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HVAC ENGINEERING TOOL
Psychrometric Calculator
Calculate moist-air properties from dry bulb, humidity and atmospheric pressure.
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Engineering Method & Assumptions
- ASHRAE 2025 saturation-pressure basis with perfect-gas psychrometric relationships suitable for normal HVAC calculations; real-moist-air RP-1485 properties are outside scope.
- Water-vapor saturation pressure uses IAPWS R14-08 over ice below the 0.01 °C triple point and IAPWS-IF97 over liquid water at and above it.
- Humidity ratio, enthalpy and specific volume are per unit mass of dry air. Moist-air density includes both dry-air and water-vapor mass.
- Enthalpy uses the conventional 0 °C reference and the approximation h = 1.006 Tdb + W(2501 + 1.86 Tdb).
- Dew point and wet bulb use bounded bisection with 0.0001 °C tolerance and at most 100 iterations.
- Pressure affects all calculated properties. Altitude gives estimated standard-atmosphere pressure, not measured station pressure.
- Phase 2A dry-bulb range is −50 to 100 °C, but its complete-state upper limit is pressure-dependent; high-temperature industrial drying requires separate validation.
- This higher-fidelity engine is separate from the simpler, liquid-water Magnus equation retained in the Phase 1 Dew Point Calculator.
WHEN TO USE THIS TOOL
Useful during psychrometrics & coils work.
- Evaluate moist-air conditions, dew point, condensation risk and cooling/dehumidification processes.
- Check how temperature, humidity and pressure assumptions affect the resulting air state.
- Support coil, cleanroom, comfort and humidity-control discussions with traceable calculations.
Frequently asked questions
Why does atmospheric pressure matter?
Humidity ratio, wet bulb, specific volume and density depend on total pressure. Use measured station pressure when it is available.
Why is dew point undefined at 0% RH?
Ideal dry air has zero water-vapor partial pressure. No finite temperature reaches saturation at exactly zero vapor pressure.
Is enthalpy per kilogram of moist air?
No. The displayed enthalpy and specific volume use a dry-air mass basis, as their labels state. Moist-air density is calculated separately on a moist-air mass basis.
Is this the same calculation as the Phase 1 dew-point tool?
No. This engine uses current IAPWS water/ice saturation-pressure equations and pressure-dependent moist-air relationships; the focused Phase 1 tool retains its documented Magnus approximation.