Psychrometric Calculator
Calculate psychrometric properties of moist air including humidity ratio, dew point, enthalpy, and specific volume from dry-bulb and wet-bulb temperatures.
What Is a Psychrometric Calculator?
A psychrometric calculator estimates properties of moist air from dry-bulb temperature, wet-bulb temperature, and atmospheric pressure. HVAC engineers, meteorologists, and process designers use these values to size equipment, analyze comfort conditions, and balance air streams.
Key Psychrometric Properties
Dry-bulb temperature is ordinary ambient air temperature. Wet-bulb temperature is read with a thermometer wrapped in a wet wick and reflects evaporative cooling. From these two readings and barometric pressure, you can find humidity ratio, dew point, relative humidity, specific volume, and enthalpy.
Partial vapor pressure follows the psychrometric relation:
$$P_v = P_w - 0.00066\,P_a\,(T_{db} - T_{wb})\,(1 + 0.0115\,T_{wb})$$Humidity ratio (mass of water vapor per mass of dry air) is:
$$\omega = 0.6219\,\frac{P_v}{P_a - P_v}$$Enthalpy of moist air in kJ/kg dry air is:
$$h = 1.006\,T_{db} + \omega\,(2499.86 + 1.84\,T_{db})$$Related tools: Mixed Air Calculator, Evaporation Rate Calculator, and Mixed Air Temperature Calculator.
Frequently Asked Questions
What is the difference between dry-bulb and wet-bulb temperature?
Dry-bulb is standard air temperature. Wet-bulb is lower when air is dry because evaporation cools the wick. A large difference means low humidity.
What is humidity ratio?
Humidity ratio is the mass of water vapor divided by the mass of dry air, usually in kg/kg. It is also called specific humidity in many psychrometric charts.
How is dew point calculated?
Dew point is found from partial vapor pressure using the inverse Magnus relation. It is the temperature where condensation begins at constant pressure.
Why does wet-bulb temperature matter in HVAC?
Wet-bulb indicates how much evaporative cooling is possible. Cooling towers, humidifiers, and comfort analysis all rely on wet-bulb readings.
Can wet-bulb exceed dry-bulb?
No. Wet-bulb equals dry-bulb only at saturation (100% relative humidity). Otherwise wet-bulb is always lower than dry-bulb.
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