Psychrometrics: choose the correct definition of relative humidity At a given temperature and pressure, select the expression that correctly defines relative humidity of moist air. Choose the correct statement.
Mechanical Engineering
Heat Transfer, Refrigeration and Air Conditioning
Difficulty: Easy
Choose an option
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AMass of water vapour present in 1 m³ of dry air
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BMass of water vapour present in 1 kg of dry air
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CRatio of actual mass of water vapour per unit mass of dry air to that at saturation at the same T and p
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DRatio of actual mass of water vapour in a given volume of moist air to that in the same volume of saturated air at the same T and p
Answer
Correct Answer: Ratio of actual mass of water vapour in a given volume of moist air to that in the same volume of saturated air at the same T and p
Explanation
Concept/ApproachRelative humidity φ is defined as the ratio of the actual water vapour content to the maximum possible (saturation) water vapour content at the same temperature and pressure.
Equivalent formsφ = pv / pvs = ρv / ρvs at the same T and p, which corresponds to comparing equal volumes.
Why option D is correctOption D states the ratio using equal volumes at the same T and p, matching φ = ρv/ρvs.
Why others are incorrect
- Option A defines absolute humidity (kg/m³).
- Option B defines humidity ratio/specific humidity (kg/kg dry air).
- Option C compares per unit mass of dry air; that ratio equals the ratio of humidity ratios, not the standard definition of relative humidity.
Final AnswerRatio of actual mass of water vapour in a given volume of moist air to that in the same volume of saturated air at the same T and p