Psychrometric Calculator

Psychrometry deals with the physical and thermodynamic properties of gas-vapor mixtures, most commonly air and water vapor. The core calculations start with measuring dry-bulb temperature (Ta) and wet-bulb temperature (Tw) alongside ambient pressure (Pa). Using standard equations, we first determine actual vapor pressure (Pv) through the psychrometric wet-bulb formula: Pv = Pw - 0.00066 * Pa * (Ta - Tw) * (1 + 0.0115 * Tw). From this foundational vapor pressure, secondary properties cascade outward. Relative humidity (Phi) emerges by comparing Pv to saturation vapor pressure (Ps). Specific humidity (Omega) relates the mass of water vapor to dry air, while enthalpy (h) and specific volume (vol) quantify the total heat energy and physical space occupied by the mixture respectively.

Let us walk through a practical scenario where we determine the relative humidity and specific humidity of an air sample. Suppose our ambient atmospheric pressure (Pa) is 101.325 kPa, our dry-bulb temperature (Ta) is 30 degrees Celsius, and our wet-bulb temperature (Tw) is 20 degrees Celsius. At 20 degrees Celsius, the saturation water vapor pressure (Pw) is 2.339 kPa, and the saturation vapor pressure at dry-bulb temperature (Ps) is 4.246 kPa. First, compute the actual vapor pressure: Pv = 2.339 - 0.00066 * 101.325 * (30 - 20) * (1 + 0.0115 * 20). This yields an actual vapor pressure of approximately 1.579 kPa. Next, calculate relative humidity: Phi = 100 * (1.579 / 4.246), resulting in 37.19 percent. Finally, find the specific humidity: Omega = 0.621945 * 1.579 / (101.325 - 1.579), which equals 0.00984 kilograms of water vapor per kilogram of dry air.

Always ensure your pressure and temperature units remain consistent before feeding them into any psychrometric formula to prevent scaling errors. When measuring wet-bulb and dry-bulb temperatures in the field, use an aspirated psychrometer with sufficient airflow across the wet bulb to guarantee accurate evaporation rates. Remember that barometric pressure changes significantly with elevation, so updating your ambient pressure input is vital for high-altitude environments.