Stay Ice Cold This Summer.
Over the last 15 years, I have made an annual trip to the FL Keys. We would do a lot of fishing and little diving and very little sleeping. One of the problems we had was ice, we would go through 15-20 bags and a couple of blocks every day. So I dusted of some of my Chemistry books and thought I would find a safe, readily available liquid that would freeze at 28 degrees F. I tested out some glycol and other types of alcohols. I found a few that might work, but they all had issues.
What I finally ended up with was easy to find and I had access to several trillion gallons. It was saltwater. Around the same time my wife started a saltwater fish tank, which required her to make the saltwater. The saltwater she made had to use pure water and have a specific gravity of 1.025. So I did a few calculation and found that this saltwater, when made with pure water would freeze at 28.4 degrees. At the next water change (something that is done once a month) I filled up 5 one gallon jugs and put them in the coffin freezer. It took 2 days, but they froze solid, which is important.
I'm sure you all have seen the trick of freezing a couple of gallons of water and using that instead of ice. The downside of using fresh water is that once the ice starts to melt in the jugs, it provides much less cooling power. Since the sounding waster in the jug is above 32 degrees. The saltwater as a freezing point of 28.4 degrees has is starts to melt the sounding water in the jug will remain below 32 degree until the saltwater ice is gone.
Now what can you do with the laws of thermodynamics. By adding one or two jugs of saltwater ice to a cooler with a few bags of ice you will preserve the bagged ice in a solid state for much longer. What will happen; as the ice begins to reach it's melting point the saltwater ice and surrounding saltwater will absorb the heat from the bags of ice keeping the bagged ice solid. Even though it is only a few degrees colder than 32 degree, you can actually make freshwater ice, by adding jugs of saltwater ice to fresh water. To test this experiment, I added 3 one gallon jugs of saltwater ice in a 12 gallon cooler. Then added about 3 gallons of water from the hose. I left it over night and the next morning the water was frozen about 3 inches thick.
By adding a few jugs of saltwater ice to your cooler, you will reduce the amount of ice you need. And as a bonus you can now bring ice cream sandwiches the next time you go fishing. Nothing taste better after a few hours of battling redfish then an ice cream sandwich.
FYI: I have tested the specific gravity of the gulf water and it comes in around 1.020 to 1.028 depending on the rains and location. If you want to use saltwater from the gulf make sure you get the water below the surface. The water on the surface could contain contaminates and oil distillates which might keep it from freezing solid.