Salt in swimming pool

Hi all, Question about how much salt it takes to lower the freezing point of water. If one has a swimming pool with about 80 cubic metres of water, and adds 18 25kg bags of salt (enough for the chlorination machine to say 'enough is enough'), how much will the freezing point of the water be affected?
Bit irrelevant now really since we have added another 'hivernage/overwintering' product aswell, but now we have thought of the question we have to have the answer :-)
TIA
-- Holly, in France Holiday Home in Dordogne http://la-plaine.chez.tiscali.fr /
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Freezing point depression is directly proportional to the concentration of the solution, and is about 6 Deg C for a 10% solution of salt in water (ie the soln will freeze at -6 C at 1 atm)...
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The freezing point is not linearly proportional to the concentration of salt.
See
http://www.madsci.org/posts/archives/feb98/888588209.Es.r.html
http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-melts-ice.shtml

--

Michael Chare



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Over the range we are talking about it will be....
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Holly, in France wrote:

Sea water is 37 grams per litre all you need to do is find out what temp that freezes and you will know. I think it is about -2 degrees Centigrade. But with that sort of concentration, the mechanics change (you will need to watch that.)
Any ice that forms will increase the concentration of the rest of the pool. As will any evapouration. As it cools it gets denser and falls to the floor. All the cold water will be at the bottom of the pond until the ice forms.
Small crystals of ice will rise and either be re-absorbed or form crystal strings on the surface, the equivalent of first year sea ice.
That will float leaving a much stronger solute nearer the floor (and vents.) The small ice crystals may cause damage as abrasive or straightforward clogging.
37g/l = 37Kgm/cubic metre (I think.) = 296 Kgm/swimming pool (ditto.) So 18 x 25kg bags of salt should be plenty.
The chances of much more more a slight frost this winter are not good. (Or rather the chances are better than good there will be no frost. Pay no attenton to anything you may have heard on the BBC a few weeks back.
Oops! France. What part of France?)
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Weatherlawyer wrote:

Well there was a slight frost in Glasgow this morning and the overnight low is forecast to be -3 today and tomorrow. In Aviemore it's forecast to be -4 and -5, so it looks fairly certain that certain parts of Britain at least will experience a fair bit of frost before the winter's out.
Regards
Helen
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snipped-for-privacy@yahoo.co.uk wrote:

Quite right. I was even allowing for some in my forecasts for this past spell. I should have qualified it as long lasting frost. And even then YMMV.
Frosts usually occur with suny weather so unless you are in a frost pocket and the depths of winter, the chances of the pool forming more than a little rime at the edges are not that good or bad or what ever. I imagine a sheet or tarpaulin would keep the worst of it at bay.
The chances are that there will be a lot of wet and windy weather this winter. In fact I think it will be just like summer only a little darker.
Especially at night.
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Weatherlawyer wrote:

You were replying to someone else here, but anyway.... We get lots of sunny weather in winter, and are in a frost pocket, and have had 6'' of ice on the lake a couple of times, so the chances of our pool surviving with only a little rime are very slim indeed! We have just put a winter cover on the pool too though, so I'm hoping that will help.

:-)))
-- Holly, in France Holiday Home in Dordogne http://la-plaine.chez.tiscali.fr /
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On Thu, 17 Nov 2005 23:13:19 +0100, Holly, in France wrote:
<snip>

It rather depends on how exposed your pool is. We live in Norfolk, where, as the saying goes, the wind can be cold enough to blow the bristles off barbed wire. Our outdoor pool is enclosed with a combination of walls and inset closeboard fencing, which I guess results in a something of a micro-climate. Not had any problems with winter frosts in the 9 years we've lived here, and I don't take any special precautions other than pumping out water from time to time.
We had a pool at our previous house down in the Thames valley, and that wasn't as sheltered, and likewise I never took any special precautions.

Err, the winter cover is really intended to keep leaves and other debris out.
--
the dot wanderer at tesco dot net

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It should also reduce evaporation at the surface and keep the (some of the) heat in.
--
John Cartmell john@ followed by finnybank.com 0845 006 8822
Qercus magazine FAX +44 (0)8700-519-527 www.finnybank.com
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The Wanderer wrote:

It's considerably colder here than there though, and the pool is exposed and in a frost pocket. But as I said before, everyone else's pools seem to survive so I expect ours will too.

Well, I know, but I thought it might have some benefit. Then again it will stop the sun getting to the ice during the daytime so it might be a mixed blessing.
In the summer you replied to a question of mine about clearing the water and I never got around to replying to your last message in the thread. So, a belated thank you for your help, I got it crystal clear in the end. Given the amount of water I have had to pump out for the overwintering, I'll probably have to refill it from the lake again in the spring so I'll have to go through the whole process all over again!.
-- Holly, in France Holiday Home in Dordogne http://la-plaine.chez.tiscali.fr /
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As a codicil to cover my lower posterior while this frosty weather lasts:
If my forecasts are wrong and there are severe frosts (for the rest of the year at least -I'll have to have another look at January and February 2006) the chances are that the planet will be subject to severe seismic disturbances instead of cyclones.
If you intend to use salt, the freezing will take place at the bottom of the pool as I said. Ice will show up on the top of it when it has shucked off most of the salt content.
You had best try "sci.physics" to get a more competent estimate of what ratios of salt you might need.
It seems to me an immersed infra-red bulb would be the thing to stave off the worst of the cold nights. But why are you bothering? If it got that cold, you'd not be using it would you?
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Weatherlawyer wrote:

.
No, we won't be using it! As I said in my initial question, we have drained it down as recommended and put in a special pool product anyway. I was just interested in the additional effect that the salt which is already in it (for the chlorination process) would have. -- Holly, in France Holiday Home in Dordogne http://la-plaine.chez.tiscali.fr /
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Weatherlawyer wrote:

Thanks, and the links that Michael gave explain this in more detail, thanks to you too Michael. I was sure that the salt wasn't going to do the job on it's own but then got interested in how much of an effect it would have.

Ah! That makes a big difference doesn't it? :-). North Dordogne, and in a frost pocket in a valley. We regularly get frosts down to -8C, -10C ish, but -16C is the lowest I have recorded. The best specialist pool product says it protects down to -14C so that is the best we can do, which should be OK. In any case the pool is drained down to the recommended level and relevant holes bunged. I'm also putting floating thingies on the top which I assume are designed to collapse and take some of the pressure of ice building up on the surface. Anyway, all the other pools seem to survive so I expect ours will too.
Thanks again
-- Holly, in France Holiday Home in Dordogne http://la-plaine.chez.tiscali.fr /
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Weatherlawyer wrote:

Yup
Yup.
Nope. For pure water the maximum density is at 4 degC (the value will be different for salt water). Once the water has cooled below this temperature, the water will be cooler at the top and warmer at the bottom
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