Psychrometric Calculator: Moist Air Properties

What is psychrometrics?

Psychrometrics deals with the properties of moist air, the mixture of dry air and water vapour. Air conditioning, ventilation, drying and dehumidification calculations all rest on these properties. The state of air is fixed by three things: pressure, dry-bulb temperature and one quantity that describes moisture. This calculator accepts moisture in four forms: relative humidity, wet-bulb temperature, dew point or humidity ratio.

Which relations are used?

First the saturation pressure of water p_ws is found (Hyland and Wexler correlation, ASHRAE Handbook of Fundamentals). Then in order:

• Vapour pressure: p_w = φ × p_ws
• Humidity ratio: W = 0.621945 × p_w / (p − p_w)
• Enthalpy: h = 1.006 × t + W × (2501 + 1.86 × t) kJ per kg of dry air
• Specific volume: v = 0.287042 × T × (1 + 1.607858 × W) / p

The dew point is the temperature whose saturation pressure equals p_w. Wet-bulb temperature is solved by iteration from the adiabatic saturation equation.

Example

At sea level, 25 °C and 50 percent relative humidity give a saturation pressure of 3169 Pa and a vapour pressure of 1585 Pa. The humidity ratio is 9.9 g/kg, enthalpy 50.3 kJ/kg, specific volume 0.858 m³/kg, dew point 13.9 °C and wet bulb 17.9 °C. The same air at 1000 m altitude holds 11.2 g/kg, because at lower pressure the same vapour pressure goes with less dry air.

Effect of altitude

A psychrometric chart is drawn for one pressure. Using a sea level chart for a high altitude site gives errors of 10 percent or more in humidity ratio and enthalpy. Enter the altitude to avoid that.

Limits

The calculation assumes ideal gas behaviour and omits the enhancement factor, which is the usual accuracy in air conditioning engineering. It is not meant for very high pressure, temperatures above 90 °C or precision measurement work.