Construction of the psychrometric chart

A psychrometric chart is a graph of the properties of moist air at a constant total pressure (normally 101.325 kPa). From any two independent properties, all the others can be read.

Axes

• Horizontal axis: dry bulb temperature (DBT).
• Vertical axis (right side): specific humidity $\omega$ (or vapour pressure).

Lines on the chart

1. Constant DBT lines: vertical.
2. Constant specific humidity lines: horizontal. The dew point is read where the horizontal through the state meets the saturation curve.
3. Saturation curve (100 % RH): the upper curved boundary; DBT, WBT and DPT are equal on it.
4. Constant RH curves: curves below the saturation curve (10 %, 20 %, ...).
5. Constant WBT lines: inclined straight lines running down to the right, ending on the saturation curve.
6. Constant enthalpy lines: nearly parallel to the WBT lines; read from a scale left of the saturation curve.
7. Constant specific volume lines: steeper inclined lines (m³/kg dry air).
8. Sensible heat factor (SHF) scale and protractor at the upper left for process lines.

 w ^                         .-' saturation
   |                   .--''   (100 % RH)
   |  DPT <--------- P *  <- w = constant
   |             .-'/ |
   |          .-'  /  |  <- DBT (vertical)
   |       .-'   WBT line
   +------------------------------> DBT

Reading a state P: vertical down gives DBT; horizontal left to the saturation curve gives DPT and horizontal right gives $\omega$; the inclined line through P reaching the saturation curve gives WBT; the curve through P gives RH; the enthalpy and volume lines nearest to P give $h$ and $v$.