The following property functions only operate with substance AIRH2O: DEWPOINT, HUMRAT, WETBULB, RELHUM. The relative amount of moisture can be indicated by providing the humidity ratio (W), relative humidity (R), dewpoint temperature (D) or wetbulb temperature (B). Pressure is one of the required properties. Note that substance AIRH2O requires three independent properties to fix the state. The property correlations are designed for use at temperatures below 100☌. The properties of air-water vapor mixtures are calculated assuming ideal gas behavior using the relation provided in Chapter 1 of the 2009 ASHRAE Handbook of Fundamentals (Eqn 28). The reference state for both enthalpy and entropy of water vapor is based on the traditional steam table reference state. Specific entropy is referenced to 0 K to be consistent with fluid AIR. The reference state sets the specific enthalpy of dry air to 0 at 0 F (in English units) and 0 C in (SI units) for consistency with most psychrometric charts. Wet-bulb is approximated with the adiabatic saturation temperature. Substance AIRH2O implements air-water vapor mixture (psychrometric) properties using thermodynamic data from the built-in AIR and STEAM property relations from Hyland and Wexler, "Formulations for the Thermodynamic Properties of the Saturated Phases of H2O from 173.15 K to 473.15 K, ASHRAE Transactions, Part 2A,Paper 2793 (RP-216), (1983).
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