I’m thinking of including a couple of plastic jars with frozen salted water into a cooler of things I need to stay cold. I’ve been reading about it and thought I’d give it a try and see if it makes a difference.
But no where does anyone mention how much salt to use in how much water.
Anyone got any ideas?
As much salt as will dissolve into the water …
Adding salt to the water won’t make things stay cold any longer; rather, it will make the mixture melt faster, thereby reaching temperatures below 0 °C. The way I’ve seen this done, you usually start with plain crushed ice which you mix with coarse salt and put in your cooling bath. Wikipedia has a few recipes: Cooling bath - Wikipedia
What are you trying to achieve? What did your sources say was the benefit of doing this?
Every year I take a couple of coolers of food and ice to a friend’s woodland cabin. Through the years I have experimented with various packing, packaging and ice configurations to see what works and lasts longest.
Last year someone asked if I’d tried salting the slab ice I make for this. Someone else suggested I try freezing the block ice right in plastic bottles, so there is less watery mess by the end.
I thought I’d combine the two and see whether it will make any difference. But, as I said, no where did anything I read specify how much salt to how much water, so I was wondering about that.
As much salt as can be dissolved into the water wasn’t the answer I was expecting though! That’s a lot of salt!
It’s just a silly experiment, nothing earth shattering!
Basically, you’re re-inventing these, which are common enough in Sweden (yes, we have a few months every year when even water turns into a liquid! :)), where you can also have them exchanged for frozen ones in most stores and gas stations for a fee.
I’ve always assumed they contain nothing but water (and in some cases a blue dye).
I’m surprised there is no Wikipedia entry for those cooling elements in English. They are common in The Netherlands as well.
Salt water has a lower melting point than just regular water … useful for ice covered roads, where the water stays frozen at 30ºF, applying salt will melt the ice and the salt water will drain off … another example is making ice cream at home, the inner cylinder holds the ice cream ingredients and this is placed in a container with a water/ice bath … turn turn turn … the water/ice bath is at 32ºF and this takes a long time to freeze the ice cream … dump a mess of salt in the bath and the temperature of the water/ice/salt bath drops to 25ºF, freezing your ice cream faster …
To your application … I’d say it doesn’t matter … if you want your ice chest to stay cold longer the only solution will be more ice … the only difference is as the ice chest warms up, you’ll reach a temperature “plateau”, at either 25ºF for salt water or 32ºF for regular water, but then after the temperature climbs again …
It will take the exact same amount of time for your ice chest to go from 10ºF to 35ºF whether you use salt water or plain water …
ETA: I’ve not verified this “fact”, but supposedly a 30% solution of salt water freezes at -15ºF … as noted above, good info is a little thin on the web …
They’re just called reusable ice packs in English. I don’t know how popular they are, but we have a few smaller ones for keeping baby bottles of milk cool in the diaper bag. At least in that application, they are fairly popular. We used to also have a big one in a flexible plastic container that we used as a cold pack for various body pains where application of cold was appropriate. But they do sell them like this as well for keeping food cool.
The ones we had were filled with some type of gel, I believe, not plain water.
Well, I’ve decided to conduct my own little experiment and see how it comes out. I’ll have half the slab ice salted, half just plain. The stuff in plastic bottles too, half salted, half not. And I’ll see if there is any difference in how long they last.
I made cooler tubes out of PVC pipe and a couple of caps. I have been using them daily in my cooler for work for years. The first one I made was just plain water and worked, but it didn’t last through a twelve hour shift. I started experimenting with salt water instead and found it to work much better.
I am no scientist, but my theory was that salt in the water would lower the freezing temperature of the water. Water freezes at 32 degrees F. Salt water freezes at a lower temperature, (depending on how much salt is in the water). I experimented with single cups of water and teaspoons of salt. I kept adding salt until the mixture wouldn’t freeze and then backed off a little to get a solid block of ice.
I always refrigerate everything the night before and put my cooler tubes in the deep freeze, (it gets colder than the freezer attached to the refrigerator). I put my cooler tubes in the cooler the next morning with my lunch/supper. I usually put a couple of sodas in the bottom next to the tubes. If I grab a soda out later in the day, it is usually ice cold, (the way I like it). When I get home the tubes are still very cold and collecting frost on the outside, but I can hear the liquid sloshing a bit.
I work indoors, but the cooler tubes will keep my stuff cold for 24 hours. If I am outside, I add a couple extra tubes and things still stay cold.
I wish I still had my tracking chart for the water/salt mixture, but I made a bunch of tubes and haven’t needed to make any since.
editForgot to add that once I found the proportions, I just ramped it up to a big batch of salt water glued one cap to each pipe, filled the tubes and glued the second cap on to seal everything in.
ISTM that the unstated underlying assumption here is that because salt water freezes at a lower temperature than fresh water, that somehow it will keep your stuff colder or cold longer. This is fairly obviously false if you think about it in terms of thermal mass. Salt water and fresh water will come out of your freezer at exactly the same temperature and for practical purposes have the same thermal mass (thermal energy density), so salting the water will make no practical difference.
You may be better off with the aforementioned freezer packs (sometimes called gel packs) because they probably do have a higher thermal mass (just a guess) and in any case they don’t melt and leave a mess. I like them and always keep a few in the freezer for occasional use.
Liquid water has about twice the heat capacity as water ice, so the wider the liquid range, the more overall capacity it has (keeping temperature constant). That said, salt is likely to make the heat capacity worse (for both liquid and solid), so this might be a negative tradeoff.
That’s an interesting point, and in itself that might (maybe) make a small difference in favor of salt water. The other side of the coin is that because of the lower freezing point, salt water releases its latent heat of fusion at a lower temperature when it freezes, and absorbs heat at a lower temperature when it melts, say in a cooler. So that when using frozen salt water in a cooler, the temperature profile will be different, but not necessarily more desirable. By making stuff colder faster, it creates a bigger initial temperature differential, whereas fresh water ice will absorb heat more gradually and its latent heat of fusion absorption will occur later, so I’m hypothesizing that frozen fresh water may actually keep things cool a bit longer.
Of course if you add so much salt that your freezer can’t freeze the salt water at all, then you’re really much worse off because the major thermal impact from latent heat of fusion won’t occur at all!
The difference is 2 J g[sup]-1[/sup] K[sup]-1[/sup] between liquid water and solid water … compared to 333 J g[sup]-1[/sup] latent heat of fusion … so for salt water that freezes at 25ºF, we’re only talking about 10 J g[sup]-1[/sup]; not a trivial amount but still fairly small compared to melting the ice …
As wolfpup notes, most of the time, the system will be melting the ice … so this time will be at 25ºF for salt water, but 32ºF for fresh water … the main downside of using salt water ice is that the beer will freeze and the bottle will break …
Is this true? (There is not obvious reason why it should be). We note that the latent heat of fusion of tin is different to sodium is different to water. Do we have a table giving the latent heat of Salt Ice?
Unrelated to salt and water as a cooling agent, have you considered dry ice for your camping needs?
I know the local Praxair near me sells small amounts for Halloween “smoking cauldron” special effects, I suspect they sell small bags or chunks for campers and hunters too.
It’s far colder than water or salted water ice but I don’t know if that extends the duration. Best of all though, never any watery mess.
Dy ice would keep things much colder (below -50ºF) and longer (750 J g[sup]-1[/sup] latent heat of sublimation) … with no nasty salt water corroding everything … just be careful handling dry ice, it can and will cause frostbite …
Just remember not to use it in an air tight container, not unless you want a big hole it anyway.
You shouldn’t have to search too hard to find dry ice. Around here at least, lots of supermarkets have it in coolers near the exit doors, usually not far from bags of regular ice.
Also, some dry ice tips from someone who’s used it in a cooler before: if you want the items in the cooler to remain frozen solid, put the dry ice on top. If you want to keep things cool, but not frozen solid, put the dry ice at the bottom of the cooler, with some insulation such as a small towel between it and the food to reduce direct contact. Getting the dry ice wet or putting it into a regular refrigerator or freezer will actually make it “melt” faster, because it’ll pick up condensation, so just keep it wrapped in plain brown paper or something.
DO NOT, DO NOT, DO NOT, and did I also say DO NOT lock/seal a cooler with dry ice in it. It expands immensely as it sublimates into CO[sub]2[/sub], and you could damage your cooler, as well as anyone standing nearby. Seriously, don’t do it.