Psychrometrics – identifying the correct term: The temperature indicated by a thermometer at the instant when the moisture present in air first begins to condense is called the dew-point temperature, not the wet-bulb temperature. State whether the given statement (“…is called wet bulb temperature”) is correct or not.
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AYes
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BNo
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COnly at 100% relative humidity
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DDepends on barometric pressure only
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EApplicable only below 0°C
Answer
Correct Answer: No
Explanation
Introduction / Context:In air-conditioning and psychrometrics, three commonly used temperatures are dry-bulb temperature, wet-bulb temperature, and dew-point temperature. The question checks whether you can distinguish between dew-point and wet-bulb properly, because mixing these terms leads to serious design and control errors in HVAC calculations.
Given Data / Assumptions:
- Air–water vapour mixture at near-atmospheric pressure.
- Standard definitions for psychrometric properties.
- Accurate, ventilated thermometers used for wet-bulb measurement with a properly wetted wick.
Concept / Approach:Dew-point temperature is defined as the temperature at which air becomes saturated (relative humidity equals 100%) when cooled at constant moisture content, and condensation begins. Wet-bulb temperature is the equilibrium temperature attained by a wetted wick thermometer due to evaporative cooling into the surrounding airstream. These are different processes and usually yield different temperatures except at saturation.
Step-by-Step Solution:
Identify the phrase in the stem: “when the moisture present in it begins to condense.”Condensation onset during cooling at constant humidity ratio defines the dew-point temperature (T_dp).Wet-bulb temperature (T_wb) is reached by adiabatic saturation on a wetted wick; it reflects the air’s evaporative capacity, not the onset of condensation.Therefore the statement that this is “wet-bulb temperature” is false; the correct term is “dew-point temperature.”Verification / Alternative check:On a psychrometric chart, moving horizontally left at constant humidity ratio reaches the saturation curve at T_dp. In contrast, lines of constant T_wb are diagonally sloped; T_wb equals dry-bulb temperature only at 100% relative humidity.
Why Other Options Are Wrong:
- Yes: Incorrect—this confuses dew-point with wet-bulb.
- Only at 100% relative humidity: Even then, the correct name at condensation onset is still “dew-point.” At saturation T_wb equals dry-bulb, but the definition remains distinct.
- Depends on barometric pressure only: Both temperatures depend on moisture content; pressure is not the sole deciding factor.
- Applicable only below 0°C: Dew-point can be above or below 0°C; freezing is not required.
Common Pitfalls:Assuming wet-bulb indicates condensation; assuming dew-point equals wet-bulb at all states; ignoring the constant-humidity-ratio condition for dew-point measurement.
Final Answer:No