I have a coil of copper pipe frozen into a block of water in my freezer. The idea was to circulate coolant through the coil and run the hoses out of the freezer to another cooling chamber, using an arduino to switch the pump on and off. I need to figure out some way to get a coolant that stays liquid in freezer temperatures (shouldn't be that hard right--freezers are maybe 0F but not -30F or anything).
I wanted to use a food-grade coolant. I could use alcohol, but ethanol is pretty expensive and it is flammable. RV antifreeze is supposed to be propylene glycol, but I don't know how concentrated it is. First I tried filling up the system with pure RV antifreeze and it freezes in the heat exchanger solid, so that the pump can't pump the coolant through the system. The RV antifreeze says "protection down to -50F" but apparently it's freezing.
Wikipedia states that mixing propylene glycol with water will actually lower the freezing point, but all the charts I can find on the freezing point of a glycol/water mix show that the freezing point monotonically decreases as the proportion of glycol increases, but they all stop at 60% glycol (-70F). It's possible that the freezing point shoots up as the proportion of glycol approaches 100% but that seems unlikely. Maybe I got ripped off and RV antifreeze is actually like 10% propylene glycol and not pure PG? Should I try watering my RV antifreeze down in hopes of reducing the freezing point or should I assume it's a weak PG solution and start adding alcohol?
Actully, you can't determine the proportions of a propylene glycol/water solution with a hydrometer, because it's possible to have different concentrations that have the same gravity.
You probably could use viscosity, but I don't have equipment to measure that.
Pure propylene glycol freezes at about -57 degrees F so you probably don't have 100% propylene glycol. I just checked the bottle I bought to winterize my RV and it has a freezing point of -50 degrees F which suggests it is around 58% propylene glycol.
Propylene glycol is way way heavier than water
Propylene glycol has a specific gravity of 1.036 which is only 0.036 greater than distilled water.
According to DOW's tables and my hydrometer, my RV antifreeze is 50%
propylene glycol. Gravity is about 1.042 which is less than the turnaround point, correct?
Which still surprises me that it slushed, but it did. Right now I have 50% ethanol, 50% Vodka, and 25% vodka and glycol solutions in the freezer to spearmint.
I'd use isopropyl alcohol, personally.
In situations like that with a really cold surface and high movement water will often freeze out of a mixture onto the wall, even when the mixture itself is far from freezing.
I think the effect has a name, check out wikipedia's article on freeze distillation for more info.
