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.
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).