Psychrometric Calculator: Method

Moist-Air Relations

Moist air is treated as a mixture of two perfect gases, dry air and water vapor, at the total pressure p of the altitude. The humidity ratio is the mass of vapor per mass of dry air, W = 0.621945 p_w/(p − p_w), with p_w the vapor's partial pressure; at saturation p_w equals the saturation pressure p_ws(T) of Hyland and Wexler, and the relative humidity is p_w/p_ws. The enthalpy per unit of dry air, h = 1.006 t + W(2501 + 1.86 t) kJ/kg with t in °C, carries the sensible heat of the air and vapor and the latent heat of the vapor; ASHRAE's USCS tables take 0 °F dry air as the datum and the tool reports Btu/lb on that basis. The specific volume per unit of dry air follows from the gas law.

The wet-bulb temperature is found from the adiabatic-saturation relation (ASHRAE eq. 35) by iteration; the dew point from the inverse of the saturation curve (eq. 39 and 40). Given the dry bulb and any one of relative humidity, wet bulb, dew point or humidity ratio, the state is fixed.

Altitude matters: at 5,000 ft the pressure is 12.2 psia and the same dry bulb and relative humidity hold 20 percent more water per pound of air than at sea level, which is why the chart is drawn at the entered pressure.

Temperatures are entered as plain numbers in the units of the selected system because their scales have offsets; every other quantity carries its unit dropdown.