I just use filtered water and those heavier bottled water bottles, get some reuse out of them.
The temp diff isnt much, and it is handy to be able to drink or just pour out onto plants the water after it melts. Or refreeze, which is most common.
So far, I have determined: 200 ml of salt to a litre of water, seems to be the correct ratio!
When I was a kid, we knew a guy who owned a paint company who had ready access to dry ice. He’d get 1/2" thick slabs of dry ice and alternate layers of dry ice & steaks wrapped in newspaper. Made for a pretty heavy cooler, but it kept meat frozen for over a week when he was camping.
I have also now determined that the bottles of salted water take much longer to freeze solid.
Overnight was not enough, though they are slushy. The freezer did freeze a slab 2" thick, 6x10" overnight, no problem.
I’m moving the salted water ones to the chest freezer in the basement!
Engineer’s Toolbox has a bunch of handy graphs. It looks like the specific heat decreases from ~4200 to ~3300 as the solution gets saturated.
PS: A mixture of salt and water has a technical name of brine in case you are looking to google for this.
Is this the 20% solution mentioned above?
Also, is Mr/Mrs Elbows looking at you in a concerned fashion?
It is.
My friends will have been at their truly woodland cabin for several weeks already. For the week we are with them, we will all eat very well indeed. The Mr, always an easy going fellow, will be eating his favourite yummy Thai Green Curry in the woods, so he’s good.
Also I have a wide latitude to be weird in all things. Such is domesticity at our house.
Salted water bottles still not frozen! Harrumph!
If you are trying to cool things down quickly, you want salted water. Heat transfer is greatly increased when there is liquid in with the ice. The salted water will melt sooner, and therefore (in simple terms) transfer the cold to the other items sooner.
If you want the ice to last longer, then fresh filtered water will stay solid longer, and therefore last longer, but won’t cool everything else down as fast.
If you really want it to last a long time, buy a small packet of disposable diapers. Open one up, and put some of the powder into a large ziplock bag. (About a level teaspoon for a gallon bag.) then fill it with water. You’ll wind up with a gel. The gel will last the longest of all.
So the ideal would be to:
- cool or freeze everything you are putting in the cooler
- put bags with frozen salt water on the bottom of the cooler.
- put bags of frozen freshwater in a layer on top.
- put bags of frozen gel on the topmost layer.
Never use dry ice for long term storage unless you’ll be able to replenish it. The reason is that you can’t tightly close the container or the expanding co2 will cause it to explode. So you basically lose more from the insulation loss than you gain from the deeper cooling ability.
Oh, and I actually second the idea of using bottled water for the freshwater component. It’s nice to have on hand when it starts to melt and saves some packing space.
Thank you, that’s very helpful information. (Though it borders on ruining my experiment, but I’ll ignore that part!)
( One cooler will be filled with already frozen solid items, and be opened only once daily very likely. The other cooler is filled with items needing to be kept cold but not frozen. It will be opened more often.)
Too salty … either spin down the freezer’s thermostat* … or use a 10% salt solution …
- = Not recommended, my appliance guy says never to do this … no idea why though …
When you get down below your design operating temperature, your 12year expected lifetime turns into your 12 month expected operating life.
The expected pump life is measure in 1000’s of hours. If it runs continously. that doesn’t take very long.
Excelent! Those are the numbers we want. At a casual glance, it looks like salt water has better effect per volume than plain water, but weighs more.
I just learned that NaCl has what’s called a negative partial molar volume, which means that the volume of liquid actually *decreases *as you add salt. So not only is it increasing mass from the salt, it’s “compressing” the water due to the influence of the ions. Even if the salt contributed nothing to the heat capacity itself, adding it would be a volumetric benefit since you can pack more water into the same volume as you add salt.
The chest freezer did the job! Both litre bottles of (20%) salted water are now frozen solid!
Fantabulous … hope you enjoy your vacation !!!
But NaCl doesn’t “contribute nothing” to the heat capacity, it substantially reduces it!
My casual reading of the previously linked charts would be that at around the 23% concentration by mass, you gain 18% density but lose 21.4% specific heat capacity, so ISTM that you’ve actually got a small net loss in cooling capacity. But maybe there’s some other factor that I’m missing. It’s understandable that salt-water ice can be used to make things colder than freshwater ice, but as already explained that’s not an advantage here.
It doesn’t seem to. Say you have 1 kg water–by the chart, that’s 4210 J/K. Compare against 1.25 kg of salt water at 20% mass concentration. 80% of that is water, or still 1 kg. The heat capacity is 3370 J/kg-K * 1.25 kg = 4212 J/K. That looks the same to me, so it seems likely that the NaCl ions are floating around doing nothing, while the water is doing its normal thing.
That said, their density chart does seem to contradict my earlier claim that adding salt decreases the volume, since it has a slope of less than 1. I’ll have to look into that again. Perhaps it’s only true for small concentrations and there is some interpolation error.