Psychrometrics guide
HVAC Psychrometrics Explained
Psychrometrics connects temperature, moisture, energy, and pressure for one stated air condition.
The direct answer
Psychrometrics is the study of moist-air properties. In HVAC it helps describe whether air must be cooled, heated, humidified, dehumidified, or simply moved.
Properties work together
| Property | Meaning | HVAC use |
|---|---|---|
| Dry bulb | Air temperature | Sensible condition |
| Relative humidity | Vapor pressure relative to saturation | State comparison, not moisture mass alone |
| Dew point | Saturation temperature for the moisture state | Condensation risk context |
| Humidity ratio | Water-vapor mass per dry-air mass | Moisture balance |
| Enthalpy | Moist-air energy per dry-air mass | Total-air process comparison |
Example
At 24°C, 50% RH, and 101.325 kPa, the Psychrometric Calculator derives a dew point, humidity ratio, enthalpy, and approximate wet bulb for one condition. Changing pressure changes humidity ratio and enthalpy; relative humidity alone is not enough.
Wet bulb versus dry bulb
Dry bulb is the measured air temperature. Wet bulb describes an evaporative cooling equilibrium and depends on pressure. The calculator labels its wet-bulb result as a constant-enthalpy approximation, not a substitute for an instrument-specific measurement method.
Common mistakes
- Comparing RH values at different dry-bulb temperatures as if they represent equal moisture content.
- Ignoring pressure or elevation when using humidity ratio.
- Using dew point or wet bulb alone to select equipment without a complete load calculation.