Displacement ???

Question
Hi Mike, I have a question about Displacement..... Can you please explain what is ment by displacement, how do you judge displacement, I just need the down to earth scoop of what is displacement...... I tried to read and get an idea of the displacement thing from my PADI book but the way it is written and the wardage used I have a haed time seeing a clear picture of the whole displacement thing!  Thank you in advance for any help you can send to me..... Eddie,  

Answer
Hi Eddie,

Let me see if I can explain displacement (displacement volume) in such a way that it makes sense to you.  This will be a visual concept so you need to use your imagination to picture what I'm going to tell you.

Take a drinking glass and fill it about half full with water and set it on the counter.  Note where the surface of the water is located.  Perhaps you could make a mark on the outside of the glass with a marker pen to show where the surface of the water is located.

Next, take a golf ball and carefully drop it into the glass of water.  you will notice that the water level in the glass will rise as the golf ball sinks below the surface.  Mark this new water level with the marker pen.

The difference between these two marks on the glass is the displacement volume of the golf ball.  If you were to take the water contained between these two marks and measure it, you would find that it is exactly the same volume as the golf ball.

Now let's go a bit farther.  Let's imagine that we take the displaced volume of water out of the glass and put it into a very small plastic bag (assume that the bag weighs nothing).  Now we've got two volumes of exactly the same size, even though the shape of the water bag may not be round like the golf ball, it's still the same volume.

Now let's place the golf ball and the water bag on the opposite ends of a balance beam (like a teeter-totter).  The golf ball will be heavier so it will push it's end of the balance beam down.  This is just like the golf ball sinking in fresh water.  In order to get the golf ball to rise back up, we would need to add some weight to the water bag on the opposite end of the balance beam.  This extra weight could be in the form of salt crystals.  When we add salt crystals to water, we get salt water.  When we add enough salt, the water bag will be heavier than the golf ball so the golf ball will then rise up as the salt water bag pushes its end of the balance beam down.  This is just like the golf ball floating in salt water.  You may be wondering if the volume of the water changes as we add a bunch of salt to it.  The answer is "no".  The salt will dissolve in the water so the volume doesn't change.

If you can visualize the explanation I've just given you then you can see why objects are more buoyant (float better) in salt water than they are in fresh water.  The saltier the water, the more buoyant objects are when they're placed in it.

Eddie, I hope this gives you a better idea of what displacement is and how it's used to determine buoyancy.  If you're still confused, please write back and I'll try again!!

Best Wishes,
Mike Giles
Mike's Dive Center
mikescuba.com