3.20K 3He 3He evaporation 4.18K sHe He 3He +He Figure 12.5 Cooling principle of the helium dilution refrigerator. Liquid .He is in equilibrium with a He-He mixture. When 4He is added to the mixture, He evaporates from the pure He fluid and absorbs heat in the process.4.18 K 3.20 K