Psychrometric state point
Moist air is fixed by two independent properties, and every other number on the chart follows. Enter dry bulb with relative humidity, wet bulb or humidity ratio, and this returns the dew point, enthalpy, specific volume and density the coil and duct calculations actually need.
| Property | Value | Note |
|---|---|---|
| Dry-bulb temperature | 24.00 °C | The number a thermometer reads |
| Relative humidity | 50.0% | Vapour pressure over saturation pressure |
| Humidity ratio | 9.30 g/kg | Mass of water per mass of dry air |
| Degrees of saturation | 49.3% | Ratio to the saturation humidity ratio |
| Vapour pressure | 1493 Pa | Partial pressure of the water vapour |
| Saturation pressure | 2985 Pa | At the dry-bulb temperature |
| Dew point | 12.95 °C | Where condensation begins on a surface |
| Wet-bulb temperature | 17.07 °C | Thermodynamic, not the sling reading |
| Enthalpy | 47.81 kJ/kg | Reference zero at 0 °C, dry air |
| Specific volume | 0.854 m³/kg | Per kilogram of dry air |
| Density | 1.181 kg/m³ | Of the moist mixture |
| Superheat | 11.05 K | Above the dew point |
| Saturation humidity ratio | 18.88 g/kg | The most moisture the air can hold here |
| Total pressure | 101.33 kPa | Site barometric pressure |
The dew point is the design number for condensation, not the relative humidity. A surface below it will be wet regardless of how low the RH reads.
W = 0.621945 p_w / (p − p_w) · The 0.621945 is the molecular-weight ratio of water to dry air.
h = 1.006 T + W(2501 + 1.86 T) · kJ per kg of dry air, T in °C.
ln p_ws = C₈/T + C₉ + C₁₀T + C₁₁T² + C₁₂T³ + C₁₃ ln T · ASHRAE eq. 6 over water; a separate fit over ice.
W = ((2501 − 2.326 T_wb) W_s* − 1.006(T − T_wb)) / (2501 + 1.86 T − 4.186 T_wb) · Solved by bisection on T_wb.
A clear path from inputs to results.
Moist air is fixed by two independent properties, and every other number on the psychrometric chart follows from them. Enter dry bulb with relative humidity, wet bulb or humidity ratio, and this returns the dew point, enthalpy, specific volume and density the coil and duct calculations actually need.
How to use
- Choose which pair of properties you know.
- Enter the dry-bulb temperature and the second property.
- Set the site barometric pressure if you are above sea level.
- Read the humidity ratio, enthalpy, dew point and wet bulb.
- Use the dew point — not the relative humidity — as the condensation threshold.
The formula
Reference: ASHRAE Handbook — Fundamentals 2017, Chapter 1: Equations 5 and 6 (Hyland–Wexler), 20, 32 and 33.
Worked example
24 °C at 50% relative humidity
- Dry-bulb temperature 24 °C.
- Relative humidity 50%.
- Sea-level pressure 101.325 kPa.
- 1
Saturation pressure at 24 °C is 2,985 Pa from the Hyland–Wexler fit.
- 2
Vapour pressure is 50% of that: 1,493 Pa.
- 3
W = 0.621945 × 1,493 / (101,325 − 1,493) = 0.00930 kg/kg, or 9.30 g/kg.
- 4
Enthalpy is 1.006 × 24 + 0.00930 × (2501 + 1.86 × 24) = 47.81 kJ/kg.
- 5
The dew point is the temperature where the saturation pressure equals 1,493 Pa: 12.95 °C.
- 6
The thermodynamic wet bulb, solved from the saturation identity, is 17.07 °C.
9.30 g/kg · 47.81 kJ/kg · dew point 12.95 °C · wet bulb 17.07 °C.
Interpreting the result
The dew point is the number that matters for condensation: any surface below 12.95 °C will be wet, whatever the relative humidity says. The wet bulb is what evaporative cooling can reach, and the gap between it and the dry bulb — 6.9 K here — is the evaporative cooling potential. The humidity ratio is what the mass and energy balances use, because it is conserved when air is heated or cooled without condensation.
Assumptions & limitations
- Saturation pressure uses the ASHRAE Hyland–Wexler formulation over water above 0 °C and the ice fit below it.
- The pressure is the site barometric pressure; assuming sea level at altitude is a common source of a few percent error in the humidity ratio.
- The wet bulb is the thermodynamic value, not the reading of a sling psychrometer, which includes a small radiation and ventilation error.
- Enthalpy uses a reference of zero for dry air at 0 °C and saturated liquid water at 0 °C.
- Above 100% relative humidity the state is supersaturated; the model clamps rather than reporting a fog.
Frequently asked questions
What is the difference between relative humidity and humidity ratio?
Relative humidity is how full the air is relative to saturation at that temperature, so it changes when you heat or cool the air without adding moisture. Humidity ratio is the actual mass of water per mass of dry air, and it only changes when moisture is added or removed. Energy balances use the ratio.
Why does altitude change the answer?
Because the humidity ratio depends on the total pressure in the denominator. At 1,500 m the pressure is about 84 kPa, so the same vapour pressure corresponds to a higher humidity ratio and a higher relative humidity for the same dry bulb.
What is the wet-bulb depression?
The dry bulb minus the wet bulb. It is the evaporative cooling available: dry air has a large depression, humid air almost none. At 24 °C and 50% it is 6.9 K, so evaporative cooling can reach about 17 °C at best.
Why does my dew point calculation differ from an online chart?
Usually a different saturation-pressure fit. The Magnus approximation used by many simple tools differs from the ASHRAE tabulated values by about 0.4% above freezing, which is a few hundredths of a degree in the dew point. Below 0 °C the difference is larger, because the ice branch matters.
How do I use this for a cooling coil?
Read the entering and leaving states, then find where the straight line between them meets saturation — that is the apparatus dew point. The bypass factor is the ratio of the leaving-to-ADP temperature difference over the entering-to-ADP difference.
Methodology last reviewed 2026-10-21. Results are informational and not financial or engineering advice — see our disclaimer.