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Psychrometrics

Wet-Bulb Temperature Calculator

Computes the thermodynamic wet-bulb temperature of moist air from its dry-bulb temperature, relative humidity and pressure, by solving the ASHRAE psychrometric balance on the Hyland–Wexler saturation pressure.

The wet-bulb temperature is the temperature an evaporating (wetted) thermometer reads — the limit to which air can be cooled by evaporating water into it. This calculator solves the thermodynamic wet-bulb temperature iteratively from the psychrometric balance, and also reports the humidity ratio, dew point, enthalpy and specific volume. 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)

Wet-bulb temperature17.9 °C
Humidity ratio9.92 g/kg da
Dew-point temperature13.87 °C
Specific enthalpy50.43 kJ/kg da
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)
Wet-bulb balance (solved iteratively for t_wb)
W = ((2501 − 2.326·t_wb)·W_s(t_wb) − 1.006·(t − t_wb)) / (2501 + 1.86·t − 4.186·t_wb)

Diagram

t_dbW100% RHstatet_wbt_db

Worked example

Air at 25 °C dry-bulb, 50 % relative humidity, 101.325 kPa. Find the wet-bulb temperature.

  1. 01Saturation pressure (Hyland–Wexler): p_ws(25 °C) = 3.169 kPa
  2. 02Enhancement factor (Buck 1981): f = 1.0043
  3. 03Partial pressure: p_w = 0.50 × 1.0043 × 3.169 = 1.591 kPa
  4. 04Humidity ratio: W = 0.621945 × 1.591 / (101.325 − 1.591) = 0.00992 kg/kg = 9.92 g/kg
  5. 05Solve the psychrometric balance for t_wb (iteration converges to t_wb ≈ 17.9 °C)
Result

The wet-bulb temperature is about 17.9 °C (humidity ratio 9.92 g/kg dry air, dew point 13.9 °C, enthalpy 50.4 kJ/kg dry air).

FAQ

Is this the thermodynamic or the sling wet-bulb temperature?
It is the thermodynamic wet-bulb temperature, solved from the psychrometric balance. For typical near-ambient air the sling (psychrometer) wet-bulb temperature is very close, but they are not identical by definition.
Can I use a pressure other than sea level?
Yes — the pressure input is adjustable over 80–110 kPa. Outside that range the calculator refuses rather than extrapolating.
Why does it refuse below 0 °C?
This version uses the over-water saturation correlation and does not model the ice / frost line, so it is validated only over 0–50 °C dry-bulb. It refuses outside that range instead of returning an extrapolated number.
What datum is the enthalpy on?
Enthalpy is per kilogram of dry air, with the enthalpy of dry air and of liquid water both taken as zero at 0 °C — the ASHRAE moist-air convention, the same datum the humid-air dataset uses.

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