Psychrometric Calculator
Calculate the thermodynamic properties of moist air, humidity ratio, relative humidity, dew point, enthalpy, specific volume, and vapor pressure. Based on ASHRAE Handbook equations. Essential for HVAC design, building science, and environmental engineering.
📊 Psychrometric Property Reference
| Property | Symbol | Description | SI Units |
|---|---|---|---|
| Dry-Bulb Temperature | DBT | Air temperature measured with a dry thermometer | °C |
| Wet-Bulb Temperature | WBT | Temperature measured with a wet wick — indicates evaporative cooling | °C |
| Humidity Ratio | W | Mass of water vapor per mass of dry air | kgv/kgda |
| Relative Humidity | RH | Ratio of actual vapor pressure to saturation vapor pressure | % |
| Dew Point | DP | Temperature at which air becomes saturated | °C |
| Enthalpy | h | Total energy content of moist air per kg of dry air | kJ/kgda |
| Specific Volume | v | Volume of moist air per kg of dry air | m³/kgda |
| Vapor Pressure | Pw | Partial pressure of water vapor in the air | kPa |
Understanding Psychrometrics
Psychrometrics is the study of the thermodynamic properties of moist air. It is fundamental to HVAC design, building science, and environmental engineering. The properties of moist air — temperature, humidity, enthalpy, and volume — are used to design heating, cooling, ventilation, and air conditioning systems.
This calculator uses the ASHRAE Handbook equations to compute key psychrometric properties. The calculations are based on the Eulerian formulation for moist air and the IAPWS formulation for the saturation pressure of water.
Key Relationships:
Humidity Ratio: W = 0.62198 × (Pw / (P − Pw))
Relative Humidity: RH = (Pw / Pws) × 100
Enthalpy: h = 1.006 × T + W × (2501 + 1.86 × T) kJ/kgda
Specific Volume: v = 0.287042 × (T + 273.15) × (1 + 1.607858 × W) / P
Dew Point: Temperature at which Pws(Tdp) = Pw
The calculator determines the humidity ratio from either wet-bulb temperature (using the psychrometric equation) or relative humidity. All other properties are then computed from the fundamental relationships above.
How the Psychrometric Calculator Works
The calculator follows these steps:
- Step 1: Compute the saturation vapor pressure at the dry-bulb temperature using the IAPWS formulation.
- Step 2: Determine the actual vapor pressure — either from relative humidity (Pw = RH × Pws) or from wet-bulb temperature using the psychrometric equation.
- Step 3: Calculate the humidity ratio from vapor pressure and total pressure.
- Step 4: Compute enthalpy, specific volume, and dew point temperature.
If both wet-bulb and relative humidity are provided, the calculator uses wet-bulb for the primary calculation and shows the relative humidity from wet-bulb as a check.
❓ Psychrometric Calculator FAQ
What is psychrometrics?
Psychrometrics is the study of the thermodynamic properties of moist air. It is used in HVAC design, building science, and environmental engineering to analyze air properties like temperature, humidity, and energy content.
What is the humidity ratio?
The humidity ratio (also called specific humidity or moisture content) is the mass of water vapor per unit mass of dry air. It is expressed in kgv/kgda or grains/lbda.
What is the difference between relative humidity and humidity ratio?
Relative humidity is the ratio of actual vapor pressure to saturation vapor pressure at a given temperature, expressed as a percentage. Humidity ratio is the actual mass of water vapor per mass of dry air. Relative humidity depends on temperature, while humidity ratio is a direct measure of moisture content.
What is the dew point temperature?
The dew point temperature is the temperature at which air becomes saturated with water vapor. If the air cools below the dew point, condensation occurs. It is a direct measure of the absolute moisture content of the air.
What is enthalpy in psychrometrics?
Enthalpy is the total energy content of moist air per unit mass of dry air. It includes both the sensible heat of the air and the latent heat of the water vapor. The formula used is: h = 1.006 × T + W × (2501 + 1.86 × T) kJ/kgda.
What is specific volume in psychrometrics?
Specific volume is the volume occupied by moist air per unit mass of dry air. It is used to size air handling equipment and ducts. The formula is: v = 0.287042 × (T + 273.15) × (1 + 1.607858 × W) / P.
What is the saturation vapor pressure?
The saturation vapor pressure is the maximum partial pressure of water vapor that can exist in air at a given temperature. Above this pressure, condensation occurs. This calculator uses the IAPWS formulation for accuracy.
How does the calculator determine humidity ratio from wet-bulb temperature?
The calculator solves the psychrometric equation: Pw = Pws(Twb) − A × P × (T − Twb), where A is the psychrometric constant. This relates wet-bulb temperature to vapor pressure.
What is the psychrometric constant?
The psychrometric constant (approximately 0.00066 kPa/°C at sea level) relates the wet-bulb depression to the vapor pressure. It depends on atmospheric pressure and the properties of water and air.
What is the difference between dry-bulb and wet-bulb temperature?
Dry-bulb temperature is the actual air temperature measured with a dry thermometer. Wet-bulb temperature is the temperature measured with a thermometer that has a wet wick — the evaporative cooling lowers the temperature. The difference between them (the wet-bulb depression) indicates the humidity of the air.
How does atmospheric pressure affect psychrometric properties?
Atmospheric pressure affects the humidity ratio, specific volume, and vapor pressure calculations. At higher altitudes (lower pressure), the humidity ratio for a given vapor pressure is higher. The calculator allows you to enter the local atmospheric pressure.
What is the IAPWS formulation?
The IAPWS (International Association for the Properties of Water and Steam) formulation is the most accurate method for calculating the saturation pressure of water. It is used as the basis for this calculator's saturation vapor pressure calculations.
What are the units used in the calculator?
You can choose between SI (metric) units and IP (imperial) units. SI uses °C, kPa, kJ/kg, and m³/kg. IP uses °F, psi, Btu/lb, and ft³/lb. The calculator converts automatically based on your selection.
How accurate is this psychrometric calculator?
This calculator uses the ASHRAE Handbook equations and the IAPWS formulation, which are the industry standards for psychrometric calculations. It provides accurate results for typical HVAC temperature ranges (-50°C to 150°C).
What is a psychrometric chart?
A psychrometric chart is a graphical representation of the thermodynamic properties of moist air. It shows the relationships between dry-bulb temperature, humidity ratio, relative humidity, enthalpy, and specific volume. This calculator can display your state point on a psychrometric chart.
How do I use this calculator for HVAC design?
Enter the dry-bulb temperature and either the wet-bulb temperature or relative humidity of the air. The calculator provides all key psychrometric properties needed for load calculations, coil sizing, and air handling unit design.
What is the difference between sensible heat and latent heat?
Sensible heat is the heat that changes the temperature of the air without changing its moisture content. Latent heat is the heat required to change the phase of water (evaporation or condensation). Enthalpy includes both components.
Can I use this calculator for cooling tower design?
Yes, psychrometric properties are essential for cooling tower design. The wet-bulb temperature is particularly important as it determines the cooling tower's approach temperature and performance.
What is the difference between absolute humidity and relative humidity?
Absolute humidity (or humidity ratio) is the actual mass of water vapor per mass of dry air. Relative humidity is the ratio of actual vapor pressure to saturation vapor pressure, expressed as a percentage. Relative humidity depends on temperature; absolute humidity does not.