Humid Air Properties

Method & assumptions

1. Calculate humidity ratio, dew/frost point, enthalpy, specific volume and moist-air density from temperature, relative humidity and absolute pressure.
2. Supported inputs: −50 to 80 °C, 0–100% RH and 20–120 kPa absolute. Total pressure must exceed saturation vapor pressure at the dry-bulb temperature.
3. ASHRAE saturation equations use ice at or below 0.01 °C and liquid water above it. RH is relative to that phase; subfreezing RH reported relative to liquid water must be converted before use.
4. Water-vapor pressure is RH/100 times saturation pressure. W = 0.621945 pv/(p − pv); specific humidity = W/(1 + W).
5. Enthalpy h = 1000[1.006T + W(2501 + 1.86T)] J/kg dry air, with T in °C and the conventional dry-air/liquid-water zero at 0 °C. It is not enthalpy per kg of moist air.
6. v = 287.042(T + 273.15)(1 + 1.607858W)/p; total density = (1 + W)/v. Assumes ideal moist air with no liquid droplets or real-gas corrections.
7. Dew/frost point inverts the saturation equation. Dry air has no finite dew point; results below −100 °C are reported outside the model range. These labels are returned as strings in the API.
8. Adapted coefficients from MIT-licensed PsychroLib; the numerical inversion, exact zero-humidity behavior and restricted operating domain differ from that library.

Sources

• PsychroLib: ASHRAE-based psychrometric equations and documentation ↗
• PsychroLib MIT license and adaptation attribution ↗