My chiller has not reached -50c and doubt it will. I have got it down to -32c and the pull down time is pretty fast less than 2 hours. My holding tank capacity is 3 gallons but I am filled only to 2 1/2 gallons. I used 1 gallon of strait antifreeze (prestone) it states that at a 70% antifreeze 30% water it is good to -84f or 64.5c. I diluted it with one gallon of pure denatured alcohol (ace hardware brand =, Methanol, Ketones, and Ethanol. SMELLY STUFF). I then added 1/2 gallon of distilled water, this really cut the smell of the denatured alcohol.

I believe that this mixture will do very well at -50c, Ethanol freezes at -114c and a 70-30 % mixture of the antifreeze and water freezes at -84f or -64.5c. Even if I did not even bring the alcohol into the equation, the antifreeze water mixture is 75-25% so it would freeze at a even lower temp (not much granted maybe -65.5c) . Then adding the gallon of denatured alcohol will bring that temp lower and thin the solution considerably at that temp, IMO. I haven’t studied much thermo dynamics but do understand the mixture solution equation is not linear, but it only has a minor curve and approximations will suffice. This solution will probably freeze around -70c or so.

6quarts + 4 quarts = 10 quarts so I have 6 tenths that freezes at -65c and 4 tenths that freeze at -114c . I believe that I can safely say that -70c is a safe freezing temp to assume the mixture will freeze at ( it could be lower but -70c is 20 degrees lower than -50c)