Psychrometric Chart
Ctrl+D to bookmark this toolCalculate air properties and plot AHU processes. Input dry bulb temperature and relative humidity — get all 7 properties instantly with an interactive chart.
Equations from CIBSE Guide C §1.4. Standard atmospheric pressure 101,325 Pa.
How It Works
Every point on a psychrometric chart represents a unique air condition. From just two known properties — dry bulb temperature and relative humidity — all other properties can be calculated using thermodynamic equations from CIBSE Guide C.
Add multiple state points to plot HVAC processes: outside air to mixed air (mixing), mixed air to off-coil (cooling), off-coil to supply (fan gain). The enthalpy difference between points gives you the coil duty.
Properties Calculated
| Property | Symbol | Unit | Description |
|---|---|---|---|
| Dry Bulb Temperature | Tdb | °C | Air temperature measured by a standard thermometer |
| Relative Humidity | RH | % | Ratio of actual to saturation vapour pressure |
| Moisture Content | g | g/kg | Mass of water vapour per kg of dry air |
| Wet Bulb Temperature | Twb | °C | Temperature at adiabatic saturation |
| Dew Point Temperature | Tdp | °C | Temperature at which condensation begins |
| Specific Enthalpy | h | kJ/kg | Total energy content per kg of dry air |
| Specific Volume | v | m³/kg | Volume per kg of dry air |
Common AHU Processes
| Process | Chart Direction | What Changes | Example |
|---|---|---|---|
| Sensible Heating | Horizontal right → | Tdb increases, g constant | Heating coil, fan heat gain |
| Sensible Cooling | Horizontal left ← | Tdb decreases, g constant | Cooling above dew point |
| Cooling + Dehumidification | Down-left ↓← | Tdb and g both decrease | Cooling coil below dew point |
| Humidification (steam) | Vertical up ↑ | g increases, Tdb constant | Steam humidifier |
| Adiabatic Humidification | Along wet bulb line | g increases, Tdb decreases | Evaporative cooler, spray washer |
| Mixing | Straight line between points | Both change proportionally | Outside air + return air |
MEP Desk includes psychrometric calculations with coil duty sizing, AHU design tools, and full project management — all connected to your project data.