Wet-bulb temperature (Twb)
What wet-bulb temperature (Twb) means, how it differs from dry-bulb and dew point, and why it matters for HVAC and evaporative cooling.
Wet-bulb temperature (Twb) is the temperature a thermometer would read if its bulb were covered with a wet wick and ventilated. Evaporation cools the bulb, so Twb is usually lower than dry-bulb temperature.
Why it matters
- Evaporative cooling limit: Twb indicates how far air can be cooled by evaporation at roughly constant pressure (important for evaporative coolers and cooling towers).
- Latent vs sensible intuition: Twb helps you reason about moisture and enthalpy changes on a psychrometric chart.
- HVAC measurements: many field workflows use Tdb + Twb to estimate other properties when RH sensors are unavailable.
Relationship to dry-bulb and dew point
For typical moist air conditions (not near freezing and at common pressures), these temperatures often satisfy:
dew point ≤ wet-bulb ≤ dry-bulb
At saturation (100% RH), all three are equal. When air is very dry, Twb can be much lower than dry-bulb.
Common pitfalls
- Pressure/altitude: psychrometric relationships depend on pressure. Use the correct pressure or altitude for your site.
- Sensor wetting/ventilation: a true wet-bulb reading requires a properly wetted wick and sufficient airflow.
- Near-freezing conditions: icing and measurement artifacts can affect readings and derived properties.
Using FluidTool
In the humid air tool, you can use Tdb + Twb as inputs and read the computed humidity ratio, dew point, enthalpy, and other properties. This is useful when you can measure a sling psychrometer or similar wet-bulb device.
Related
Dew Point
What dew point temperature (Tdp) means, how it relates to RH and humidity ratio, and why it matters for condensation and HVAC design.
RH vs Humidity Ratio (W)
Relative humidity (RH) is temperature-dependent; humidity ratio (W) is an absolute moisture measure. Learn when each matters on a psychrometric chart.