Psychrometric Chart

A practical guide to reading a psychrometric chart (moist air): dry-bulb temperature, humidity ratio, RH, dew point, and common HVAC processes.

A psychrometric chart is a map of moist air properties at a given pressure. It’s widely used in HVAC to reason about comfort, ventilation, dehumidification, and evaporative processes.

Two core axes

Dry-bulb temperature (Tdb): the “normal” air temperature you measure with a dry thermometer.
Humidity ratio (W): the mass of water vapor per mass of dry air (often shown as g/kg or kg/kg). This is a convenient measure of absolute moisture content.

Common lines you’ll see

Relative humidity (RH): curved lines (0–100%) that show how close air is to saturation.
Dew point: the temperature where condensation begins if air is cooled at constant moisture content.
Enthalpy: useful for load calculations and mixing intuition (often diagonal).
Wet-bulb temperature: useful for evaporative processes and cooling tower intuition.

How to read dew point on a psychrometric chart

Locate the current air-state point from the two known properties.
Move horizontally at constant humidity ratio toward the 100% RH saturation curve.
Read the dry-bulb temperature at that intersection. This is the dew-point temperature.
Use a chart drawn for the actual site pressure, because altitude changes the property relationships.

For a numerical check, enter dry-bulb temperature and RH in the Open Humid Air Calculator. The calculated dew point should match the chart reading within the chart's graphical resolution.

Pressure matters (altitude and weather)

Psychrometric relationships depend on air pressure. The same temperature and RH can imply different humidity ratios at different pressures. For accurate results, use the correct pressure (or altitude) for your location.

Using FluidTool

FluidTool’s Humid Air Calculator lets you compute moist air properties and visualize points on a psychrometric chart. If you’re validating HVAC measurements, start with the inputs you actually have (e.g., Tdb + RH) and ensure the pressure mode matches your site (sea level vs altitude).

Open Humid Air Calculator

A psychrometric chart is a map of moist air properties at a given pressure. It’s widely used in HVAC to reason about comfort, ventilation, dehumidification, and evaporative processes.

Two core axes

Dry-bulb temperature (Tdb): the “normal” air temperature you measure with a dry thermometer.
Humidity ratio (W): the mass of water vapor per mass of dry air (often shown as g/kg or kg/kg). This is a convenient measure of absolute moisture content.

Common lines you’ll see

Relative humidity (RH): curved lines (0–100%) that show how close air is to saturation.
Dew point: the temperature where condensation begins if air is cooled at constant moisture content.
Enthalpy: useful for load calculations and mixing intuition (often diagonal).
Wet-bulb temperature: useful for evaporative processes and cooling tower intuition.

How to read dew point on a psychrometric chart

Locate the current air-state point from the two known properties.
Move horizontally at constant humidity ratio toward the 100% RH saturation curve.
Read the dry-bulb temperature at that intersection. This is the dew-point temperature.
Use a chart drawn for the actual site pressure, because altitude changes the property relationships.

For a numerical check, enter dry-bulb temperature and RH in the Open Humid Air Calculator. The calculated dew point should match the chart reading within the chart's graphical resolution.

Pressure matters (altitude and weather)

Using FluidTool

Open Humid Air Calculator

Two core axes

Common lines you’ll see

How to read dew point on a psychrometric chart

Pressure matters (altitude and weather)

Using FluidTool

Table of Contents

Psychrometric Chart

Two core axes

Common lines you’ll see

How to read dew point on a psychrometric chart

Pressure matters (altitude and weather)

Using FluidTool

Table of Contents