Humidity and Saturation Questions and Answers

Answer: A

Using dry air as the denominator gives a quantity that does not change when water is added or removed, which is exactly what a drying calculation needs. The moist air basis changes with every addition and is much less convenient.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the advantage. The dry air passes through a humidifier, drier or cooler unchanged in quantity, so it serves as a tie component: expressing humidity per kilogram of dry air means the denominator is constant through the unit and only the numerator changes. Referring it to moist air would make both terms vary and the algebra needlessly awkward.

Answer: C

It is a pressure ratio expressed as a percentage and reaches a hundred at saturation. The second option defines absolute humidity, which is a quite different quantity with different units. Both are quoted as percentages, which invites the confusion.

Answer: D

Correct answer: A-2, B-4, C-1, D-3

Four measures in two pairs. Absolute humidity is a mass ratio and molal humidity the same quantity in moles - both absolute amounts of water per unit of dry air. Relative humidity and percentage humidity are both fractions of saturation, the first taken on partial pressures and the second on humidities. The distinction between the last two is the subject of the next item.

Answer: A

Correct answer: False

They are close but not equal. Relative humidity is the ratio of partial pressures, \( p/p_s \); percentage humidity is the ratio of absolute humidities, \( Y/Y_s \). Because absolute humidity depends on \( p/(P-p) \) rather than on \( p \) alone, the two differ by a factor of \( (P-p_s)/(P-p) \) - negligible at low humidity and appreciable near saturation and at raised temperature.

Answer: B

Two divided by four point two four is about forty-seven per cent. The saturation pressure must be read at the actual air temperature, not at some standard condition. The ratio is dimensionless and independent of the total pressure.

Answer: C

Correct answer: the air becomes saturated on cooling

Cooling moist air at constant pressure leaves the water content unchanged while the saturation capacity falls, and the dew point is where the two meet. Cool it any further and water condenses out. The dew point is therefore a direct measure of the absolute humidity, which is why it stays constant under heating and why it is used to specify dryness in compressed air systems.

Answer: D

Both are true and the reason gives the mechanism: it is the local air temperature at the surface that matters, which is why a cold pipe sweats while the room as a whole remains far from saturated. Insulation raises the surface temperature above the dew point.

Answer: A

The dew point is by definition the temperature whose saturation pressure equals the actual partial pressure. Reading the steam table backwards from the partial pressure is the whole calculation. No other property of the air is needed for it.

Answer: B

Evaporation from the wick cools the bulb until the heat arriving by convection balances the latent heat leaving. Adequate air velocity over the wick is essential or the reading is wrong. A sling psychrometer supplies that movement by swinging.