Question about ice:

I have an above ground swimming pool, 7ish meters across, circular. My question is about how ice freezes. I bought a winterizing kit from our local pool store and one of the pieces was a 4 foot balloon.

The clerk said something I wondered about:
I thought the balloon was simply to raise up the pool cover to make it easier to remove winter sludge, water, dirt, leaves, etc. that accumulates in the cold season.

Nay nay, he said the purpose of the balloon was to make sure the ice that forms in winter pushes the balloon together and to not expand outwardly to maybe burst the outside wall of the pool.

Does that make sense? If ice expands isotropically, that is equally in X, Y, and Z directions, would a balloon actually help reduce pressure on the sides during the creation of ice on the surface layer? We get typically about 10 cm of ice forming in winter here, sometimes 15 cm.

Originally posted by sonhouse
I have an above ground swimming pool, 7ish meters across, circular. My question is about how ice freezes. I bought a winterizing kit from our local pool store and one of the pieces was a 4 foot balloon.

The clerk said something I wondered about:
I thought the balloon was simply to raise up the pool cover to make it easier to remove winter sludge, water ...[text shortened]... the surface layer? We get typically about 10 cm of ice forming in winter here, sometimes 15 cm.

It sounds like the balloon is there to take up volume for the ice to expand into. Water expands in volume when it freezes. Gases can be squeezed, so the balloon provides room for the ice.

Originally posted by AThousandYoung
It sounds like the balloon is there to take up volume for the ice to expand into. Water expands in volume when it freezes. Gases can be squeezed, so the balloon provides room for the ice.

That is the story the clerk said, what I wondered if that was true. I know water expands when it freezes but if the balloon is in place would it actually stop or slow down the expansion to the outside like he said? It could be just an old wives tale. I wanted to know if that was true, would it protect the outside skin, preferentially expanding into the center where the balloon is.

Originally posted by sonhouse
That is the story the clerk said, what I wondered if that was true. I know water expands when it freezes but if the balloon is in place would it actually stop or slow down the expansion to the outside like he said? It could be just an old wives tale. I wanted to know if that was true, would it protect the outside skin, preferentially expanding into the center where the balloon is.

There must be a way to test it with a small balloon in a box of water in your freezer (with a separate control box without a balloon).

Originally posted by sonhouse
That is the story the clerk said, what I wondered if that was true. I know water expands when it freezes but if the balloon is in place would it actually stop or slow down the expansion to the outside like he said? It could be just an old wives tale. I wanted to know if that was true, would it protect the outside skin, preferentially expanding into the center where the balloon is.

It absolutely makes sense to me. In addition to what I wrote - gases shrink when they get cold, which conveniently makes this process even easier.

Originally posted by AThousandYoung
It absolutely makes sense to me. In addition to what I wrote - gases shrink when they get cold, which conveniently makes this process even easier.

What has 'gasses shrink when cold' have to do with the ice thing?

Also, it would seem to me if the balloon thing actually did its job and saved the outer skin from being overstretched, wouldn't it be size related? Suppose you had a lake sized pool, or olympic size, wouldn't the outer edge past a certain size start up expanding again after you pass a certain size pool? I guess I am saying the balloon thing would only work up to X size whatever that would be.

Originally posted by sonhouse
What has 'gasses shrink when cold' have to do with the ice thing?

Also, it would seem to me if the balloon thing actually did its job and saved the outer skin from being overstretched, wouldn't it be size related? Suppose you had a lake sized pool, or olympic size, wouldn't the outer edge past a certain size start up expanding again after you pass a cert ...[text shortened]... ? I guess I am saying the balloon thing would only work up to X size whatever that would be.

The balloon exists to take up volume, because when water freezes it's volume increases. Gases are compressible; solids and liquids are not. If there is no gas in the tub the expansion of the ice will break the tub; it has to expand into new volume, period. By putting a gas filled balloon with an elastic rubber skin in, you take up space with a compressible object/substance. This provides extra volume for the ice to expand into. It will squash the balloon, but that's ok; gases will compress. The pressure just increases, but the nothing will break.

The fact that gases shrink when cooled means less compression is necessary; it shrinks even as the ice expands into it's space.

Originally posted by AThousandYoung
The balloon exists to take up volume, because when water freezes it's volume increases. Gases are compressible; solids and liquids are not. If there is no gas in the tub the expansion of the ice will break the tub; it has to expand into new volume, period. By putting a gas filled balloon with an elastic rubber skin in, you take up space with a comp ...[text shortened]... oled means less compression is necessary; it shrinks even as the ice expands into it's space.

Yeah, I get all that. One question, if the balloon was not under a pool cover it would not do its job, right? When it is floating on the water, the expansion would just push the balloon up in the air and it would continue to expand outwards, I would think, so the pool cover must be part of the solution also.

I am not yet convinced that it will work. If you put an open bottle of water in the freezer, some of the ice will over flow out the top, and some will push the sides out.
The sideways expansion must be when it is too solid to simply expand upwards.
Surely if you expand a solid ring, it will expand outwards not inwards? If you heat a metal washer, its hole gets bigger. If the ice can flow enough to fill the whole, it can flow enough to expand upwards.

Why do you keep water in the pool anyway? You surely aren't going swimming in all that ice. Is it safer, or does the pool last longer with water in it?

Well of course if the top is open the ice will expand upwards. I assumed it was a confined space. I don't know what happens if water freezes with the top open.

Originally posted by twhitehead
I am not yet convinced that it will work. If you put an open bottle of water in the freezer, some of the ice will over flow out the top, and some will push the sides out.
The sideways expansion must be when it is too solid to simply expand upwards.
Surely if you expand a solid ring, it will expand outwards not inwards? If you heat a metal washer, its ho ...[text shortened]... n't going swimming in all that ice. Is it safer, or does the pool last longer with water in it?

In an above ground pool, the liner would quickly crack and lose waterproofing with no water in it during the winter so you just drain enough water to be under the skimmer to keep that plastic stuff from being cracked by ice. There is a cover on top of the pool to keep out debris and a balloon under the cover to supposedly allow ice to expand into the balloon space and save the outside from over-expansion.

So one question is, does ice expand isotropically, that is to say, equally in all directions, X,Y, and Z?

Some thoughts;
On isotropic expansion of the ice: The weight of gravity (plus increasing weight of the the mass of forming ice) vertically will act against vertical expansion and I would envisage lateral expansion to be more than vertical. But there is also the flotation forces. Mmm? A bottle is a different shape container than a pool, with different dynamics.
On the balloon:
Taking some of the expansion appears valid (for the reasons already explained in posts), but the keeping the balloon in the correct lateral alignment with the ice as it expands may be a problem. Being a sphere I would imagine a tendency for it to possibly rise above the ice as it presses in somewhat, and if not lateral to the ice the effect will be lessened or lost. (I am not sure on that, being in Australia; we don't see too many frozen pools here. 🙂)
There may be tethering with the balloon but it hasn't been mentioned. If not, it may assist in keeping the balloon level with the expansion layer.

Correction. Density changes not the mass, or I read, perhaps a tad lighter even.
Discount that.

Originally posted by Taoman
Some thoughts;
On isotropic expansion of the ice: The weight of gravity (plus increasing weight of the the mass of forming ice) vertically will act against vertical expansion and I would envisage lateral expansion to be more than vertical. But there is also the flotation forces. Mmm? A bottle is a different shape container than a pool, with different dynamic ...[text shortened]... t been mentioned. If not, it may assist in keeping the balloon level with the expansion layer.

You may be right about gravity effecting the expansion of ice. The balloon is somewhat restrained in vertical movement, not a whole lot, but some, by the weight of the pool cover so I imagine it would have to rise up but at least if the idea is correct there would be some help keeping the expansion from bursting the pool sides. Thing is, I have used just a cover for more than 5 years with no balloon and saw no sign of the sides bursting, it never occurred to me that could happen before I talked to the clerk at the pool store. One thing that might have helped, I put half inch foam plastic insulation around the edge of the pool vertical wall under the liner with the idea I could maybe get some benefit keeping more heat in maybe extending the swim time a few days past summer, so it may have been saving the pool from bursting, don't know for sure. If it got compressed in winter, don't know if it could recover in spring when temps go back up above freezing. If it does, I probably don't need a balloon since the insulation would do the same job, absorbing expansion forces rather than keeping expansion to the center. Does anyone know how much physical expansion there would be on unmodified water 7 meters wide, circular pool and a meter or so deep when freezing?

Disassemble the pool every fall and you won't have to question either their or your hypothesis.