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All psychrometric properties from two inputs
Moist air at a known atmospheric pressure is a two-property thermodynamic system. Any two independent properties fully define the state, and all others follow from the psychrometric equations. This calculator accepts six common second-property choices alongside dry-bulb temperature: relative humidity, wet-bulb temperature, dew point temperature, humidity ratio, specific enthalpy, or specific volume. It then computes the complete state point using ASHRAE Fundamentals equations, built on the same water thermophysical reference data NIST maintains, and presents every psychrometric quantity in both metric and imperial units simultaneously.
The core calculation path starts with resolving the humidity ratio W (kg/kg of dry air) from whatever two properties you've entered. Once W is known, all other properties follow directly: vapor pressure Pw = W × P / (0.62198 + W), saturation pressure Psat = ASHRAE polynomial in TK, relative humidity RH = Pw / Psat, enthalpy h = 1.006 × T + W × (2501 + 1.86 × T), specific volume v = 0.28705 × TK × (1 + 1.6078 × W) / P, and air density ρ = (1 + W) / v. Wet-bulb and dew point are resolved by bisection on the saturation and psychrometer equations. Every property shown includes both metric and imperial values for direct use in equipment specifications and load calculations.
Using the full state point for HVAC coil and AHU sizing
A complete state point from this calculator feeds directly into the coil analysis tools. For cooling coil sizing, run this calculator for both the design entering condition (ASHRAE outdoor design Tdb and Twb, available from the design temperature lookup) and the target leaving condition (supply air Tdb and RH), then take those two enthalpy values into the coil load calculator with the design airflow to get total, sensible, and latent coil capacity. For ADP and bypass factor analysis, take the W values from both state points to the apparatus dew point calculator. For air handling unit mixing calculations, run this calculator once for outdoor air and once for return air, then use the mixing air calculator with those two complete state points and the mixing ratio.
Altitude correction for Canadian cities above 500 m
The City quick-fill dropdown sets the site elevation automatically for Calgary (1045 m), Edmonton (671 m), Kelowna (344 m), Red Deer (549 m), and other higher Canadian cities. At Calgary's 89.4 kPa atmospheric pressure, the same Tdb and RH produces a 13 to 14% higher specific volume than at sea level, meaning fans deliver less mass flow per unit of volumetric flow; cross-check the resulting duct velocities with the duct velocity pressure calculator. The density correction factor visible in the output tells you directly how much to derate fan and coil performance from sea-level catalog data. All psychrometric properties, including enthalpy, humidity ratio, dew point, and wet-bulb, stay correct at the entered elevation pressure without any manual correction needed. Use the specific volume calculator to explore altitude effects across a range of conditions.
Frequently Asked Questions
Psychrometric properties describe the thermodynamic state of moist air. Dry-bulb temperature is the air temperature measured by a standard thermometer. Wet-bulb is the temperature reached by adiabatic evaporation and is always at or below dry-bulb. Dew point is the temperature at which moisture begins to condense. Relative humidity is the ratio of actual to saturation vapor pressure, as a percentage. Humidity ratio (W) is mass of water vapor per unit mass of dry air in g/kg or gr/lb. Enthalpy is total heat content per unit mass of dry air in kJ/kg or BTU/lb. Specific volume is volume per unit mass of dry air in m³/kg or ft³/lb. Air density is the inverse relationship to specific volume. Vapor pressure is the partial pressure of water vapor. Any two independent properties fully define the state at known pressure. Use the psychrometric chart calculator to visualize all these properties on the ASHRAE chart.
Run this calculator for your design entering air condition (use the ASHRAE design Tdb and Twb from the ASHRAE 169 climatic data for your city) to get all entering state point properties. Then run it again for your target leaving condition (Tdb and RH setpoint). Take both enthalpy values and the airflow to the coil load calculator for total, sensible, and latent capacity. Take both W values to the apparatus dew point calculator for the ADP and bypass factor needed for coil row count selection. For higher Canadian cities like Calgary or Edmonton, enter the site elevation here first, since the specific volume correction ensures your mass flow and coil load calculations are accurate for the lower air density.