Psychrometrics

Psychrometrics or psychrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures.

The principles of psychrometry apply to any physical system consisting of gas-vapor mixtures. The most common system of interest, however, are mixtures of water vapor and air because of its application in heating, ventilating, and air-conditioning and meteorology.

Mixtures of air and water vapor are the most common systems encountered in psychrometry. The psychrometric ratio of air-water vapor mixtures is approximately unity which implies that the difference between the adibatic saturation temperature and wet bulb temperature of air-water vapor mixtures is small. This property of air-water vapor systems simplifies drying and cooling calculations often performed using psychrometic relationships.

Psychrometric chart

A psychrometric chart is a graph of the physical properties of moist air at a constant pressure or often equated to an elevation relative to sea-level. The chart graphically expresses how various properties relate to each other, and is thus a graphical 'equation of state'.

Willis Carrier, considered the 'father' of modern air-conditioning, rearranged the Mollier diagram for moist air (its T-s chart) to allow these graphical solutions. Many variations and improvements to the psychrometric charts have occurred since, and most charts do not show the specific entropy (s) like the Mollier diagram. ASHRAE now publishes what are considered the modern, standard psychrometric charts, in both I-P and SI units, for a variety of elevations or air pressures.