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TECHNICAL TOOL
Wet Bulb Calculator
Calculate thermodynamic wet-bulb temperature from dry bulb and RH or dew point, with local pressure for evaporative cooling, cooling-tower and coil-inlet reviews.
CALCULATED AIR STATE
- Thermodynamic wet bulb
- 19.47 °C
- Dew / frost point
- 16.70 °C
- Humidity ratio · dry-air basis
- 11.90 g/kg
- Absolute humidity · vapour density
- 13.82 g/m³
Example: 25°C dry bulb, 60% RH, 101.325 kPa.
Check all inputs. The temperatures and pressure must describe a physical air state in the supported range.
ENGINEERING USE
Use wet bulb as a process reference.
Evaporative cooling
Compare dry bulb with wet bulb to understand the available evaporative cooling potential. Actual leaving-air temperature also depends on equipment effectiveness and water conditions.
Cooling towers
Record entering-air wet bulb alongside water temperatures and flow. A wet-bulb calculation alone does not predict tower approach or heat-rejection capacity.
Coil inlet conditions
Pair dry bulb and wet bulb when reviewing HVAC coil duty. Add airflow and target leaving-air state before calculating moisture or energy removal.
DEFINITIONS
Three different measurements and calculations
| Term | Meaning for this task |
|---|---|
| Thermodynamic wet bulb | An air-state property defined by adiabatic saturation at constant pressure. This is the calculated output here. |
| Ventilated psychrometer reading | A wetted sensor exchanges heat and moisture with moving air. Wick condition, airflow, radiation and instrument corrections matter; the reading is not automatically identical to thermodynamic wet bulb. |
| WBGT | Wet-bulb globe temperature uses additional environmental measurements, including globe temperature. This calculator does not calculate WBGT or assess heat stress or health. |
Calculation method and sources
Tdry + RH / Tdew + p → pᵥ → w → Twet
The shared air-property library solves the ASHRAE thermodynamic water/ice energy balance for wet bulb, following PsychroLib. At 25°C, 60% RH and 101.325 kPa, wet bulb is 19.47°C, dew point 16.70°C and humidity ratio 11.90 g/kg dry air.
ASHRAE Fundamentals (2017), Chapter 1, equations 33 / 35 · PsychroLib
Saturation pressure uses ice at or below 0.01°C and liquid water above; the wet-bulb energy balance changes phase at 0°C. Near freezing, check the instrument’s phase convention. Dew/frost-point input uses the same ice/water convention.
Have a measured wet-bulb reading?
For measured dry bulb T and ventilated wet bulb Tw, record local station pressure P, not a sea-level-corrected weather value. The NWS approximation below uses °C and hPa; it illustrates the reverse calculation, from measured temperatures to RH. It is separate from the thermodynamic solver above.
es = 6.112 × exp(17.67T / (T + 243.5))
ew = 6.112 × exp(17.67Tw / (Tw + 243.5))
e = ew − P × (T − Tw) × 0.00066 × (1 + 0.00115Tw)
RH = 100e / es
ew = 6.112 × exp(17.67Tw / (Tw + 243.5))
e = ew − P × (T − Tw) × 0.00066 × (1 + 0.00115Tw)
RH = 100e / es
| Inputs | Intermediate pressures | Calculated RH |
|---|---|---|
| T = 30°C; Tw = 25°C; P = 1013.25 hPa | es ≈ 42.46 hPa; ew ≈ 31.67 hPa; e ≈ 28.23 hPa | ≈ 66.5% |
NWS calculation reference — This is a rounded calculation example, not a field measurement. Use the method and corrections specified for your instrument.
Check wick wetting, ventilation, radiation shielding and simultaneous readings before interpreting a small difference. Record the pressure source with the result. WMO humidity measurement guidance, Chapter 4.
Check the same air state on a chart
On a chart for the actual pressure, the dry-bulb and wet-bulb lines identify the air state. Read RH at that point. To find dew point, follow constant humidity ratio toward saturation; holding dry bulb constant would change the moisture content. Check units and measurement conditions before comparing a chart with a field psychrometer.
What this tool does not decide
It does not select cooling equipment, establish tower performance, calibrate a psychrometer, or determine worker exposure limits. Use measured airflow, actual equipment data and the required process conditions for equipment review.
NEXT / PROJECT REVIEW
Bring the operating conditions to an equipment review.
Share inlet and target air states, room volume, outdoor airflow, moisture sources and installation constraints.
Review project requirements