Wet-bulb temperature (Twb)
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.