How much will my pool water rise when it turns to ice?

[pkguy57]

My 21' above ground pool in northeast Ohio freezes each winter. There is 52" of water in it now and that is 3" below the wall caps. I usually don't have it this full, but we've had a LOT of rain lately. I was told to drain some water out of it so that the rising ice will not destroy the top pieces of the walls. How low from the top of the rails do I have to drain? I really don't like the idea of wasting water and then paying to fill it again in spring. But I also don't like the idea of buying new wall caps.

As a related question, I usually put an "air pillow" under the winter cover because I was told this helps keep the expanding ice from pushing out against the walls. Is this bunk or a justified practice?

 

[Gordianus]

Ice volume is somewhat 10% larger than the same mass of fluid water

 

[pkguy57]

10%, huh? That means that the level will rise about 5 inches. But the sides of the pool don't bulge out 10% when frozen, so will the level rise more?

 

[Yuqing]

10%, huh? That means that the level will rise about 5 inches. But the sides of the pool don't bulge out 10% when frozen, so will the level rise more?

You're right, the expansion is isotropic and in a rigid container the level will definitely rise more.

 

[mrspeedybob]

10%, huh? That means that the level will rise about 5 inches. But the sides of the pool don't bulge out 10% when frozen, so will the level rise more?

If each linear dimension expanded by 10% then the total increase in volume would be 33%. In a non rigid container a volumetric expansion of 10% would mean that the linear dimensions would expand by about 3%. Since your pool is mostly rigid all the expansion takes place along its vertical axis, so a 10% increase in volume means a 10% increase in water height.

 

[Manis]

You don't need to think more. Just the height of water level inscrease by
(density of water/density of ice) * original height of pool

Note-While formulating this equation the pool is regarded as rigid. So length and breadth doesn't alters.

 

[cragar]

Why don't you just stick a cup of water in the freezer and find out.
And also this reminds me of an interesting concept in physics, let's say i have a glass of ice water and all the ice melts, How does the water level change in the glass?

 

[pkguy57]

Thanks to everyone who is trying to help. I did put a cup of water and a water filled balloon in the freezer just now. They have roughly the same volume of water so I will let interested parties know the results here. (Cup is 2.75" x 1" and the balloon has a circum. of 7.5")

Can I then safely use that info and say that the ice in my pool will rise to the same percentage as the cup?

And does anyone have any insight into the utilization of the "air pillow"?