Psychrometric Calculator
Units
I also know the
Sets the air pressure: 101.33 kPa
Moist air properties
- Relative humidity
- 50.0%
- Dew point
- 18.4 °C
- Wet-bulb temperature
- 22.1 °C
- Humidity ratio
- 13.28 g/kg
- Enthalpy
- 64.13 kJ/kg
- Specific volume
- 0.8771 m³/kg
- Vapour pressure
- 2118 Pa
- Absolute humidity
- 15.14 g/m³
Per kilogram (or pound) of dry air. Saturation pressure from the Magnus formula (within about 0.4% from −40 to 50 °C); wet bulb from the ventilated-psychrometer equation; enthalpy and volume from the ASHRAE ideal-gas relations.
Calculate the properties of moist air from the dry-bulb temperature and one more measurement (relative humidity, wet-bulb temperature or dew point): dew point, wet bulb, humidity ratio, enthalpy, specific volume, vapor pressure and absolute humidity, at any elevation, in metric or US units.
Formula
Worked Example
Saturation pressure at 30 °C is 4.24 kPa, so 50% RH means a vapor pressure of 2.12 kPa, which saturates at 18.4 °C, the dew point. Each kilogram of dry air carries 13.28 g of water vapor.
Frequently Asked Questions
What is the difference between dew point and wet-bulb temperature?
The dew point is the temperature at which the air's moisture would start to condense if it were cooled. The wet-bulb temperature is what a thermometer wrapped in wet cloth reads in moving air, cooled by evaporation. Both equal the air temperature only at 100% humidity.
Why does relative humidity change during the day when the moisture doesn't?
Relative humidity compares the water vapor present with the most the air could hold at its temperature. As the air warms in the afternoon its capacity rises and RH falls, while the dew point and humidity ratio, which measure the actual moisture, stay about the same.
What is enthalpy used for in HVAC?
It is the total heat content of the air, sensible plus latent, per kilogram of dry air. Multiplying the enthalpy difference across a cooling coil by the mass airflow gives the coil's total cooling load, including the moisture it removes.