Psychrometry and humidity measurement in process engineering:\nWhat parameter does a psychrometer directly determine when used with gas streams in drying or air-conditioning operations?

Introduction / Context:
Psychrometry deals with the thermodynamic properties of gas–vapour mixtures, most commonly air–water vapour. In chemical and mechanical engineering, a psychrometer is a simple, robust instrument used to assess how much vapour is carried by a gas stream. This question checks whether you can link the instrument to the property it is designed to measure in day-to-day plant practice.

Given Data / Assumptions:

• A psychrometer uses two thermometers: a dry-bulb and a wet-bulb.
• The gas is well mixed and flows adequately over both bulbs during measurement.
• Standard psychrometric relations (charts or formulas) connect the readings to humidity measures.

Concept / Approach:
The wet-bulb thermometer cools by evaporation; the rate of evaporation (and therefore the temperature depression from dry-bulb to wet-bulb) depends on the vapour content of the gas. By combining both readings, one can determine relative humidity, humidity ratio (kg water per kg dry gas), and other related properties such as enthalpy and dew point. The essential outcome is the humidity of gases; other properties (dew point, enthalpy) are secondary results derived from the same data.

Step-by-Step Solution:

Verification / Alternative check:
A sling or aspiration psychrometer yields consistent readings in the field; values map to standard ASHRAE or chemical-engineering psychrometric charts for humidity ratio and relative humidity, confirming the measurement’s purpose.

Why Other Options Are Wrong:

• Moisture content of solids: Requires oven drying or moisture analysers, not a psychrometer.
• Water of crystallisation: Determined by thermal analysis or chemical tests.
• Hygroscopic nature of solids: Inferred from sorption isotherms, not directly by psychrometry.
• Dew point of steel surfaces only: Psychrometers are general gas-phase instruments, not surface-specific tools.

Common Pitfalls:
Confusing humidity (gas property) with moisture in solids; also assuming a single thermometer can give humidity without a wet-bulb reference.

Final Answer:
Humidity of gases