Psychrometric Calculator

Moist air is treated as an ideal mixture of dry air and water vapor, and every psychrometric property follows from three independent state variables: dry bulb temperature, one moisture measure, and total pressure. The humidity ratio W = 0.62198 x Pw / (P - Pw) contains the constant 0.62198, which is nothing more than the ratio of the molar mass of water (18.015 g/mol) to that of dry air (28.966 g/mol). Saturation vapor pressure Pws depends on temperature alone and is estimated here with a Magnus-type exponential, 0.61078 x exp(17.27 t / (t + 237.3)) in kPa. Specific enthalpy h = 1.006 t + W (2501 + 1.86 t) separates sensible from latent content: 1.006 and 1.86 kJ/kg-K are the specific heats of dry air and water vapor, and 2501 kJ/kg is the latent heat of vaporization of water at 0 degrees C, the chosen datum. Typical design values give a feel for the scale. Indoor comfort conditions of 24 degrees C and 50 percent relative humidity yield about 9.3 g/kg humidity ratio, a dew point near 12.9 degrees C, and roughly 47.8 kJ/kg enthalpy; hot humid outdoor air at 35 degrees C and 80 percent RH reaches about 28.9 g/kg and 109 kJ/kg. Dew point is the practical condensation threshold, so any duct, window, or chilled-water pipe surface below it will sweat. Caveats: the ideal-gas treatment and the Magnus fit degrade outside roughly minus 40 to 50 degrees C, the Stull wet bulb approximation is tuned to sea-level pressure, and results say nothing about air quality or contaminant loads.

Quantifying atmospheric moisture began long before HVAC existed. Horace-Benedict de Saussure built a practical hair hygrometer in 1783, and in the nineteenth century Ernst Ferdinand August coined the term psychrometer for the wet-and-dry-bulb instrument, while Gustav Magnus published the empirical saturation vapor pressure relation in 1844 whose modern coefficient forms are still used in this calculator. These instruments and correlations let meteorologists and later engineers describe air as a two-component mixture rather than a single fluid. The engineering discipline itself dates to Willis Carrier, who installed a controlled-humidity system at a Brooklyn printing plant in 1902 and then, in a paper on rational psychrometric formulae presented to the American Society of Mechanical Engineers in 1911, laid out the equations relating dew point, wet bulb, humidity ratio, and enthalpy and published the chart that made them usable at a drafting table. In Germany, Richard Mollier's enthalpy-based diagrams from the same era became the parallel European convention, and the h-x chart is still standard there. ASHRAE later took over maintenance of the property tables and charts in its Handbook of Fundamentals, extending them to different barometric pressures for high-altitude work and eventually replacing graphical lookup with the closed-form and iterative routines that software implementations use today.