Psychrometric Calculator (Moist Air Properties)

Computes the state of moist air the way HVAC engineers do, with the equations of the ASHRAE Handbook — Fundamentals (2017), chapter 1. Give the dry-bulb temperature and any one humidity measure — relative humidity, thermodynamic wet-bulb temperature, dew point or humidity ratio — plus the barometric pressure, or an altitude from which the standard pressure is derived, and it returns all the others: relative humidity, wet bulb, dew point, humidity ratio in g/kg and grains per pound, specific enthalpy, specific volume, moist-air density, absolute humidity, vapour and saturation pressures and the degree of saturation. Saturation pressure uses the Hyland–Wexler formulas, over ice below the triple point and over water above it, so frost points and wet bulbs below freezing come out right; the wet bulb and dew point are solved to far below a thousandth of a degree. The results match PsychroLib, an independent implementation of the same handbook equations, and reproduce the handbook's own worked example (40 °C dry bulb, 20 °C wet bulb gives 14 % RH and 56.7 kJ/kg). Limits worth knowing: the equations treat air and vapour as ideal gases, which is accurate to a fraction of a percent at ordinary pressures but not in compressed-air systems; the wet bulb is the thermodynamic one, which a real sling psychrometer only approaches when the airflow is fast enough; and a state wetter than saturation (fog) is refused rather than computed.