Psychrometrics – the study of moist air – underpins air conditioning, drying, cooling towers and greenhouse design. A sling psychrometer’s wet- and dry-bulb readings give everything else.
How to use the Psychrometric Calculator
- Pressure from
- Altitude
- Barometric pressure
- Dry-bulb temperature
- Humidity from
- Wet-bulb temperature
- Relative humidity (%)
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the altitude (or pressure), the dry-bulb temperature and either the wet-bulb temperature or relative humidity.
Inputs explained
| Field | What to enter |
|---|---|
| Pressure from | Choose one: Altitude, Barometric pressure |
| Altitude | Units: m, ft |
| Barometric pressure | Units: kPa, hPa, inHg, psi |
| Dry-bulb temperature | Units: °C, °F, K |
| Humidity from | Choose one: Wet-bulb temperature, Relative humidity |
| Wet-bulb temperature | Units: °C, °F, K |
| Relative humidity (%) | A number |
What each result means
- Saturation vapour pressure = 6.112×exp(17.62×([Dry-bulb temperature])÷(243.12 + ([Dry-bulb temperature])))÷10
- Relative humidity = [Vapour pressure]÷[Saturation vapour pressure]×100
- Humidity ratio = 622×[Vapour pressure]÷([Barometric pressure] – [Vapour pressure])
- Degree of saturation = ([Vapour pressure]÷([Barometric pressure] – [Vapour pressure]))÷([Saturation vapour pressure]÷([Barometric pressure] – [Saturation vapour pressure]))×100
- Specific enthalpy = 1.006×[Dry-bulb temperature] + [Humidity ratio]÷1000×(2501 + 1.86×[Dry-bulb temperature])
- Specific volume = 0.287042×([Dry-bulb temperature] + 273.15)×(1 + 1.607858×[Humidity ratio]÷1000)÷[Barometric pressure]
Formula
p = 101.325·(1 − 2.25577×10⁻⁵·Z)^{5.2559} kPa, e = e_s(T_wb) − 6.6×10⁻⁴·(1 + 1.15×10⁻³T_wb)·p·(T − T_wb)
W = 0.622·e/(p − e), h = 1.006T + W·(2501 + 1.86T) kJ/kg, v = 0.287042·(T + 273.15)·(1 + 1.6078W)/p
Worked example
Inputs — Pressure from: Altitude; Altitude: 0 m; Dry-bulb temperature: 25 °C; Humidity from: Wet-bulb temperature; Wet-bulb temperature: 18 °C.
Results — Barometric pressure = 101.325 kPa; Saturation vapour pressure = 3.16006 kPa; Vapour pressure = 1.58132 kPa; Relative humidity = 50.0408 %; Dew point = 13.8642 °C; Humidity ratio = 9.86107 g/kg; Degree of saturation = 49.2487 %; Specific enthalpy = 50.27107 kJ/kg; Specific volume = 0.85802 m³/kg.
Key terms
- Psychrometric chart
- Graph of moist-air properties.
Frequently asked questions
What is the wet-bulb temperature?
The temperature of a thermometer cooled by evaporation from a wet wick.
Why does altitude matter?
Lower pressure changes the humidity ratio and specific volume.
What is enthalpy used for?
Calculating heating and cooling loads of air.
What is the humidity ratio?
Grams of water vapour per kilogram of dry air.