Psychrometric chart

How to read a psychrometric chart

From reading the six line families to finding a state point, cooling, heating, humidifying and mixing, the SHF line and protractor, and the cooling plot order used in exams. Each step links to the calculator with the example already drawn.

10 20 30 40 50 60 70 80 90 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 °C g/kg OA RA M SA
Figure. Cooling example: outdoor OA (33 °C, 60 %) and room RA (26 °C, 50 %) mixed at 30 % outdoor air to M, then through the cooling coil to supply SA. The dashed line is the SHF 0.8 line through RA; curves are RH, diagonals are enthalpy.

Reading the six line families

Dry-bulb lines (vertical)
Straight vertical lines from the bottom scale. Drop straight down from the point and read the axis.
Humidity ratio lines (horizontal)
Horizontal lines to the right-hand g/kg scale. Move right from the point and read. Points on one horizontal line share the same dew point.
Relative humidity curves
The top saturation curve is 100 %, with 90 %, 80 % … below in the same shape. Between two curves, interpolate by distance.
Wet-bulb lines (diagonals)
Diagonals starting on the saturation curve and running down to the right. Follow the line through the point up to the saturation curve and read the temperature there. Nearly parallel to enthalpy lines, but not the same.
Enthalpy lines (shallow diagonals)
Diagonals drawn from the enthalpy scale outside the top left edge (kJ/kg dry air). Lay a ruler through the point parallel to them and read the outer scale.
Specific volume lines (steep diagonals)
Steep diagonals every 0.01 m³/kg; interpolate between two of them. Used to turn airflow into mass flow (kg/h = m³/h ÷ v).

Dew point has no line of its own: move left from the point at constant humidity ratio until you hit the saturation curve, and read the temperature.

Finding a state point

  1. Pick the two values you know: usually dry bulb and RH, or dry bulb and wet bulb.
  2. The state point is where the two lines cross. Example: the 26 °C vertical and the 50 % RH curve.
  3. Read the rest along each line family. 26 °C, 50 % gives 10.5 g/kg, 52.9 kJ/kg, wet bulb 18.7 °C, dew point 14.8 °C, 0.862 m³/kg.

Open this point in the calculator

Reading processes

Sensible heating and cooling
Moisture stays constant, so the point moves horizontally: right for heating, left for cooling above the dew point. RH falls on heating and rises on cooling.
Cooling and dehumidifying
When the coil surface is colder than the dew point, vapor condenses and the point moves down and to the left. Leaving air is usually near 90 to 95 % RH. Walls and pipes colder than the dew point collect condensation the same way.
Humidifying
Steam humidification moves almost straight up; water spray (evaporative) moves up and to the left along a wet-bulb line, so the air cools.
Mixing
The mixed state lies on the straight line between the two states, closer to the larger airflow. With 30 % outdoor air it sits 30 % of the way from the room point to the outdoor point. 3,000 m³/h at 33 °C, 60 % plus 7,000 m³/h at 26 °C, 50 % gives 28.1 °C, 54.7 %.

Open the mixing exampleOpen a constant wet-bulb direction lineOpen a condensation check

SHF and the protractor

Sensible heat ratio SHF = sensible ÷ (sensible + latent). If the room SHF is 0.8, the supply air must lie on the "SHF 0.8 line" through the room point to remove both loads.

By hand, join the centre of the protractor (top left of the chart) to the 0.8 mark to get the slope, then slide that line with set squares until it passes through the room point. Supply states lie on it.

Open an SHF line

Cooling plot, step by step

Most exam chart problems follow this order. Example: outdoor 33 °C, 60 %; room 26 °C, 50 %; 30 % outdoor air; room sensible 10 kW and latent 2.5 kW; 10 K supply temperature difference.

  1. Plot the outdoor point OA (33 °C, 60 %) and the room point RA (26 °C, 50 %).
  2. Join OA and RA, and mark the mixed point M 30 % of the way from RA toward OA.
  3. Take SHF = 10 ÷ (10 + 2.5) = 0.8 on the protractor and draw the SHF line through RA.
  4. With a 10 K difference, SA is where the 26 − 10 = 16 °C vertical crosses the SHF line. If a coil leaving RH (say 90 %) is given instead, use where that curve crosses the SHF line.
  5. Supply airflow = room sensible ÷ (ρ × cp × Δt) ≈ 10 × 3,600 ÷ (1.2 × 1.006 × 10) ≈ 2,980 m³/h.
  6. The line from M to SA is the cooling coil process. Coil load = mass flow × (h_M − h_SA), moisture removed = mass flow × (W_M − W_SA).

Open this example in the cooling calculation

The calculator’s cooling and heating calculation takes the same inputs and returns the mixed point, supply point, coil load and supply airflow, so you can check a hand plot. Heating goes mix, then heat (horizontal), then humidify (steam vertical, spray along a wet-bulb line), in the heating mode of the same panel.

Sources

Frequently asked questions

Why do wet-bulb and enthalpy lines look almost the same?

In adiabatic saturation (water evaporating into the air) the air’s enthalpy stays almost constant. It differs by the enthalpy of the evaporated water, so accurate charts draw both. Reading enthalpy along a wet-bulb line gives a small error.

Why does the mixed point lie on a straight line?

Dry air mass, water mass and enthalpy are each conserved, and the chart axes are nearly linear in enthalpy and humidity ratio. Strictly the split is by dry air mass, so for exact values convert airflow to kg/h with the specific volume.

How do I read an older chart in kcal units?

The lines and reading method are the same; only the scales differ. 1 kcal/kg = 4.1868 kJ/kg and 1 kg/kg = 1,000 g/kg of dry air. Blank SI charts are at https://psychart.kr/en/download/

Last updated 2026-10-12.