Graph |link| — Psychrometric
These are sweeping parabolic curves. Each curve represents a percentage of saturation (0% to 100%). The 100% RH line is the Saturation Line . Air cannot hold more moisture beyond this line; it will start to condense (dew, fog, rain).
These are straight horizontal lines measuring the actual mass of water vapor per mass of dry air (grains/lb or g/kg). This tells you how much moisture is in the air. psychrometric graph
The term comes from the Greek psukhros (cold) and metron (measure). While it measures "cold," its real power lies in measuring change. To read the chart, you must identify its seven fundamental components. Most charts are plotted with Dry Bulb Temperature (the temperature your thermometer reads) on the horizontal axis. These are sweeping parabolic curves
Often described as a "graph of the physics of air," the psychrometric chart might look like a chaotic spiderweb of lines at first glance. However, once you learn its language, it becomes an indispensable map for controlling comfort, preserving food, drying paint, or even keeping a data center cool. Psychrometry is the study of the thermodynamic properties of gas-vapor mixtures—specifically, air and water vapor. The chart doesn't just plot temperature; it plots the relationship between heat, moisture, and energy. Air cannot hold more moisture beyond this line;
These are straight vertical lines (or horizontal, depending on the chart's orientation). They represent the sensible heat level of the air—the heat you can feel.
These are slightly sloped diagonal lines indicating the volume occupied by a unit mass of dry air (cubic feet per pound). This is critical for sizing fans and ducts.
Once you learn to see the air through these lines, you will never look at a cloudy window or a sweat glass of iced tea the same way again. You are seeing psychrometrics in action. Standard psychrometric charts are published by ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) and are available for various altitudes.
