Engineering & Science
Psychrometric Humidity Ratio (g/kg) & Moist Air Specific Enthalpy (ASHRAE / Arden Buck)
Calculate saturation vapor pressure Pws via the Arden Buck equation, humidity ratio W = 0.621945 * Pw / (Patm - Pw) in g/kg dry air, and moist-air specific enthalpy h in kJ/kg dry air.
Replace paper psychrometric charts with exact ASHRAE equations!
Sizing HVAC cooling coils, dehumidifiers, and cleanroom air handlers requires converting dry-bulb temperature T (°C), relative humidity RH (0 to 1), and barometric pressure Patm (kPa, sea level = 101.325 kPa) into humidity ratio W (mass of water vapor per mass of dry air) and specific enthalpy h (total sensible + latent heat per kg of dry air)!
First compute saturation vapor pressure Pws (kPa) above liquid water using the high-accuracy Arden Buck equation: Pws = 0.61121 * EXP((18.678 - T/234.5) * (T / (257.14 + T))), and actual vapor pressure Pw = RH * Pws. From ideal-gas molecular weight ratio Mw/Mda = 18.01528 / 28.966 = 0.621945, humidity ratio is W = 0.621945 * Pw / (Patm - Pw) kg/kg (multiply by 1,000 for g/kg)! Finally, ASHRAE moist-air enthalpy is h = 1.006*T + W * (2501 + 1.86*T) kJ/kg dry air—giving W = 9.879 g/kg and h = 50.32 kJ/kg at standard 25 °C, 50% RH!
| Air State (Patm = 101.325 kPa) | Dry-Bulb T & RH | Humidity Ratio (g/kg_da) | Moist Air Enthalpy h (kJ/kg_da) |
|---|---|---|---|
| Standard Room Return Air | 25 °C @ 50% RH | 9.879 g/kg dry air | 50.32 kJ/kg dry air |
| Tropical Outdoor Makeup Air | 32 °C @ 70% RH | 21.136 g/kg dry air | 86.31 kJ/kg dry air |
| Invalid RH > 100% or Boiling | RH > 1 or Pw >= Patm | Guarded Input Error | "Invalid Psychrometric State" |
Humidity Ratio W in Grams Water per kg Dry Air (g/kg_da)
Computes Arden Buck saturation pressure Pws, partial vapor pressure Pw = RH * Pws, and 1000 * 0.621945 * Pw / (Patm - Pw).
=IF(OR(B3 < 0, B3 > 1, B4 <= 10), "Invalid Psychrometric State", ROUND(1000 * 0.621945 * (B3 * 0.61121 * EXP((18.678 - (B2 / 234.5)) * (B2 / (257.14 + B2)))) / (B4 - (B3 * 0.61121 * EXP((18.678 - (B2 / 234.5)) * (B2 / (257.14 + B2))))), 3))Moist Air Specific Enthalpy h (kJ per kg Dry Air)
Sums dry-air sensible heat 1.006*T and water-vapor latent + superheat W * (2501 + 1.86*T).
=IF(OR(B3 < 0, B3 > 1, B4 <= 10), "Invalid Psychrometric State", ROUND((1.006 * B2) + ((0.621945 * (B3 * 0.61121 * EXP((18.678 - (B2 / 234.5)) * (B2 / (257.14 + B2)))) / (B4 - (B3 * 0.61121 * EXP((18.678 - (B2 / 234.5)) * (B2 / (257.14 + B2)))))) * (2501 + (1.86 * B2))), 2))Try the example
Use the original values or change them to test your understanding.