อัปเดต 2026-09-07

Psychrometric Properties

ASHRAE Fundamentals 2021 Ch.1 — Hyland-Wexler moist air

Full moist-air state (dry-bulb, wet-bulb, dew-point, relative humidity, humidity ratio, enthalpy, specific volume) from any consistent pair of inputs, at a given total or altitude-derived atmospheric pressure.

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Evaluates the Hyland-Wexler saturation-pressure correlation, derives the humidity ratio from the partial/total pressure ratio, and closes the remaining state via the ASHRAE moist-air enthalpy relation; wet-bulb and dew-point are recovered by bracketed bisection / Newton-Raphson root-finding on their respective ASHRAE energy-balance and saturation relations (converges in a few iterations, never throws on unsolvable/supersaturated input).

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Humidity Ratio

W = 0.621945 \frac{p _{w}}{p - p _{w}}

ASHRAE 2021 Ch.1 eq.22; W in kg water vapor / kg dry air; 0.621945 = M_water/M_dry-air.

Specific Enthalpy of Moist Air

h = c_{p a} t + W (h_{f g 0} + c_{p v} t)

ASHRAE 2021 eq.30; c_pa=1.006, h_fg0=2501, c_pv=1.86 kJ/(kg·K); reference state dry air + liquid water at 0°C.

Saturation Vapor Pressure (Hyland-Wexler, over liquid water)

ln p_{w s} = \frac{- 5800.2206}{T} + 1.3914993 - 4.8640239 \times 1 0^{- 2} T + 4.1764768 \times 1 0^{- 5} T^{2} - 1.4452093 \times 1 0^{- 8} T^{3} + 6.5459673 ln T

T in K, valid T≥0°C (ASHRAE 2021 eq.6); a parallel eq.5 over-ice branch is used below 0°C.

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