Free Interactive Psychrometric Chart

Tutorials & how-to videos

Step-by-step tutorials and how-to videos for the psychrometric chart are coming soon.

A free, interactive psychrometric chart for analyzing moist air. Plot state points, read every air property at a glance, draw HVAC process lines, mix air streams, overlay ASHRAE comfort zones, and export to SVG or PNG.

What is a psychrometric chart?

A psychrometric chart is a graphical map of the thermodynamic properties of moist air. It plots dry-bulb temperature along the horizontal axis against humidity ratio (mass of water vapor per unit mass of dry air) on the vertical axis. Overlaid on that grid are curves for relative humidity, wet-bulb temperature, enthalpy, and specific volume. Because every moist-air state is fixed by any two independent properties, a single point on the chart lets you read all the others — which makes it the everyday tool for HVAC load, coil, and ventilation analysis.

The saturation curve and the properties you can read

The curved left boundary is the saturation curve (100% relative humidity); air cannot hold more moisture beyond it. From any state point you can read:

• Dry-bulb temperature — the vertical gridlines.
• Humidity ratio — the horizontal gridlines.
• Relative humidity — the curves sweeping up from left to right.
• Wet-bulb temperature and enthalpy — the diagonal lines descending to the right.
• Dew point — follow the humidity ratio left to the saturation curve.
• Specific volume — the steeper diagonal lines.

Plotting HVAC processes

Most HVAC processes are simple paths across the chart:

• Sensible heating / cooling — a horizontal line; only the temperature changes.
• Humidification / dehumidification — a vertical move; the moisture content changes.
• Cooling and dehumidification — down and to the left toward the coil's apparatus dew point.
• Evaporative (adiabatic) cooling — along a constant wet-bulb line.
• Mixing two air streams — the result lands on the straight line between them, weighted by mass flow.

The slope of a coil process line is set by the sensible heat ratio (SHR) — the fraction of total load that is sensible — so SHR lines let you size cooling coils directly on the chart.

What this tool does

• ASHRAE comfort zones overlaid for summer and winter.
• Process lines and air mixing drawn directly on the chart.
• Sensible-heat-ratio (SHR) lines for coil sizing.
• Adjustable pressure / altitude for high-elevation work.
• SI and IP units with a single toggle.
• Export to SVG or PNG for reports and presentations.

Using the chart controls

The sidebar lets you build an analysis directly on the chart:

• Conditions — a named group that sets the color of the state points and process lines you plot. Create several conditions (for example "Design Day" and "Part Load") and switch between them to overlay and compare multiple operating scenarios on the same chart.
• Airflow — enter the volumetric air flow for a process. Combined with the temperature and humidity change across the process, it gives the sensible, latent, and total capacity of that step (and the condensate rate when moisture is removed).
• Sensible coil (with condensate / frost) — model a coil by its surface temperature. While the coil surface stays above the air's dew point the process is purely sensible — a horizontal line at constant humidity, with no moisture removed. If the coil surface is below the dew point, moisture condenses out (cooling and dehumidification) and the process bends down toward the coil's apparatus dew point (≈ the coil surface temperature); if the surface is below freezing, that condensate forms frost on the coil.
• Air Mixing — combine two air streams at different states. Enter each stream's airflow and the mixed-air state is plotted on the straight line between the two points, positioned by the flow ratio — the larger stream pulls the mix toward itself.

Properties are computed with PreciseFluids using ASHRAE equations.

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