The IAPWS Formulation 1995 for the Thermodynamic properties of ordinary water substance for general and scientific use by W. Wet-bulb temperature is estimated by converging dewpoint to ambient temperature until saturated water vapor pressure at the estimated temperature is approximately equal to actual water vapor pressure with selected precision.Where h is the enthalpy and ρ is the density of air. Energy per unit volume (E v): E v = h × ρ.Where the specific gas constant for dry air, R d = 287.058 Īnd the specific gas constant for water vapor, R w = 461.495 The pressure of wet air is the sum of the pressure of dry air (P d) and the water vapor pressure (P w): Note, that n ⁄ V is the density (ρ), then the ideal gas law can be re-written as follows: P = ρRT, and the formula for density is simply this: ρ = P ⁄ (RT). The density of air calculation makes use of the ideal gas law: PV = nRT, where P, V and T is the pressure, volume and temperature respectively n is the amount of substance, and R is the ideal gas constant.Where P is the entered ambient pressure and P w is the water vapor pressure. Pressure of dry air (P d): P d = P − P w.Where P w is water vapor pressure, RH is relative humidity (%), P ws is saturated water vapor pressure. Water vapor pressure (P w): P w = RH × P ws ⁄ 100.Where P is ambient pressure, P ws is saturated water vapor pressure. Saturated mixing ratio (W s): W s, g/kg = 621.97 × P ws ⁄ (P − P ws).Note: Replace 621.97 with 4354 to calculate the ratio in grains per pound (gr/lb or GPP). Where P w is water vapor pressure, P is ambient pressure. Where T is ambient temperature in ☌elsius, W is mixing ratio (mass of water vapor ⁄ mass of dry air) in g/kg. Where T is the ambient temperature in ☌elsius, RH is relative humidity (%), A 1 = 17.625, B 1 = 243.04☌. Dewpoint or dew point temperature (T d).This formula is derived from Ideal gas law, and adapted for water vapor. Where P w is the water vapor pressure, T is ambient temperature in Kelvin, R w is specific gas constant for water vapor and it is equal to 461.5. Absolute humidity (AH): AH, kg/m³ = P w ⁄ (R w × T).The attributes include mass of water vapor per mass of dry air mixing ratio, enthalpy, water vapor volume and mass as compared to dry and wet air in parts per million, dew point and wet-bulb temperatures, absolute humidity, water vapor and vapor saturation pressures at ambient and wet-bulb temperatures, the density of air and energy per unit volume. The calculator computes numerous humidity attributes using relative humidity (RH), ambient temperature and pressure. Humidity is defined as the amount of water vapor in the air.
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