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HVAC & Building Services

Psychrometric Properties Calculator

Humidity ratio, dew point, wet bulb, enthalpy and specific volume of moist air from dry-bulb temperature and relative humidity.

Inputs

Simplified psychrometric chart with dry-bulb temperature on the horizontal axis and humidity ratio on the vertical axis at the right. A steep saturation curve (100% RH) and a lower 50% RH curve. A state point on the 50% curve. A horizontal line from the point to the saturation curve marks the dew point, and a sloping line up to the curve marks the wet bulb.100% RH50% RHstate pointWhumidityratiot_dbt_wbt_dpdry-bulb temperature →
A simplified psychrometric chart. The state point (dry bulb and RH) fixes the humidity ratio W. Moving left at constant W to the saturation curve gives the dew point; following a wet-bulb line up to the curve gives the wet-bulb temperature. Diagram not to scale.

Ordinary air temperature. Valid range −100 °C to +200 °C.

More than 0% and up to 100%.

From −500 m to 11 000 m. 0 m gives the standard sea-level pressure 101.325 kPa.

Result

Humidity ratio (moisture content)
13.3102
Dew-point temperature
18.4466
Wet-bulb temperature (thermodynamic)
22.005
Enthalpy (SI basis, per kg dry air)
64.2115kJ/kg
Enthalpy (inch-pound basis, per lb dry air)
35.27Btu/lb
Specific volume (per kg dry air)
0.877168
Density of the moist air
1.15521
Partial pressure of water vapour
2.12302
Saturation pressure at the dry-bulb temperature
4.24603
Degree of saturation
0.4893
Atmospheric pressure used
101.325
Working
p_ws = 4.246 kPa at 30 °C (Hyland–Wexler, over water) p_w = RH × p_ws = 0.5 × 4.246 = 2.123 kPa W = 0.621945 × p_w ÷ (p − p_w) = 0.621945 × 2.123 ÷ (101.33 − 2.123) = 0.01331 kg/kg h = 1.006 t + W (2501 + 1.86 t) = 64.212 kJ/kg v = 0.287042 (t + 273.15)(1 + 1.607858 W) ÷ p = 0.87717 m³/kg

This air holds 13.3102 g/kg of water vapour per kilogram of dry air. Its dew point is 18.4466 °C and its wet-bulb temperature is 22.005 °C.

Educational estimate — check the assumptions below.

About this calculator

Psychrometrics is the study of moist air — the mixture of dry air and water vapour that air-conditioning systems heat, cool, humidify and dehumidify. From the dry-bulb temperature, the relative humidity and the air pressure, this calculator finds the other properties you would read from a psychrometric chart: humidity ratio, dew point, wet-bulb temperature, enthalpy, specific volume and density.

Unlike a printed chart, it works at any altitude or pressure, which matters for sites well above sea level.

Formula

pw = RH × pws(t)    W = 0.621945 × pw ÷ (p − pw)

h = 1.006 t + W (2501 + 1.86 t)   kJ/kg dry air

v = 0.287042 (t + 273.15)(1 + 1.607858 W) ÷ p   m³/kg dry air (p in kPa)

pws = saturation pressure of water vapour, from the Hyland–Wexler equations (over ice at or below 0.01 °C, over liquid water above), valid from −100 °C to +200 °C. Dew point: the temperature td at which pws(td) = pw. Wet bulb t*: solved from the ASHRAE energy balance below (with the ice-bulb form below 0 °C), where Ws* is the saturation humidity ratio at t*.

W = [(2501 − 2.326 t*) Ws* − 1.006 (t − t*)] ÷ (2501 + 1.86 t − 4.186 t*)

Pressure from altitude Z (m), standard atmosphere:

p = 101.325 (1 − 2.25577 × 10⁻⁵ Z)5.2559 kPa

The SI enthalpy is zero for dry air at 0 °C. The inch-pound enthalpy, h = 0.240 t + W (1061 + 0.444 t) Btu/lb with t in °F, is zero for dry air at 0 °F, so the two values are not a simple unit conversion of each other. Enthalpy differences do convert directly: 1 Btu/lb = 2.326 kJ/kg.

Worked example

  • Air at 30 °C and 50% RH at sea level (101.325 kPa): pws = 4.246 kPa, pw = 2.123 kPa, W = 0.621945 × 2.123 ÷ (101.325 − 2.123) = 13.3 g/kg; dew point 18.4 °C; wet bulb 22.0 °C; h = 1.006 × 30 + 0.01331 × (2501 + 1.86 × 30) = 64.2 kJ/kg; v = 0.877 m³/kg.
  • The same air at 1500 m altitude (84.6 kPa) has a higher humidity ratio, 16.0 g/kg, because the same vapour pressure is mixed with less dry air.

Assumptions and limitations

  • Moist air is treated as a mixture of ideal gases (the standard ASHRAE method). This is accurate for normal HVAC conditions; above about 90 °C, or at high pressure, results are approximate.
  • Valid ranges: dry bulb −100 °C to +200 °C; relative humidity above 0% up to 100%; pressure 20 kPa to 120 kPa; altitude −500 m to 11 000 m. Inputs outside these ranges are rejected.
  • At or below 0.01 °C, saturation is over ice. Relative humidity and dew point are then relative to ice; many weather reports use relative humidity over water.
  • The wet-bulb temperature is the thermodynamic (adiabatic saturation) wet bulb. It is not given when the dry bulb is above the boiling point of water at the given pressure. A sling or aspirated psychrometer reads very close to it; an unventilated wet bulb reads higher.
  • Altitude is converted to pressure with the standard atmosphere. Real barometric pressure changes with the weather by a few percent; enter it directly if you know it.

References

  • ASHRAE Handbook — Fundamentals, chapter “Psychrometrics” (Hyland–Wexler saturation pressure, humidity ratio, enthalpy, specific volume, wet-bulb equations, standard atmosphere).
  • Hyland, R. W. and Wexler, A. (1983), Formulations for the thermodynamic properties of the saturated phases of H₂O from 173.15 K to 473.15 K, ASHRAE Transactions 89(2A).