qualities of the material, it makes the water sun cut and therefore less dense.

ID

 

[mattiedread]

Very good answers, I have a couple of comments that address the original question.

F=MA, the universal truth of classical physics.

Whether an object floats or sinks is not uniquely dependant upon its density (Mass/volume; both measures of measuring volume described above are very good) but rather how much water it diplaces. The 'minus four' example above may be a good example for regulators, but something of lesser density or even a boat hull made of the same density material, may float. If it diplaces an equal amount of weight of water it floats; less and it sinks.

Density is frequently thought of in terms of the density of water, and although this is not a perfect physical reply it's incorrectness lies in the simplicity.

If an object's density is greater than water (say 1.01 X the density of an equal volume of water) it will sink. If it is less, it will float, if it is exactly equal (1.00 in this example) it is neutrally boyant.

Again, F=MA. So, if the water exerts a greater force on the object than the object exerts on the water, it floats.

There is apparently a cement wreck off the Jersey coast (During WWII steel was scarce). We all know that cement is used for underwater support systems because given the right form it's density not only alows it to sink, but it gives it stability relative to the forces (F=MA!!) of the ocean. It's density is considerably higher than water's. (Say for arguments sake, 2.0 in the above example). But, if you form it in a manner so that it displaces an equal volume of water to it's own mass, it will float. Boats made of steel or cement are good examples of this.

I love classical physics. This is a very simple illustration. Think about an object's bouyancy relative to how much water it will displace; and how much that water weighs relative to the object.

 

[Mario S Caner]

Don,
Looks like I'm a bit late on this one but here is a couple of more tidbits for ya:

Archimedes Principle:
Any object wholly or partly immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object.

Remember that a cuft (10x10x10 cm) of Seawater weighs 64 lbs, where a cuft of freshwater weighs 62.4 lbs.

So an object will float if it displaces more than it weighs and it will sink if it doesn't.

 

[Rick Murchison]

Great day in the mornin'!
From the replies you can readily see that the "answer" varies depending on who you talk to and how you define "weight" and "mass" and all that - when what we *really* need to know in Scuba is how much lead we need to carry to sink something, or how much lead we can take off if we carry something that sinks.
For that, you need some lead weights of varying value, some line, some water (swimming pool will do nicely), and all the equipment you want to "weigh". I use a mesh bag tied about a foot and a half from the end of a line, with four four ounce fishing weights tied along the line from the end to the mesh bag. For a floating object like a wet suit, I tie the wetsuit to the line and add weights to the mesh bag until the whole thing just sinks, and see how many of the fishing weights end up on the bottom, which gives me the lead needed to sink that particular piece of gear to the nearest quarter pound.
For things that sink, I tie the object to a line attached to one end of a four foot bar suspended in the center from the diving board, and tie the weight line to the other end of the bar, adding weight until I get a balance, which tells me how much weight I can remove for that object.
I keep all these numbers in my log book.
Then, for any set of gear, I just add and subtract according to what I'm wearing and carrying to come up with the precise weighting I need for that set of gear - no guessing, it's always exactly right (for fresh water). For salt water, add 2 pounds for each 100 pounds total weight of you and your system, (for me that's typically 5 pounds), and once again you'll be perfectly weighted every time.
The only thing you can't use my system for is a drysuit, which you'll actually have to put on and just see how much you need to sink it and you.
Rick