Water Displacement

[kpogue]

I have a 150 gal tank with 160 lbs of LS and approx 210 lbs of LR. Due to displacement, how much water do I actually have in the tank?
Thanks,
Keith

 

[pstanley]

As far as I know there isn't an actual formula for figuring that out (but I could be wrong). There is too many things to tank into consideration besides weight of the rock. Again, I might be wrong though.

 

[kpogue]

I would think it would be 1lb of solid displaces "X" gals of water. How else would they be able to figure out ships' weight displcaement, etc. ?

 

[mb4000]

One way to tell would be to take everything out of your tank exept say half the water. Figure out exactly how much water you have. put everything back in. Then figure out how much more it would take to fill your tank. Then add the two numbers together.

 

[kpogue]

A little impractical at this point! What I amy do is take a graduated 5 gal bucket and add one pound weights to it.

 

[marcandkelly]

sw = 8.5lb/g - value from www.aqua.org/animals/setupaquarium.html
displacement in gallons = g/lb * lb of displament
actual gallons in tank = total gallons - displaced gallons
150 G volume in empty tank
370 pounds of displacement (160 LS + 210LR)
1/8.5* 370 = 43.5G of displacement
150 - 43.4= 106.6G of water in tank
This is my lunch break math so it is prone to errors...

 

[kpogue]

Close enough! Using your formula I came out w/ 118.5 g total water after displacement.
Thanks!!!!

 

[iechy]

I could be wrong but wouldn't other factors such a density affect the displacement? A 5 pound piece of steel would take up less room than 5 pounds of pourous rock and presumably cause less displacement.
I could be wrong but it makes sense to me.

 

[rockster]

The total volume of the solids you added in the tank is the same volume of water that had displaced. If you can calculate the volume of the solids inside you tank, archimedes will be happy for you...eureka!!!
Oh, Iechy, the density of a solid under water is not as important as the total volume of that solid when it comes to water displacement. For example, a 10 lb solid steel will displace less water than a 10-lb porous rock...wouldn't you agree? Ergo, the denser the solid, the less volume it occupies (However, liquids are different. Same volume of liquids with different densities will weight differently)

 

[kpogue]

Good question.......

 

[iechy]

Exactly what I was thinking Rockster i just stated it a little less eloquently. It is the volume that needs to be considered rather than the weight. Things with the same weight obviously don't always have the same volume and that is what affects displacement. Well done!

 

[broomer5]

I'm not so sure about that.
Displacement of water by a solid object - you must take into consideration not only the weight of the object you place into the tank - but also it's mass, density and the actual volume of space or volume that object takes up.
Example:
I can displace an amount of water in a tank by placing a 10 pound chunk of lead in the tank.
I can also displace an amount of water in the tank by placing a 10 pound chunk of porous live rock in the tank.
The lead is much more dense per cubic inch than the live rock, and will have a greater density. It will displace a smaller amount of water than the rock will, because it's volume is less.
I'm pretty sure you'd need to know the density per cubic inch, the mass ( weight/density ? ) to accurately calculate how much water will be displaced.
You'd need to know this information on the sand as well.
Weight alone will not get you an accuraate answer - but may be good enough for what you're doing.
Not 100% sure

 

[broomer5]

I typed too slow - good answers