The following Water Vapor Saturation Data is required in order to plot the Saturation Curve (Relative Humidity =100%) of the Psychrometric Chart.
The basic information used to construct a psychrometric chart is the water vapor saturation data (Tsat, Pg) which is obtained from steam tables over the range from Tsat = 0.01°C through 50°C. The specific humidity is then evaluated using the relative humidity as a parameter to produce the various relative humidity curves (blue lines) as follows:
The saturation curve (100% relative humidity) is also known as the dew point curve is drawn as a red line. Notice that on the saturation curve the wet and dry bulb temperatures have the same values.
For more information refer to Chapter 10b on the Psychrometric Chart
Source of the (Tsat, Pg) data: NIST Chemistry WebBook - accessed Feb 2008
The major simplifying assumption in the construction of the chart is that the enthalpy of the mixture is assumed to be constant throughout the adiabatic saturation process (described in Chapter 10a). This implies that the evaporating liquid added does not significantly affect the enthalpy of the air-vapor mixture, leading to the constant slope wet bulb temperature / enthalpy lines defined by:
Note that this simplyfied linear equation results in an error of less than 1% over the temperature range 0.01°C to 50°C.
Finally the specific volume curves which complete the chart are given by:
Refer to the complete MATLAB program for Plotting the Psychrometric Chart using these equations.
Engineering Thermodynamics by Israel Urieli is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License