Psychrometric limit: At 100% relative humidity, the wet-bulb temperature is __________ the dry-bulb temperature.

Introduction / Context:Wet-bulb temperature (WBT) reflects evaporative cooling potential. At saturation (RH = 100%), air cannot accept more moisture, so evaporation from a wetted wick ceases and WBT converges to dry-bulb temperature (DBT).

Given Data / Assumptions:

• Air–water vapour mixture at atmospheric pressure.
• Thermometer bulb properly wetted and ventilated.
• State point lies on the saturation curve (RH = 100%).

Concept / Approach:At saturation, the partial pressure of water vapour equals the saturation pressure at that temperature. No net evaporation occurs from the wick, eliminating evaporative cooling. Therefore, WBT = DBT at the saturation state.

Step-by-Step Solution:

Verification / Alternative check:On a psychrometric chart, lines of constant WBT meet the saturation curve exactly at the same point as DBT, confirming equality at RH = 100%.

Why Other Options Are Wrong:“Lower” or “higher” occur only for unsaturated air; “unrelated” and “always 0°C” are physically incorrect.

Common Pitfalls:Assuming some evaporation persists at 100% RH; in ideal measurement, none does.

Final Answer:the same as