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TECHNICAL TOOL / 06
Interactive Psychrometric Chart Calculator
Use this psychrometric calculator to read the air state and compare moisture and energy changes between two conditions.
STATE A
- Relative humidity
- 60.00 %
- Dew / frost point
- 16.70 °C
- Thermodynamic wet bulb
- 19.47 °C
- Humidity ratio (dry-air basis)
- 11.895 g/kg
- Enthalpy (SI reference datum)
- 55.45 kJ/kg
- Specific volume (dry-air basis)
- 0.8608 m³/kg
- Vapour partial pressure
- 1.9015 kPa
STATE B
Check every input: temperatures, pressure and the selected moisture parameter must describe an unsaturated or saturated air state within the stated range.
01 / PLOT
Two coordinates. A complete air state.
Blue: RH curves (outer boundary = 100%). Grey long dashes: constant enthalpy. Green short dashes: constant wet bulb. The A–B line joins the endpoints; it is not a simulated equipment path. Swipe horizontally on a phone.
Humidity ratio is water mass per mass of dry air, not water per room volume. IP enthalpy is converted from the SI 0°C dry-air datum; absolute values differ from charts using a 0°F datum. Δh uses the same energy difference.
02 / READ THE CHART
Follow temperature, moisture, then energy.
Locate the state
Read dry-bulb temperature on the horizontal axis. Move vertically to the selected RH curve. The right-hand scale gives humidity ratio.
Read the dew point
Move left at constant humidity ratio until the 100% boundary. This is the dew or frost point at the selected pressure.
Compare A → B
Negative Δw means moisture removal. Multiply −Δw in kg/kg by dry-air mass flow for kg/h removed. Δh requires the same dry-air basis; a point difference alone is not equipment power.
03 / DEHUMIDIFICATION
The route depends on how water leaves the air.
| Process | Direction on this chart | Check before selection |
|---|---|---|
| Cooling and condensation | Left at nearly constant w until condensation; then left and down as water condenses. Reheat moves right at constant w. | Coil temperature, bypass, defrost and actual removal at inlet conditions. |
| Desiccant adsorption | Typically down and right: moisture falls while adsorption and carry-over heat warm the process air. A constant-enthalpy direction is only an approximation. | Regeneration heat, wet-air exhaust and post-cooling requirements. |
Common reading mistakes
Lower RH does not necessarily mean less water: reheating can lower RH at constant w. Wet-bulb and enthalpy lines are close, but not identical. Pressure changes the chart. Never read a sea-level chart as an altitude-corrected result.
Calculation method and example
pᵥ = RH × pws(T) / 100
w = 0.621945 × pᵥ / (p − pᵥ)
h = 1.006T + w(2501 + 1.86T)
v = 287.042(T + 273.15)(1 + 1.607858w) / p
w = 0.621945 × pᵥ / (p − pᵥ)
h = 1.006T + w(2501 + 1.86T)
v = 287.042(T + 273.15)(1 + 1.607858w) / p
Pressure is in Pa, T in °C, w in kg/kg dry air, h in kJ/kg dry air and v in m³/kg dry air. At 25°C, 60% RH and 101.325 kPa: w ≈ 11.90 g/kg, h ≈ 55.45 kJ/kg. Dew point is found by inverting saturation pressure; wet bulb solves the thermodynamic water/ice energy balance.
Source: ASHRAE Handbook—Fundamentals (2017), Chapter 1 · PsychroLib. Hyland–Wexler saturation equations use ice up to 0.01°C and liquid water above. The interface deliberately limits dry bulb to −20…60°C. Ideal-gas engineering approximations and rounded chart lines are not a laboratory calibration.
Estimate the room moisture load and required dehumidifier capacity
NEXT / PROJECT REVIEW
Turn the air states into equipment requirements.
Bring inlet and target states, airflow, moisture sources and installation constraints.
Review project requirements