Ask A Scientist© Environmental Earth Science Archive

name         Jaspreet
status       student
age          14

Question -   I am making a pot in pot refrigerator. i get two pots and
place one inside the other. Then I put sand in the space between them
two. Then I add water to keep the jars cool when the water evaporates
from the sand. Which makes the temperature in the pots down. I was
wondering if there was a liquid that will evaporate more slowly than
water, so I didn't need to pour more liquid ,between the pots to keep
them cool, so frequently. So can you please name some liquids that have a
higher evaporating point. If there are any.
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Jaspreet-  there are lots of liquids that evaporate slower,
but most of them you wouldn't want reeking up the air in your house, and some are fire 
hazards,
and the rest you're mother wouldn't let you play with this year.
That's why nobody has any around the house.

You can use glycerin, but it basically won't evaporate at all.  And you won't get any 
refrigeration.
Same for drug-store mineral oil, and cooking vegetable oil.

Water carries more cold per cubic centimeter than almost any other fluid,
and it's cheaper, safer, and more environmental than anything else too..
If I were you I'd just try to make your water evaporate slower.
I'm sure you can make that happen.

Maybe dissolve a small measured amount of salt or sugar or glycerin in the water.
After that dries up, you never need to add more solute, just add more water.

Or cover some fraction of the outer surface of your pot,
with aluminum foil or plastic wrap or a bigger non-porous pot.
Or put it in a place where there is less air movement.
Maybe you can find ways to slow down the water leaving, that will keep cold inside, too.

I think it's OK to use a lot of water, so long as you don't have to refill it very often.
If the pot takes water at a certain rate, from a large volume bottle, it will last longer.
If this is OK with you,
you need to figure out a way to make a wet connection between a big gallon jug and your 
sand.
If it works right it will feed water to the sand slowly, only as fast as it needs to stay 
wet.
Something like:
   - a fat wick made of rolled-up paper towel, covered with plastic wrap so it won't try to 
   dry up,
   - a skinny wick made of 1 or 2 strings sheltered in straws or twisted scotch tape,
   - a real skinny siphon tube, maybe made out of straws, or
   - a slow dripper, about 1-10 seconds per drop.
Try these, find out which one works for you.

Jim Swenson
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Water is cheap, and is probably the most efficient liquid for evaporative
cooling.  If you want to reduce the evaporation rate you can use a barrier
(say, plastic wrap) to block the evaporation.  However, the cooling effect
of evaporation is proportional to the rate at which the water evaporates.
If you reduce the rate of evaporation, then the inner pot will not stay as
cool.

Bob Erck
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