Psychrometric Properties Calculator
Computes moist-air properties — dew point, wet-bulb temperature, enthalpy, or humidity ratio — from dry-bulb temperature and relative humidity. Essential for HVAC design, weather analysis, and industrial drying processes.
About this calculator
Psychrometrics describes the thermodynamic properties of mixtures of dry air and water vapour. Saturation vapour pressure is estimated using the Magnus formula: psat = 0.61078 · exp(17.27 · T / (237.3 + T)) kPa. Humidity ratio is W = 0.622 · (φ · psat) / (p − φ · psat) kg/kg, where φ is relative humidity (0–1) and p is atmospheric pressure. Dew point is approximated as Td ≈ T − (100 − RH) / 5 °C for quick estimates. Specific enthalpy of moist air is h = 1.006·T + W·(2501 + 1.86·T) kJ/kg_dry. These relationships are used in psychrometric charts and allow engineers to determine how much moisture air holds, when condensation will occur, and how much energy is needed to condition air to desired comfort or process conditions.
How to use
At T = 25 °C, RH = 60%, p = 101.325 kPa: First compute psat = 0.61078 · exp(17.27 × 25 / (237.3 + 25)) = 0.61078 · exp(1.6453) = 0.61078 × 5.183 ≈ 3.166 kPa. Humidity ratio: W = 0.622 × (0.60 × 3.166) / (101.325 − 0.60 × 3.166) = 0.622 × 1.900 / 99.425 ≈ 0.01189 kg/kg. Enthalpy: h = 1.006 × 25 + 0.01189 × (2501 + 1.86 × 25) = 25.15 + 0.01189 × 2547.5 ≈ 25.15 + 30.24 ≈ 55.4 kJ/kg. Dew point ≈ 25 − (100 − 60)/5 = 17 °C.
Frequently asked questions
What is the difference between dew point and wet-bulb temperature in psychrometrics?
Dew point is the temperature at which air becomes saturated (RH reaches 100%) if cooled at constant pressure without adding moisture — it is purely a function of the amount of water vapour present. Wet-bulb temperature is measured by a thermometer whose bulb is covered in a wet wick; evaporative cooling lowers its reading, and it reflects both the moisture content and the capacity of the air to absorb more vapour. Dew point is always lower than or equal to wet-bulb temperature, which is in turn lower than or equal to dry-bulb temperature. HVAC engineers use wet-bulb temperature to size cooling coils and evaporative coolers.
How does atmospheric pressure affect psychrometric calculations?
Atmospheric pressure appears in the denominator of the humidity ratio formula — lower pressure (e.g., at high altitude) means air can hold relatively more water vapour for the same dry-bulb temperature and relative humidity. At Denver (altitude ~1600 m, p ≈ 84 kPa) versus sea level (101.3 kPa), the humidity ratio is about 20% higher for the same 25 °C / 60% RH conditions. This is why HVAC systems designed for sea level may dehumidify inadequately at high altitude, and why psychrometric charts are sometimes published for different standard elevations.
Why is specific enthalpy of moist air important for HVAC system design?
Specific enthalpy captures the total heat content of moist air — both the sensible heat (temperature) and the latent heat (moisture) components. Cooling or heating equipment must handle both, and latent loads can dominate in humid climates, sometimes accounting for 30–50% of total cooling load. Engineers use enthalpy differences between supply and return air states to calculate coil duties and verify that equipment capacity is sufficient. Without accounting for enthalpy, a system sized only on temperature difference will be undersized in humid conditions, leading to poor comfort and mould risk.