A psychrometric chart is a graphical tool used across thermodynamics, meteorology, and Heating, Ventilation, and Air Conditioning (HVAC) to model the physical properties of moist air. Since air temperature and moisture content profoundly affect each other, a simple measurement of one property is insufficient to fully characterize the air’s state. The chart simplifies analysis by presenting multiple thermodynamic properties simultaneously, allowing users to plot an air sample and determine all its characteristics from just two known values.
Understanding the Core Axes and Boundaries
The structure of the psychrometric chart is defined by two primary, orthogonal properties: Dry-Bulb Temperature and Humidity Ratio. The horizontal axis represents the Dry-Bulb Temperature (DBT), which is the standard air temperature measured by a typical thermometer. Lines of constant DBT are vertical lines, moving from cooler temperatures on the left to warmer temperatures on the right. The vertical axis, usually located on the right side, represents the Humidity Ratio (W), sometimes called specific humidity. This value quantifies the mass of water vapor present per unit mass of dry air, indicating the absolute moisture content of the air sample.
Any single point on the chart, known as the “state point,” is uniquely defined by the intersection of a specific DBT and a specific Humidity Ratio. The upper-left boundary of the chart is a distinct curved line known as the Saturation Curve. This curve represents air that is 100% saturated with water vapor. At any point on this curve, the Dry-Bulb Temperature, Wet-Bulb Temperature, and Dew Point Temperature are all equal.
Interpreting the Curved and Sloping Property Lines
Beyond the core axes, the chart features non-orthogonal lines that represent the secondary properties of the air mixture. One family of lines is the Relative Humidity (RH) curves, which arc upward and to the right, terminating at the 100% saturation curve. Relative humidity expresses the percentage of moisture the air holds relative to the maximum amount it could hold at that same temperature.
The Wet-Bulb Temperature (WBT) is represented by a set of steeply sloping lines that run parallel to each other, downward from the saturation curve to the right. The WBT is measured by a thermometer covered in a water-soaked cloth and is the lowest temperature air can reach through evaporative cooling. These lines are conceptually linked to Enthalpy, which is the total heat energy contained within the air (sensible and latent heat).
Enthalpy is often read along a scale adjacent to the WBT lines because the lines of constant Wet-Bulb Temperature and constant Enthalpy are nearly identical. The chart also includes lines of Specific Volume, which are moderately sloping lines that run from the upper-left to the lower-right. Specific Volume defines the volume occupied by a unit mass of dry air, a property inversely related to air density.
Step-by-Step Guide to Plotting Air States
To determine the full state of an air sample, one must first identify two independent properties, such as the Dry-Bulb Temperature and the Relative Humidity. The process begins by locating the measured DBT on the bottom horizontal axis and drawing a vertical line straight up from that point. Next, the measured Relative Humidity percentage must be found by tracing the corresponding curved line across the chart.
The intersection of the vertical Dry-Bulb line and the curved Relative Humidity line establishes the air’s state point. Once this point is plotted, all other properties can be read by following the appropriate property lines outwards to their respective scales. For example, to find the Wet-Bulb Temperature, one follows the diagonal WBT line that passes through the state point up to the saturation curve or its dedicated scale.
To determine the Dew Point Temperature, one follows a horizontal line directly from the state point to the left until it intersects the saturation curve. The corresponding temperature value on the saturation curve is the Dew Point, which is the temperature at which condensation begins. Reading the humidity ratio involves following the horizontal line from the state point to the right to intersect the vertical Humidity Ratio scale.
Visualizing Common Air Conditioning Processes
The psychrometric chart is invaluable for visualizing how air properties change during common air conditioning and ventilation processes, which are represented by movements between two state points. Sensible Heating is represented by a horizontal movement to the right, as the Dry-Bulb Temperature increases while the Humidity Ratio remains constant. Conversely, Sensible Cooling is a horizontal movement to the left, indicating a decrease in temperature without a change in moisture content.
Processes that change the air’s moisture level are represented by vertical movement. Humidification, such as adding steam to the air, is shown by a vertical line moving upwards, signifying an increase in the Humidity Ratio at a constant Dry-Bulb Temperature. Dehumidification, where moisture is removed without changing the temperature, is represented by a vertical line moving downwards.
Most real-world air conditioning processes involve both heating/cooling and humidification/dehumidification, resulting in diagonal or multi-segment lines. For instance, the common Cooling and Dehumidification process, which occurs across a standard cooling coil, is represented by a line sloping down and to the left. This movement shows that both the air temperature and the moisture content are decreasing simultaneously, moving the air toward a cooler, drier state.