How to Read and Use a Psychrometric Chart

A psychrometric chart shows the properties of moist air on one plot. This page tells you how to read the chart, then how to use the interactive psychrometric chart in HVAKR to plot air conditions and HVAC processes.

To try a chart without opening the app, use the free online psychrometric chart on the HVAKR website.

How to Read a Psychrometric Chart

Each point on the chart is one air condition, called a state point. Two independent properties fix a state point. When you know two properties, you can read all the others from the chart.

1. Find the first property. For dry-bulb temperature, find the value on the horizontal axis and follow the vertical line up.
2. Find the second property. Follow its line family until it crosses the first line. For relative humidity, use the curved lines. For wet-bulb temperature, use the diagonal lines that slope down to the right. The crossing is the state point.
3. Read the other properties from the state point:
   • Humidity ratio: move horizontally to the right, to the vertical axis.
   • Dew point: move horizontally to the left, to the saturation curve (100% relative humidity). Read the dry-bulb temperature at that point.
   • Wet-bulb temperature: follow the wet-bulb line up and to the left, to the saturation curve.
   • Enthalpy: follow the enthalpy line to the enthalpy scale.
   • Specific volume: interpolate between the two nearest specific-volume lines.

For example, the seeded Return Air point is 75 °F dry bulb and 50% relative humidity at sea level. From that point, the chart gives a wet bulb of about 62.6 °F, a dew point of about 55.2 °F, a humidity ratio of about 0.0092 lb/lb, an enthalpy of about 28.1 BTU/lb, and a specific volume of about 13.7 ft³/lb.

In HVAKR, you do not have to read the lines by hand. Enter the dry-bulb temperature and the wet-bulb temperature or relative humidity, and HVAKR calculates the other properties. See Placing Points.

Understanding the Chart

Axes and Lines

Dry-Bulb Temperature

The horizontal axis shows dry-bulb temperature (°F or °C). This is the temperature measured by a standard thermometer.

Humidity Ratio

The vertical axis shows humidity ratio (lb/lb or g/kg). This is the mass of water vapor per mass of dry air.

Relative Humidity Lines

Curved lines showing constant relative humidity from 10% to 100% (saturation line).

Wet-Bulb Lines

Diagonal lines showing constant wet-bulb temperature, used for evaporative cooling analysis.

Enthalpy Lines

Lines of constant enthalpy (BTU/lb or kJ/kg), representing total heat content.

Specific Volume Lines

Steep diagonal lines of constant specific volume (ft³/lb or m³/kg), the volume the moist air occupies per unit mass of dry air.

State Points

A state point on the chart represents a specific air condition:

• Dry-bulb temperature
• Wet-bulb temperature
• Dew point
• Humidity ratio
• Relative humidity
• Enthalpy
• Specific volume

Using the Chart

What a New Chart Starts With

A new chart is not empty. It opens on a worked mixed-air example (four state points and three process lines) that you can edit or delete:

Point	Label	Condition
Outside Air	OA	100 °F dry bulb, 78 °F wet bulb
Return Air	RA	75 °F dry bulb, 50% relative humidity
Mixed Air	MA	The mix of 300 CFM outside air and 1,000 CFM return air, not draggable
Supply Air	SA	55 °F dry bulb, 55 °F wet bulb (saturated)

The three process lines are Outside Air → Mixed Air at 300 CFM, Return Air → Mixed Air at 1,000 CFM, and Mixed Air → Supply Air at 1,300 CFM on the Saturated Cooling path.

Display Options

The chart toolbar sits above the plot. Use the View control to show or hide each overlay: the eye icon next to an item marks whether it is currently shown:

• Dry Bulb Temperature, Wet Bulb Temperature, Enthalpy, Specific Volume, Relative Humidity, and Humidity Ratio: the constant-property line families.
• ASHRAE 55 Comfort Zones: the shaded comfort regions from ASHRAE Standard 55, labeled 1.0 clo (winter clothing) and 0.5 clo (summer clothing). They are standard guidance, not your project’s setpoints. Hover a zone to see its name and its dry-bulb and relative-humidity limits. Unit and space charts in Configuration shade your project’s indoor design ranges instead.
• Cursor Intersection Lines: guide lines from the pointer to the axes.

All overlays are shown by default. The toolbar carries two more controls next to View: Elevation, covered in Elevation Adjustment below, and Fullscreen, which expands the chart to fill the window.

Placing Points

Add a state point in either of two ways:

• Click anywhere on the chart to drop a point at that condition, or
• Click Create State Point in the State Points panel to add one you can type exact values into. It starts at 75 °F dry bulb, at the wet bulb for 50% relative humidity at sea level, and is named Point 5, Point 6, and so on

Set the dry-bulb temperature plus either the wet-bulb temperature or relative humidity: the remaining properties (dew point, humidity ratio, enthalpy, and specific volume) are calculated and shown in the point’s details. Each point also takes a Name and an optional Label. The chart draws the first two characters of the label, or of the name when the point has no label, which is why the seeded points read as OA, RA, MA, and SA. Toggle Draggable to control whether the point can be repositioned by dragging it on the chart.

Use Delete in a point’s section to remove it, or right-click the point on the chart and choose Delete name. Deleting a point also removes every process line that used it.

Moving Points

Drag points to see how properties change:

• Move horizontally for temperature change
• Move vertically for humidity change
• All properties update in real-time

A drag is held inside the chart: the dry-bulb temperature stops at the ends of the axis, and the humidity ratio stops at the saturation line, so a point cannot be dragged past 100% relative humidity. A point with Draggable off (the seeded Mixed Air point) does not move; edit its temperatures in the State Points panel instead. Hover or click any point to read all seven of its properties without opening the panel.

Connecting Points

Draw a process line between two state points:

1. Click Create Process Line in the Process Lines panel
2. Choose the From and To points
3. Click Create: the process line is drawn between them

Each process line takes a Flow Rate (CFM or L/s), the volume of the stream that drives the loads, and a Path, which is Straight unless you switch it to Saturated Cooling. Sensible Heat, Latent Heat, and Total Heat appear once the flow rate is set; a line with no flow rate is drawn on the chart but reports no loads. Delete in the line’s section removes it. You need at least two state points before a process line can be created.

HVAC Processes

Sensible Cooling

Horizontal line to the left:

• Temperature decreases
• Humidity ratio stays constant
• Common for cooling coils above dew point

Sensible Heating

Horizontal line to the right:

• Temperature increases
• Humidity ratio stays constant
• Common for heating coils

Cooling with Dehumidification

Line toward saturation then left:

• Air cools to dew point
• Moisture condenses
• Continues along saturation line

Humidification

Line upward (higher humidity ratio):

• Steam injection (isothermal)
• Evaporative (adiabatic)

Mixing

Straight line between two air streams:

• Mixed condition on the line
• Position based on airflow ratios

Elevation Adjustment

Psychrometric properties vary with altitude, because atmospheric pressure falls as elevation rises.

Set the site elevation with the Elevation control in the chart toolbar. It takes a length in your display units (ft or m) and starts at 0 (sea level). Type 0 again to return to sea level.

The elevation you set drives the atmospheric pressure behind everything on screen: the constant-property line families, every derived point property, the ASHRAE 55 comfort zones, and the process-line loads. It belongs to the chart, not to a project, and is stored with the chart when you save.

Practical Applications

Coil Analysis

Visualize cooling coil performance:

• Entering air condition
• Leaving air condition
• The coil load, split into sensible and latent heat by the process line between them

Economizer

Analyze mixed air conditions:

• Outdoor air point
• Return air point
• Mixed air result

Dehumidification

Evaluate moisture removal:

• Coil leaving condition
• Reheat requirements
• Latent capacity

Saving & Exporting

Copy as PNG

Hover the chart and use the copy control in the corner to put a PNG of the current chart (including state points and process lines) on the clipboard. The capture leaves out the passing chrome: the cursor guide lines, their labels, and any open point tooltip.

Save the chart

Save asks for a name, then stores the chart with its points, process lines, elevation, and view settings. Saving a chart you opened from History updates that chart in place rather than adding a copy.

Saved charts are listed in the History sidebar, where you can reopen, rename, or delete them. The History button stays disabled until you have saved at least one chart.

New Chart discards what is on screen and starts over from the seeded four-point example described in What a New Chart Starts With.

Frequently Asked Questions

Which two properties do I need to find a state point?

Any two independent properties, for example dry bulb and relative humidity, dry bulb and wet bulb, or dry bulb and dew point. Dew point and humidity ratio together do not fix a point, because both set only the moisture content. Wet-bulb and enthalpy lines are almost parallel, so that pair gives a poor intersection. In HVAKR, enter the dry-bulb temperature with the wet-bulb temperature or the relative humidity.

How do I find the dew point on a psychrometric chart?

Start at the state point and move horizontally to the left until you reach the saturation curve. The dry-bulb temperature at that point is the dew point. Air at 75 °F and 50% relative humidity has a dew point of about 55 °F.

What is the difference between wet-bulb and dew-point temperature?

The wet-bulb temperature is the temperature that air reaches when water evaporates into it until it is saturated. The dew point is the temperature at which moisture starts to condense when you cool the air at a constant humidity ratio. The wet bulb is always between the dew point and the dry bulb. At saturation, the three temperatures are equal.

How do I find the mixed-air condition of two air streams?

Draw a straight line between the two state points. The mixed-air point is on that line, nearer to the larger air stream. Its position is set by the ratio of the dry-air mass flows, which is close to the ratio of the airflows in CFM. With 300 CFM of outside air at 100 °F and 1,000 CFM of return air at 75 °F, the mixed-air point is about 22% of the way from the return air point to the outside air point. The CFM ratio (300 / 1,300) gives about 23%, because the warmer outside air is less dense. A new chart in HVAKR starts with this example. See What a New Chart Starts With.

Does elevation change the psychrometric chart?

Yes. Atmospheric pressure falls as elevation rises, so the same dry-bulb and wet-bulb temperatures give a higher humidity ratio at altitude than at sea level. Set the site elevation in HVAKR to get the correct properties. See Elevation Adjustment.