Discussion
Consider the following statements about psychrometry:
1. Humidity ratio is unchanged when air is heated without adding moisture.
2. Relative humidity falls when air is heated at constant humidity ratio.
3. Dew point rises when air is heated at constant humidity ratio.
Which of the statements given above is or are correct?
Answer: C
Correct answer: 1 and 2 only
Statement 1 is correct: heating adds no water, so the humidity ratio is unchanged. Statement 2 is correct: relative humidity is \( p_v/p_{sat} \), and heating raises \( p_{sat} \) steeply while \( p_v \) stays put, so the ratio falls.
Statement 3 is false. The dew point depends only on the actual vapour pressure, which is fixed by the humidity ratio - so heating at constant humidity ratio leaves the dew point exactly where it was. This is a useful check: heating changes relative humidity dramatically and dew point not at all.
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