Working with Humid-Air Measurements

Start with the quantities you know

Use Humid Air Properties when you know dry-bulb temperature, relative humidity and absolute pressure. Use Humid Air from Dry and Wet Bulb for a thermodynamic wet-bulb input instead. Neither input set can be reduced to two temperatures alone without specifying pressure.

The wet-bulb model is based on an adiabatic saturation balance. It treats evaporating liquid water at wet-bulb temperatures of 0°C or above and uses an ice balance below 0°C. A physical thermometer reading can be affected by ventilation, radiation, wick condition and freezing; the calculator does not correct those effects.

Keep mass bases distinct

Humidity ratio W is water-vapor mass per dry-air mass. Specific humidity is W/(1+W), water-vapor mass per total moist-air mass. They are different quantities even though both are mass ratios.

The tools report enthalpy in J/kg of dry air and specific volume in m³/kg of dry air. Total density includes both air and vapor mass. Use the matching mass basis when converting these results into heat rates or mass flows.

Compare two constructed states

Both cases below use 95,461 Pa absolute pressure. These calculated examples also correspond to the warm and cold wet-bulb cases in PsychroLib’s SI verification tests.

The subfreezing case uses an ice-referenced saturation pressure at dry bulb. The warm case uses liquid water. A humidity sensor that reports subfreezing RH relative to liquid water is using a different reference; do not substitute that number into an ice-referenced calculation without conversion.

Mix streams using dry-air mass flow

The two-stream air-mixing calculator conserves dry-air mass, water-vapor mass and enthalpy at a common pressure. Weight humidity ratio and enthalpy by dry-air flow. Relative humidity is then calculated at the resulting temperature; it is not the average of the inlet RH values.

Constructed case: combine 1 kg dry air/s at 10°C and 60% RH with 3 kg dry air/s at 25°C and 50% RH, at 101,325 Pa. The model gives 21.277°C, 54.30% RH and 0.00854970 kg water/kg dry air.

If inlet flow is measured in m³/s, divide it by that inlet’s specific volume in m³/kg dry air before entering it. This conversion uses each inlet state separately. A volume-flow fraction generally differs from a dry-air mass fraction.

The model excludes heat transfer, work and pressure loss. When the vapor-only balance predicts supersaturation, it stops: the final state needs a condensation or ice calculation including the phase-change energy. See the ASHRAE mixing reference below for the conservation framework.

Continue from mixing to an ideal cooling process

Enter the mixed temperature, RH and 4 kg dry air/s from the preceding example into Air Heating, Cooling and Condensate. At 101,325 Pa with a 10°C target outlet, the ideal process removes 55.212 kW and drains 13.243 kg of liquid water per hour.

Cooling below dew point requires moisture removal to reach a vapor-only outlet at that temperature. This model returns a saturated outlet and assumes the condensate drains at the outlet temperature. The water leaving carries energy, so the heat removed is less than the reduction in the air stream’s enthalpy alone.

Heating or dry cooling keeps humidity ratio constant. Positive reported heat means removal; negative means addition. These thermal rates are not electrical consumption. Real coil bypass, pressure loss, fan heat and frost require additional models, and subfreezing outlet temperatures are excluded here.

Recognize invalid pairs and model boundaries

Wet bulb cannot exceed dry bulb in this unsaturated model. A very large wet-bulb depression can imply negative moisture content even when the temperatures are in order. For example, 25°C dry bulb and 0°C wet bulb at 101,325 Pa is rejected by this model.

At equal dry and wet bulb, the model returns 100% RH. The supported dry-bulb range is −50 to 80°C, wet bulb is at least −50°C, and pressure is 20–120 kPa absolute. Total pressure must exceed saturation pressure at dry bulb. These bounds do not cover fog, supersaturation or every industrial air process.

PsychroLib supplies the ASHRAE-based coefficients under the MIT license. Engivault deliberately rejects negative inferred moisture instead of imposing PsychroLib’s small positive humidity floor. Numerical checks include both phase branches, saturation limits and an independent energy-balance residual.

References

• ASHRAE Fundamentals 2025: sensible processing, dehumidification and airstream mixing (accessed 2026-09-08)
• PsychroLib ASHRAE-based psychrometric functions (accessed 2026-09-08)
• PsychroLib SI reference tests, version 2.5.0 (accessed 2026-09-08)