Humidity chart is useful for the solution of problems concerning condensation, vaporisation and air conditioning. At a given dry bulb temperature, value of humidity obtained from the humidity chart directly, is in terms of __________ humidity.

Solution (By Examveda Team)

A humidity chart (psychrometric chart) is a graphical tool used to study the properties of air-water vapor mixtures in processes like condensation, vaporisation, and air conditioning.

At a given dry bulb temperature, the value of humidity that is read directly from the vertical axis of the chart is the humidity ratio (also called absolute humidity), which represents the mass of water vapor per unit mass of dry air.

This corresponds to molal humidity when expressed in molar terms (moles of water vapor per mole of dry air).

Relative humidity is also present on the chart as curved lines and can be inferred or estimated, but it is not the direct vertical-axis value at a fixed dry bulb temperature.

Percentage humidity is a vague and non-standard term not used in technical interpretations of the chart.

Therefore, the value of humidity obtained directly at a given dry bulb temperature from the chart is in terms of molal humidity.

Correct Answer: Option A: Molal

Option B: Relative Humidity ❌

Relative humidity is shown on the psychrometric (humidity) chart as a series of curved lines, not as values that are directly read from the main axes.

To determine relative humidity at a specific dry bulb temperature, you need another parameter like wet bulb temperature, dew point, or enthalpy.

Hence, while relative humidity is present on the chart, it is not the direct value obtained from a dry bulb temperature alone.

Option C: Percentage Humidity ❌

Percentage humidity is a vague and imprecise term. It is sometimes used interchangeably with relative humidity, but that usage is technically incorrect.

The psychrometric chart does not use the label "percentage humidity" as a direct measure.

Also, the chart doesn't plot any axis explicitly labeled with this term, so it cannot be considered a correct direct reading.

Option D: None of These ❌

This is incorrect because Option A (Molal humidity) is valid.

The humidity chart allows direct reading of the humidity ratio or absolute/mass-based humidity (kg water vapor per kg dry air), which is conceptually aligned with molal humidity when expressed in moles.

Therefore, saying “none of these” ignores the valid and correct option available in the list.

Conclusion: The only correct and directly readable form of humidity from the chart (at a given dry bulb temperature) is molal humidity, making Option A the right answer.