Relative Humidity Calculator | Psychrometric Properties
Relative Humidity Calculator
Process Conditions
Actions
Psychrometric Results
Relative Humidity (RH):
- %
Dew-Point Temperature (Tββ):
- Β°C
Vapor Pressure (Pα΅₯):
- kPa
Humidity Ratio (W):
- kg/kg
Saturation Pressure (Pββ):
- kPa
Saturation Humidity Ratio (Wβ):
- kg/kg
Degree of Saturation (ΞΌ = W/Wβ):
-
Moist Air Enthalpy (h):
- kJ/kgβdry airβ
Specific Volume (v):
- mΒ³/kgβdry airβ
Moist Air Density (Ο):
- kg/mΒ³
Absolute Humidity (Οα΅₯):
- g/mΒ³
Grains of Moisture:
- grains/lbβdry airβ
Wet-Bulb Depression (Tββ β Tββ):
- Β°C
Technical Notes
Variable Definitions
- Patm: Atmospheric pressure (kPa, absolute).
- Tdb: Dry-bulb temperature (Β°C).
- Twb: Wet-bulb temperature (Β°C).
- Pws(T): Saturation vapor pressure at temperature T (kPa).
- Pv: Water vapor partial pressure (kPa).
- RH: Relative humidity = 100Β·Pv/Pws(Tdb) (%).
- W: Humidity ratio (kg water/kg dry air).
- Ws: Saturation humidity ratio at Tdb.
- ΞΌ: Degree of saturation = W/Ws.
- h: Moist-air enthalpy (kJ/kg dry air).
- v: Specific volume (mΒ³/kg dry air).
- Ο: Moist-air density (kg/mΒ³).
Formulas / Logic
- Saturation vapor pressure: Buck (1981) correlation (over water for T β₯ 0Β°C; over ice for T < 0Β°C).
- Psychrometric relation (ventilated wet-bulb): Pv = Pws(Twb) β AΒ·PatmΒ·(Tdb β Twb)
- Psychrometric coefficient: A = 0.00066Β·(1 + 0.00115Β·Twb) (1/Β°C).
- Humidity ratio: W = 0.621945Β·Pv/(Patm β Pv).
- Enthalpy (ASHRAE form): h = 1.006Β·Tdb + WΒ·(2501 + 1.86Β·Tdb).
- Specific volume: v = RdaΒ·TKΒ·(1 + 1.607858Β·W)/Patm, with Rda = 0.287042 kPaΒ·mΒ³/(kgΒ·K).
- Dew point: solved by numerical inversion of Pws(T) to match calculated Pv.
Assumptions / Notes
- Wet-bulb relation assumes a well-ventilated (aspirated) psychrometer and steady evaporation.
- Inputs are treated in SI units (kPa, Β°C). Ensure Twb β€ Tdb for typical psychrometric states.
- Ideal-gas approximations are used for v, Ο and absolute humidity; accuracy is suitable for HVAC/engineering screening.
- For high altitude or unusual pressure conditions, enter the correct local Patm for best accuracy.
Standards / References
- ASHRAE psychrometric relationships (enthalpy, humidity ratio, specific volume forms used in HVAC practice).
- Buck (1981) saturation vapor pressure correlation (widely used meteorological/psychrometric formulation).
- For critical design, validate against an ASHRAE psychrometric chart or project-approved psychrometric library.