Vapor sulfur hexafluoride SF6 at pressures of about 1,600 kPa is used as a dielectric in large primary circuit breakers for electric transmission systems. As liquids, SF6 and H2O are essentially immiscible, and it is therefore necessary to specify a low enough moisture content in the vapor SF6 so that if condensation occurs in cold weather a liquid-water phase will not form first in the system. For a preliminary determination, assume the vapor phase an ideal gas and prepare the phase diagram for H2O(1)/SF6(2) at 1,600 kPa in the composition range up to 1,000 parts per million of water (mole basis). The following approximate equations for vapor pressure are adequate:

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In Pist/kPa=19. I 478- n Pst kPa 14.65 11-2,048.97 T/K T/K