In applied thermodynamics and HVAC testing, humidity of ambient air is commonly determined using which field instrument that compares dry-bulb and wet-bulb temperatures while being whirled through the air?

Introduction / Context:
Humidity measurement is fundamental in air-conditioning design, drying operations, meteorology, and environmental monitoring. Many instruments exist to analyze air composition, but only a few are practical for routine field measurement of moisture content (relative humidity). The classic and widely taught tool is the sling psychrometer, which directly links temperature depression to humidity through psychrometric relationships.

Given Data / Assumptions:

• The task is routine field measurement of atmospheric humidity.
• Low-cost, portable equipment is implied by “determined by.”
• We seek the most appropriate, standard instrument from the options given.

Concept / Approach:
A sling psychrometer has two thermometers: one measures dry-bulb temperature, the other has a wetted wick and measures wet-bulb temperature. Whirling the device promotes evaporation from the wet wick. Evaporation cools the wet-bulb thermometer; the temperature depression (dry-bulb minus wet-bulb) depends on the moisture already in the air. Using psychrometric charts or formulae, operators convert the measured pair into relative humidity, humidity ratio, and dew point.

Step-by-Step Solution:
Hold the instrument and wet the wick with clean water.Whirl the psychrometer at steady speed for a prescribed time.Read stabilized dry-bulb and wet-bulb temperatures.Use a psychrometric chart/table to compute relative humidity and other properties.

Verification / Alternative check:
Dew-point hygrometers and electronic capacitive sensors can also measure humidity, but for textbooks and basic labs, the sling psychrometer remains the canonical method due to simplicity, low cost, and traceable calculations. Its readings can be cross-checked against a digital thermo-hygrometer for validation.

Why Other Options Are Wrong:
Chromatograph / mass spectrometer: Laboratory analyzers, not practical field humidity meters.Polarimeter / refractometer: Optical concentration tools for optically active solutions or refractive index, not atmospheric humidity.

Common Pitfalls:
Insufficient wetting of the wick, contaminated water, or inadequate air velocity around the wet bulb cause errors. Also, using warm hands to hold bulbs can bias readings. Calibrating thermometers and shielding from radiant heat improves accuracy.

Final Answer:
Sling psychrometer