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/

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)...

Holly, in France wrote:

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?

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?)

"Phil" wrote in message
oups.com...
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)...

The freezing point is not linearly proportional to the concentration of salt.

See

http://www.madsci.org/posts/archives...8209.Es.r.html

http://antoine.frostburg.edu/chem/se...elts-ice.shtml



--

Michael Chare

Weatherlawyer wrote:

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.

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

Weatherlawyer wrote:
Holly, in France wrote:

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?

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.)

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.

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?)

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/

Over the range we are talking about it will be....

wrote:
Weatherlawyer wrote:

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.

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.

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.

Weatherlawyer wrote:
Holly, in France wrote:

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?

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.
Yup
As will any evapouration.
Yup.
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.
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

Weatherlawyer wrote:

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.

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.

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.

:-)))

--
Holly, in France
Holiday Home in Dordogne
http://la-plaine.chez.tiscali.fr/

On Thu, 17 Nov 2005 23:13:19 +0100, Holly, in France wrote:

snip

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!

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.

We have just put a winter
cover on the pool too though, so I'm hoping that will help.

Err, the winter cover is really intended to keep leaves and other debris
out.

--
the dot wanderer at tesco dot net

In article , The Wanderer
wrote:
We have just put a winter cover on the pool too though, so I'm hoping
that will help.

Err, the winter cover is really intended to keep leaves and other debris
out.

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
Qercus - the best guide to RISC OS computing

Weatherlawyer wrote:
Weatherlawyer wrote:

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.
..

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?

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/

The Wanderer wrote:
On Thu, 17 Nov 2005 23:13:19 +0100, Holly, in France wrote:

snip

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!

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.

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.

We have just put a winter
cover on the pool too though, so I'm hoping that will help.

Err, the winter cover is really intended to keep leaves and other
debris out.

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/