For unsaturated air (relative humidity less than 100%), how does the dew-point temperature compare with the wet-bulb temperature?
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AEqual to the wet-bulb temperature
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BLess than the wet-bulb temperature
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CGreater than the wet-bulb temperature
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DUnrelated; could be higher or lower randomly
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EAlways equal to the dry-bulb temperature
Answer
Correct Answer: Less than the wet-bulb temperature
Explanation
Introduction / Context:Interpreting psychrometric measurements requires knowing the order of characteristic temperatures. In unsaturated air, these temperatures follow a consistent inequality that aids quick checks and diagnostics.
Given Data / Assumptions:
- Air–water vapour mixture at RH < 100%.
- Proper instrument use and steady conditions.
Concept / Approach:For unsaturated air, the standard relationship is: dew-point temperature (DPT) < wet-bulb temperature (WBT) < dry-bulb temperature (DBT). Dew point is the temperature at which saturation just occurs for the existing moisture content; wet-bulb reflects adiabatic evaporation potential; dry-bulb is the actual air temperature.
Step-by-Step Solution:
Recognize unsaturated condition → potential for evaporation from the wet-bulb.Evaporation cools the wet-bulb → WBT below DBT.Dew point is colder still because cooling to DPT is required for saturation at current moisture content.Hence DPT < WBT < DBT.Verification / Alternative check:Plot a point inside the psychrometric chart’s field (not on the saturation curve): lines of constant DPT lie left of the state point, and constant WBT lines intersect with WBT greater than DPT but less than DBT.
Why Other Options Are Wrong:
Equality holds only at saturation (RH = 100%).“Greater than” contradicts established ordering in unsaturated air.“Unrelated” is incorrect; the values are physically linked.
Common Pitfalls:Instrument lag or poor ventilation around the psychrometer wick can distort WBT readings; still, the theoretical order remains DPT < WBT < DBT when RH < 100%.
Final Answer:
Less than the wet-bulb temperature