Psychrometric Chart
The psychrometric chart is an interactive tool for visualizing and analyzing air conditions in HVAC systems.
Understanding the Chart
Section titled “Understanding the Chart”Axes and Lines
Section titled “Axes and Lines”Dry-Bulb Temperature
Section titled “Dry-Bulb Temperature”The horizontal axis shows dry-bulb temperature (°F or °C). This is the temperature measured by a standard thermometer.
Humidity Ratio
Section titled “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
Section titled “Relative Humidity Lines”Curved lines showing constant relative humidity from 10% to 100% (saturation line).
Wet-Bulb Lines
Section titled “Wet-Bulb Lines”Diagonal lines showing constant wet-bulb temperature, used for evaporative cooling analysis.
Enthalpy Lines
Section titled “Enthalpy Lines”Lines of constant enthalpy (BTU/lb or kJ/kg), representing total heat content.
Specific Volume Lines
Section titled “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
Section titled “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
Section titled “Using the Chart”What a New Chart Starts With
Section titled “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
Section titled “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 summer and winter comfort regions.
- 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
Section titled “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
Section titled “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
Section titled “Connecting Points”Draw a process line between two state points:
- Click Create Process Line in the Process Lines panel
- Choose the From and To points
- 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
Section titled “HVAC Processes”Sensible Cooling
Section titled “Sensible Cooling”Horizontal line to the left:
- Temperature decreases
- Humidity ratio stays constant
- Common for cooling coils above dew point
Sensible Heating
Section titled “Sensible Heating”Horizontal line to the right:
- Temperature increases
- Humidity ratio stays constant
- Common for heating coils
Cooling with Dehumidification
Section titled “Cooling with Dehumidification”Line toward saturation then left:
- Air cools to dew point
- Moisture condenses
- Continues along saturation line
Humidification
Section titled “Humidification”Line upward (higher humidity ratio):
- Steam injection (isothermal)
- Evaporative (adiabatic)
Mixing
Section titled “Mixing”Straight line between two air streams:
- Mixed condition on the line
- Position based on airflow ratios
Elevation Adjustment
Section titled “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
Section titled “Practical Applications”Coil Analysis
Section titled “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
Section titled “Economizer”Analyze mixed air conditions:
- Outdoor air point
- Return air point
- Mixed air result
Dehumidification
Section titled “Dehumidification”Evaluate moisture removal:
- Coil leaving condition
- Reheat requirements
- Latent capacity
Saving & Exporting
Section titled “Saving & Exporting”Copy as PNG
Section titled “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
Section titled “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.