Psychrometry — when is wet-bulb depression zero? At what relative humidity does the wet-bulb temperature equal the dry-bulb temperature (i.e., zero wet-bulb depression)?

Introduction / Context:
Wet-bulb depression is the difference between dry-bulb and wet-bulb temperatures, indicating the air’s capacity to absorb additional moisture by evaporation. It is widely used in comfort analysis and evaporative cooler performance.

Given Data / Assumptions:

• Standard atmospheric pressure and typical psychrometric relationships.
• Well-ventilated sling or aspirated wet-bulb measurements.

Concept / Approach:
At saturation (relative humidity = 1.0), the air cannot accept more moisture by evaporation; thus the evaporative cooling from a wetted wick does not occur. Consequently, the wet-bulb temperature equals the dry-bulb temperature, and the wet-bulb depression is zero.

Step-by-Step Solution:
Define RH = p_v / p_vs(T_db).At RH = 1.0, p_v = p_vs and the air is saturated.No evaporation from the wick → no latent cooling → T_wb = T_db.Hence wet-bulb depression (T_db − T_wb) = 0.

Verification / Alternative check:
On a psychrometric chart, saturation states lie on the 100% RH curve where constant dry-bulb and constant wet-bulb lines intersect, confirming equality of temperatures at saturation.

Why Other Options Are Wrong:
(a), (b), (c), and (e) are subsaturated states; evaporation occurs and T_wb is lower than T_db, producing a positive wet-bulb depression.

Common Pitfalls:
Assuming a small wet-bulb depression always implies near-comfort conditions; temperature level and enthalpy also matter.

Final Answer:
1.0