This calculator evaluates a single moist-air state using common psychrometric relations. Pressure is the total barometric pressure.
- Saturation vapor pressure: Psat(T) from a Buck-type exponential equation.
- Vapor pressure from RH: Pv = (RH/100) · Psat(Tdb)
- Humidity ratio: W = 0.621945 · Pv / (P − Pv)
- Enthalpy: h = 1.006·Tdb + W·(2501 + 1.86·Tdb)
- Specific volume: v = 0.287042·(Tdb+273.15)·(1 + 1.607858·W) / P
- Wet-bulb (iterative): solves Pv = Psat(Twb) − A·P·(Tdb − Twb), with A = 0.00066·(1 + 0.00115·Twb).
Results are approximations for engineering use; extreme conditions may require standards tables.
- Select the Known Pair that matches your measurements.
- Choose units, then enter P and Tdb.
- Enter the second input for the selected mode.
- Click Calculate. Results appear above the form.
- Use Download CSV or Download PDF to save outputs.
| Case | P (kPa) | Tdb (°C) | RH (%) | Expected W (kg/kg) | Expected h (kJ/kg dry air) |
|---|---|---|---|---|---|
| Comfort cooling | 101.325 | 30 | 60 | ≈ 0.016 | ≈ 71 |
| Dry indoor air | 101.325 | 22 | 35 | ≈ 0.006 | ≈ 37 |
| Humid outdoor air | 100.000 | 32 | 75 | ≈ 0.023 | ≈ 92 |
Psychrometric Chart Point Guide
1) What a psychrometric point represents
A psychrometric chart point is a complete thermodynamic state of moist air at a given pressure. One point fixes temperature, moisture content, and energy level, allowing consistent HVAC design, commissioning, and lab reporting. This calculator reconstructs that state from common field pairs.
2) Why barometric pressure matters
Psychrometric properties depend on total pressure. At sea level, pressure is about 101.325 kPa, while many high-altitude sites operate near 85–90 kPa. Lower pressure increases specific volume and slightly changes humidity ratio for the same vapor pressure, affecting airflow calculations.
3) Typical comfort and process ranges
Comfort cooling commonly targets 22–26 °C with 40–60% RH, where humidity ratio often sits near 0.007–0.012 kg/kg dry air. Warm humid outdoor conditions such as 32 °C and 75% RH can exceed 0.020 kg/kg, increasing latent load and dehumidification demand.
4) Saturation vapor pressure and RH
Saturation vapor pressure rises rapidly with temperature. For example, saturation at 30 °C is around a few kilopascals, so a 60% RH condition means the actual vapor pressure is 60% of that saturation value. This relationship anchors the chart’s curved RH families and guides control logic.
5) Humidity ratio as the moisture coordinate
Humidity ratio W (kg water per kg dry air) is the vertical chart coordinate used for mass balance. Small changes matter: increasing W by 0.002 kg/kg at 1 kg/s dry-air flow corresponds to 0.002 kg/s of added moisture, which drives coil sizing and reheat energy.
6) Dew point for condensation risk
Dew point indicates when surfaces will sweat. If indoor air has a dew point of 18 °C, any surface colder than 18 °C can condense water. Engineers use dew point to set chilled-water temperatures, verify vapor barriers, and prevent mold growth.
7) Wet-bulb temperature for evaporative processes
Wet-bulb temperature reflects evaporative cooling potential and is used for cooling tower and evaporative cooler performance. In hot-dry climates, wet-bulb may be far below dry-bulb, enabling significant cooling without refrigeration, while humid climates narrow that gap.
8) Enthalpy and specific volume for HVAC sizing
Enthalpy (kJ/kg dry air) helps quantify total load across coils and air handlers. A typical comfort state may be 35–55 kJ/kg, while humid outdoor air can exceed 80–90 kJ/kg. Specific volume supports fan and duct sizing by converting mass flow to volumetric flow.
Frequently Asked Questions
1) Which input pair should I use?
Use the pair you measured most reliably: Tdb+RH for sensors, Tdb+Twb for a sling psychrometer, Tdb+Tdp for condensation studies, or Tdb+W when humidity ratio is known from process data.
2) What pressure should I enter?
Enter local barometric pressure. If you do not have a reading, 101.325 kPa is a reasonable sea-level default. High-altitude locations often fall between 80 and 95 kPa.
3) Why is my wet-bulb result blank?
Wet-bulb is solved iteratively and may fail for inconsistent inputs, such as unrealistically high vapor pressure or a wet-bulb higher than dry-bulb. Recheck units, pressure, and the selected mode.
4) How accurate are these results?
They are engineering approximations suitable for HVAC calculations and education. Small differences versus printed charts occur due to rounding and the vapor pressure formulation. For compliance work, follow the standard required by your project.
5) What is percent saturation?
Percent saturation compares your humidity ratio to the saturation humidity ratio at the same dry-bulb temperature. It differs from relative humidity and can help interpret moisture content in mixing and drying processes.
6) Can I use °F and psi?
Yes. Select °F and psi in the unit menus. The calculator converts internally and reports results in your chosen units for temperature and pressure, while humidity ratio remains on a mass basis.
7) How do I interpret enthalpy for load calculations?
Compute enthalpy for outdoor air and supply air, then multiply the enthalpy difference by dry-air mass flow to estimate total load. This supports coil sizing, ventilation analysis, and energy benchmarking.