in psychrometrics, the term quantifies the total volume of both the dry air and water vapour per unit mass of dry air. Psychrometric ratio 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: where: = Psychrometric ratio, dimensionless = convective heat transfer coefficient, W m−2 K−1 = convective mass transfer coefficient, kg m−2 s−1 = humid heat, J kg−1 K−1 The psychrometric ratio is an important property in the area of psychrometry, 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 psychrometric relationships. Humid heat Humid heat is the constant-pressure specific heat of moist air, per unit mass of the dry air. The Humid Heat is the amount of Heat required to change the temperature of unit mass of a Water Vapor - Air Mixture by 1 °C. Pressure Many psychrometric properties are dependent on pressure concept: vapor pressure of water; atmospheric pressure at the location of the sample. Psychrometric charts Terminology A psychrometric chart is a graph of the thermodynamic parameters of moist air at a constant pressure, often equated to an elevation relative to sea level. The ASHRAE-style psychrometric chart, shown here, was pioneered by Willis Carrier in 1904. It depicts these parameters and is thus a graphical equation of state. The parameters are: Dry-bulb temperature (DBT) is that of an air sample, as determined by an ordinary thermometer. It is typically plotted as the abscissa (horizontal axis) of the graph. The SI units for temperature are kelvins or degrees Celsius; other units are degrees Fahrenheit and degrees Rankine. Wet-bulb temperature (WBT) is that of an air sample after it has passed through a constant-pressure, ideal, adiabatic saturation process, that is, after the air has passed over a large surface of liquid water in an insulated channel. In practice this is the reading of a thermometer whose sensing bulb is covered with a