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Psychrometrics

Humidity Ratio & Enthalpy Calculator

Computes the humidity ratio and specific enthalpy of moist air from its dry-bulb temperature, relative humidity and pressure, using the Hyland–Wexler saturation pressure and the ASHRAE perfect-gas psychrometric relations.

The humidity ratio is the mass of water vapour per kilogram of dry air, and the specific enthalpy is the heat content of moist air per kilogram of dry air — the two numbers a coil duty turns on. This calculator computes both from the Hyland–Wexler saturation pressure and the ASHRAE perfect-gas psychrometric relations, and also reports the specific volume, dew point and wet-bulb temperature. Enthalpy is per kilogram of dry air, on the dry-air/liquid-water 0 °C datum.

TypeInteractive engineering calculator

Calculator

Air state
°C

Validated range 0–50 °C

%

1–100 %

kPa

80–110 kPa (101.325 = sea level)

Humidity ratio9.92 g/kg da
Specific enthalpy50.4 kJ/kg da
Wet-bulb temperature17.88 °C
Dew-point temperature13.87 °C
Specific volume0.8581 m³/kg da
Audit trail
  • Saturation pressure p_ws = 3.1692 kPa (Hyland–Wexler 1983, over water)
  • Enhancement factor f = 1.00434 (Buck 1981)
  • Partial pressure p_w = f·(RH/100)·p_ws = 1.5915 kPa
  • Humidity ratio W = 0.621945·p_w/(P − p_w) = 9.9246 g/kg dry air
  • Dew point t_dp = 13.868 °C (inverse of f·p_ws = p_w, iterative)
  • Wet-bulb t_wb = 17.881 °C (psychrometric balance, iterative)
  • Enthalpy h = 1.006·t + W·(2501 + 1.86·t) = 50.433 kJ/kg dry air
  • Specific volume v = 287.042·T·(1 + 1.607858·W)/P = 0.8581 m³/kg dry air
Copyable summary

At the standard 25 °C dry-bulb / 50 % RH state (an exact node on the humid-air dataset), moist air at sea level has a humidity ratio of 9.9 g/kg dry air, a wet-bulb temperature of 17.9 °C, a dew point of 13.9 °C and a specific enthalpy of 50.4 kJ/kg dry air.

Sea-level over-water psychrometrics, computed live; cross-checked against the humid-air dataset. Properties and calculations only — not equipment selection or building-services design guidance.

Related: Wet-bulb temperature · Dew point · Humidity ratio & enthalpy · Humid-air properties · Water & steam

Formulas

Saturation pressure (Hyland–Wexler 1983)
ln(p_ws) = C8/T + C9 + C10·T + C11·T² + C12·T³ + C13·ln(T)
Water-vapour partial pressure (with Buck 1981 enhancement factor f)
p_w = f · (RH/100) · p_ws
Humidity ratio
W = 0.621945 · p_w / (P − p_w)
Moist-air enthalpy (per kg dry air)
h = 1.006·t + W·(2501 + 1.86·t)
Specific volume (per kg dry air)
v = 287.042·(t + 273.15)·(1 + 1.607858·W) / P

Diagram

t_dbW100% RHstateW, ht_db

Worked example

Air at 25 °C dry-bulb, 50 % relative humidity, 101.325 kPa. Find the humidity ratio and enthalpy.

  1. 01Saturation pressure (Hyland–Wexler): p_ws(25 °C) = 3.169 kPa
  2. 02Enhancement factor (Buck 1981): f = 1.0043; p_w = 0.50 × 1.0043 × 3.169 = 1.591 kPa
  3. 03Humidity ratio: W = 0.621945 × 1.591 / (101.325 − 1.591) = 0.00992 kg/kg = 9.92 g/kg
  4. 04Enthalpy: h = 1.006×25 + 0.00992×(2501 + 1.86×25) = 25.15 + 25.28 = 50.4 kJ/kg dry air
  5. 05Specific volume: v = 287.042×298.15×(1 + 1.607858×0.00992) / 101 325 = 0.858 m³/kg dry air
Result

The humidity ratio is 9.92 g/kg dry air and the enthalpy is 50.4 kJ/kg dry air (specific volume 0.858 m³/kg dry air, dew point 13.9 °C, wet-bulb 17.9 °C).

FAQ

Per kilogram of what — dry air or moist air?
Both the humidity ratio and the enthalpy are per kilogram of DRY air, which is the ASHRAE convention. It keeps the basis constant as water vapour is added or removed.
What datum is the enthalpy on?
The enthalpy of dry air and of liquid water are both taken as zero at 0 °C — the ASHRAE moist-air datum, the same one the humid-air dataset uses, so the two are directly comparable.
Can I use a pressure other than sea level?
Yes — the pressure input is adjustable over 80–110 kPa. Humidity ratio depends on total pressure, so the pressure matters. Outside that range the calculator refuses rather than extrapolating.
Why include the enhancement factor?
The Buck (1981) enhancement factor (≈ 1.004–1.006) corrects the ideal partial pressure for real-gas air–water interaction, bringing the result into agreement with the reference ASHRAE RP-1485 moist-air dataset to better than 0.1 %.

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