Saturation Curve. A saturation curve is a locus representing a series of state points of saturated moist air. For ASHRAE’s chart, the enthalpy of saturation vapor over liquid water or over ice at a certain temperature is calculated by the formula recommended in the Hyland and Wexler paper. The humidity ratios of the saturated moist air ws between 0 and 100°F ( 17.8 and 37.8°C) in the psy- chrometric chart can also be calculated by the following simpler polynomial: