Humid Air from Dry and Wet Bulb

Method & assumptions

1. Calculate an ideal moist-air state from dry bulb, thermodynamic wet bulb and absolute pressure, using ASHRAE relationships adapted from MIT-licensed PsychroLib.
2. Dry bulb: −50 to 80 °C. Wet bulb: −50 °C through the dry-bulb temperature. Pressure: 20–120 kPa absolute, above saturation pressure at dry bulb. Pairs implying negative moisture content are rejected rather than clamped to a small positive value.
3. Let Ws = 0.621945 ps(Tw)/(p − ps(Tw)). For Tw ≥ 0 °C, W = [(2501 − 2.326Tw)Ws − 1.006(Td − Tw)] / [2501 + 1.86Td − 4.186Tw].
4. For Tw < 0 °C, use the ice balance: W = [(2830 − 0.24Tw)Ws − 1.006(Td − Tw)] / [2830 + 1.86Td − 2.1Tw]. The wet-bulb energy-balance branch changes at 0 °C; the saturation-pressure function changes at the 0.01 °C triple point.
5. pv = pW/(0.621945 + W); RH = 100pv/ps(Td). RH uses the saturation reference phase at dry bulb, which can differ from the wet-bulb balance phase. Enthalpy and specific volume are per kg of dry air; specific humidity and total density include water vapor.
6. Thermodynamic wet bulb is an adiabatic-saturation property. A physical wet-bulb thermometer can be affected by ventilation, radiation, wick condition and ice formation; this tool does not correct an instrument reading. No fog, liquid droplets, supersaturation or real-gas correction is included.
7. For a state given by relative humidity instead, use the Humid Air Properties calculator. Neither tool solves a full HVAC process or selects equipment.

Sources

• PsychroLib: humidity ratio from wet bulb, ASHRAE equations 33 and 35 ↗
• PsychroLib MIT license and adaptation attribution ↗