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/