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 term derives from the Greek psuchron (ψυχρόν) (cold) and metron (μέτρον) (means of measurement).
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.
The psychrometric ratio is the ratio of the heat transfer coefficient to the product of mass transfer coefficient and humid heat at a wetted surface. It may be evaluated with the following equation^{[1]}^{[2]}:
Humid heat is the constant-pressure specific heat of moist air, per unit mass of dry air.^{[3]}
The psychrometric ratio is an important property in the area of psychrometrics as it relates the absolute humidity and saturation humidity to the difference between the dry bulb temperature and the adiabatic saturation temperature.
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 adiabatic 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.
A psychrometric chart is a graph of the physical properties of moist air at a constant pressure (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. The thermophysical properties found on most psychrometric charts are:
The versatility of the psychrometric chart lies in the fact that by knowing three independent properties of some moist air (one of which is the pressure), the other properties can be determined. Changes in state, such as when two air streams mix, can be modeled easily and somewhat graphically using the correct psychrometric chart for the location's air pressure or elevation relative to sea level. For locations at or below 2000 ft (600 m), a common assumption is to use the sea level psychrometric chart.
The relationship between DBT, WBT, and RH is given by the Mollier diagram (pressure-enthalpy) for water in air, developed by Richard Mollier. Willis Carrier, considered the 'father' of modern air-conditioning, rearranged the Mollier diagram for moist air (its T-s chart) to allow such 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.
The most common chart used by practitioners and students alike is the "ω-t" (omega-t) chart in which the dry bulb temperature (DBT) appears horizontally as the abscissa and the humidity ratios (ω) appear as the ordinates.
In order to use a particular chart, for a given air pressure or elevation, at least two of the six independent properties must be known (DBT, WBT, RH, humidity ratio, specific enthalpy, and specific volume). This gives rise to possible combinations.
DBT: This can be determined from the abscissa on the x-axis, the horizontal axis
DPT: Follow the horizontal line from the point where the line from the horizontal axis arrives at 100% RH, also known as the saturation curve.
WBT: Line inclined to the horizontal and intersects saturation curve at DBT point.
RH: Hyperbolic lines drawn asymptotically with respect to the saturation curve which corresponds to 100% RH.
Humidity ratio: Marked on the y-axis.
Specific enthalpy: lines of equal values, or hash marks for, slope from the upper left to the lower right.
Specific volume: Equally spaced parallel family of lines.
Common thermometers measure what is known as the dry-bulb temperature. Electronic temperature measurement, via thermocouples, thermistors, and resistance temperature devices (RTDs), for example, have been widely used too since they became available.
The thermodynamic wet-bulb temperature is a thermodynamic property of a mixture of air and water vapor. The value indicated by a simple wet-bulb thermometer often provides an adequate approximation of the thermodynamic wet-bulb temperature.
A wet-bulb thermometer is an instrument which may be used to infer the amount of moisture in the air. If a moist cloth wick is placed over a thermometer bulb the evaporation of moisture from the wick will lower the thermometer reading (temperature). If the air surrounding a wet-bulb thermometer is dry, evaporation from the moist wick will be more rapid than if the air is moist. When the air is saturated no water will evaporate from the wick and the temperature of the wet-bulb thermometer will be the same as the reading on the dry-bulb thermometer. However, if the air is not saturated water will evaporate from the wick causing the temperature reading to be lower.
The accuracy of a simple wet-bulb thermometer depends on how fast air passes over the bulb and how well the thermometer is shielded from the radiant temperature of its surroundings. Speeds up to 5,000 ft/min (60 mph) are best but dangerous to move a thermometer at that speed. Errors up to 15% can occur if the air movement is too slow or if there is too much radiant heat present (sunlight, for example).
A wet bulb temperature taken with air moving at about 1-2 m/s is referred to as a screen temperature, whereas a temperature taken with air moving about 3.5 m/s or more is referred to as sling temperature.
A psychrometer is a device that includes both a dry-bulb and a wet-bulb thermometer. A sling psychrometer requires manual operation to create the airflow over the bulbs, but a powered psychrometer includes a fan for this function.