Psychrometrics fundamentals: The wet-bulb temperature (WBT) is the temperature indicated by a thermometer whose bulb is covered with a wetted wick and exposed to moving air, reflecting cooling by evaporation.

Introduction / Context:
Wet-bulb temperature (WBT) is a key psychrometric property used in HVAC, meteorology, and cooling tower analysis. It reflects the combined effect of temperature and humidity through evaporative cooling at the thermometer bulb.

Given Data / Assumptions:

• Thermometer bulb is wrapped with a wetted wick (usually clean water).
• Air moves past the bulb (natural or forced draft) to enable evaporation.
• Evaporation rate depends on air humidity and velocity.

Concept / Approach:
When water evaporates from the wick, it absorbs latent heat from the bulb. The thermometer cools below the ambient dry-bulb temperature until an energy balance is reached between latent heat needed for evaporation and sensible heat supplied from the air to the bulb. The stabilized reading is the wet-bulb temperature.

Step-by-Step Solution:

Verification / Alternative check:
On a psychrometric chart, lines of constant wet-bulb temperature are close to lines of constant enthalpy for unsaturated air. Sling psychrometers and aspirated psychrometers exploit the same principle.

Why Other Options Are Wrong:

• “Not affected by moisture” is incorrect; WBT explicitly depends on humidity.
• “Begins to condense” describes dew-point conditions, not WBT.
• “Dry cotton” prevents evaporation and measures dry-bulb temperature.

Common Pitfalls:
Poor ventilation around the bulb, dirty water, or a dry wick will lead to inaccurate WBT readings.

Final Answer:
Its bulb is covered by a wet cloth (wick) and exposed to air flow