Question

At ________ relative humidity, the dry bulb temperature, wet bulb temperature and dew point temperature are same.

• A. 10 percent
• B. 50 percent
• C. 100 percent
• D. Difficult to predict

Hint:

At $100\%$ relative humidity, the air is completely saturated. Because no water can evaporate into saturated air, there is no evaporative cooling, making $\text{DBT} = \text{WBT} = \text{DPT}$.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Psychrometrics is the study of the thermodynamic properties of moist air.
Key parameters include:
- Dry Bulb Temperature (DBT): The actual air temperature measured by a standard thermometer.
- Wet Bulb Temperature (WBT): The temperature measured by a thermometer whose bulb is covered by a wet wick, reflecting cooling caused by water evaporation.
- Dew Point Temperature (DPT): The temperature at which air must be cooled to become saturated with water vapor, causing condensation.
Detailed Explanation:
- At less than $100\%$ relative humidity, the air is unsaturated. This allows water to evaporate from the wet wick of a wet-bulb thermometer, cooling it below the dry-bulb temperature ($\text{WBT} < \text{DBT}$).
- At $100\%$ relative humidity (C), the air is completely saturated with water vapor and cannot hold any more moisture.
- Because no water can evaporate from the wet wick under these conditions, no evaporative cooling occurs, and the wet-bulb temperature equals the dry-bulb temperature.
- Additionally, because the air is already saturated, any further cooling would cause immediate condensation, meaning this temperature is also the dew point temperature.
Therefore, under fully saturated conditions ($100\%$ RH):
\[ \text{DBT} = \text{WBT} = \text{DPT} \]

Step 2: Final Answer:
The dry bulb, wet bulb, and dew point temperatures are identical at 100 percent relative humidity.