Hong Life CalculatorsUnderstanding the psychrometric calculator — concept and principle
This calculator turns dry-bulb and wet-bulb temperatures and barometric pressure into the main properties of moist air: relative humidity, humidity ratio, enthalpy, dew point, vapour pressure, specific volume, density and degree of saturation. HVAC engineers, meteorologists and growers use it with a sling or aspirated psychrometer.
How it is calculated
W = [(2501 − 2.326 t*) W_s* − 1.006 (t − t*)] ÷ (2501 + 1.86 t − 4.186 t*)
W_s* = 0.621945 p_ws ÷ (p − p_ws) at the wet-bulb temperature t*
p_w = p W ÷ (0.621945 + W); RH = p_w ÷ p_ws(t)
h = 1.006 t + W (2501 + 1.86 t) kJ/kg; v = 287.042 (t + 273.15)(1 + 1.607858 W) ÷ p
Dew point: the temperature where p_ws = p_w
- Saturation pressures use the Hyland–Wexler equations as in the ASHRAE Handbook.
The principle
A wet-bulb thermometer is cooled by evaporation from its wet wick. The drier the air, the faster water evaporates and the larger the gap between dry and wet bulb. An energy balance on the wick, the heat to evaporate water equals the heat drawn from the air, gives the humidity ratio. Once the amount of water vapour is known, every other moist-air property follows from gas laws and the saturation pressure curve.
Worked example
Default inputs: dry bulb 25 °C, wet bulb 18 °C, 101.325 kPa.
- Humidity ratio W = 10.02 g/kg dry air.
- Vapour pressure = 1.6062 kPa; relative humidity = 50.7%.
- Dew point = 14.07 °C.
- Enthalpy = 50.67 kJ/kg; specific volume = 0.8582 m³/kg; density = 1.1769 kg/m³.
- Degree of saturation = 49.9%.
How to read the result
About 50% RH at 25 °C is a comfortable indoor condition. The dew point shows the surface temperature below which condensation forms. Enthalpy is what air-conditioning loads are calculated from. Readings need good air movement over the wet bulb and clean water on the wick.
Common mistakes
- Using a wet bulb without enough air flow, which reads too high.
- Forgetting to adjust pressure at altitude.
- Confusing relative humidity with degree of saturation, which differ slightly.
Frequently asked questions
What is the difference between dew point and wet bulb?
The wet bulb is cooled by evaporation; the dew point is where air would become saturated by cooling alone, and is lower.
Why does pressure matter?
The humidity ratio depends on the partial pressures, which change with altitude.
What is enthalpy of moist air?
The heat content per kilogram of dry air, including the water vapour.
What is the humidity ratio?
Grams of water vapour per kilogram of dry air.
Sources: ASHRAE Handbook — Fundamentals (2017), chapter 1 "Psychrometrics" · D. Meyer and D. Thevenard, "PsychroLib", Journal of Open Source Software 4(33), 1137 (2019)