Which instrument uses two thermometers to determine relative humidity?

Which instrument uses two thermometers to determine relative humidity?

• A. Barometer
• B. Thermometer
• C. Psychrometer
• D. Hygrometer

Answer: (C)

Relative humidity tells us how close the air is to being saturated with water vapor. A psychrometer uses two thermometers—one read as a plain air temperature (dry-bulb) and another with a wick kept wet (wet-bulb). As water evaporates from the wet wick, it cools the wet-bulb thermometer. The rate of evaporation depends on how much moisture is already in the air: in dry air, evaporation is faster and the wet-bulb reads much cooler than the dry-bulb; in humid air, evaporation slows, so the two readings are closer together. By comparing these two temperatures (often with a chart or calculation), you determine the relative humidity. The other instruments measure different things: a barometer gauges air pressure, a thermometer measures temperature, and a hygrometer measures humidity directly—often with a single sensor or material response—not by comparing two thermometer readings.

Relative humidity tells us how close the air is to being saturated with water vapor. A psychrometer uses two thermometers—one read as a plain air temperature (dry-bulb) and another with a wick kept wet (wet-bulb). As water evaporates from the wet wick, it cools the wet-bulb thermometer. The rate of evaporation depends on how much moisture is already in the air: in dry air, evaporation is faster and the wet-bulb reads much cooler than the dry-bulb; in humid air, evaporation slows, so the two readings are closer together. By comparing these two temperatures (often with a chart or calculation), you determine the relative humidity.

The other instruments measure different things: a barometer gauges air pressure, a thermometer measures temperature, and a hygrometer measures humidity directly—often with a single sensor or material response—not by comparing two thermometer readings.