Which instrument uses dry-bulb and wet-bulb temperatures to determine humidity?

Which instrument uses dry-bulb and wet-bulb temperatures to determine humidity?

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

Answer: (C)

Using dry-bulb and wet-bulb temperatures to determine humidity is done with a psychrometer. It consists of two thermometers mounted together: the dry-bulb thermometer reads the ambient air temperature, while the wet-bulb thermometer has its bulb wrapped in a moist wick. As air flows over the wet wick, evaporation cools the wet-bulb, so its reading drops below the dry-bulb temperature. The rate of evaporation—and thus how much the wet-bulb is cooled—depends on how much moisture is already in the air. When humidity is low, evaporation is rapid and the wet-bulb is much cooler than the dry-bulb; when humidity is high, evaporation slows and the two readings are closer together. By comparing these two temperatures—often with a psychrometric chart or a simple calculation—you can determine relative humidity and other humidity properties. Other instruments measure different quantities: a hygrometer generally measures humidity directly, not by comparing two temperatures; a barometer measures air pressure; an anemometer measures wind speed.

Using dry-bulb and wet-bulb temperatures to determine humidity is done with a psychrometer. It consists of two thermometers mounted together: the dry-bulb thermometer reads the ambient air temperature, while the wet-bulb thermometer has its bulb wrapped in a moist wick. As air flows over the wet wick, evaporation cools the wet-bulb, so its reading drops below the dry-bulb temperature. The rate of evaporation—and thus how much the wet-bulb is cooled—depends on how much moisture is already in the air. When humidity is low, evaporation is rapid and the wet-bulb is much cooler than the dry-bulb; when humidity is high, evaporation slows and the two readings are closer together. By comparing these two temperatures—often with a psychrometric chart or a simple calculation—you can determine relative humidity and other humidity properties.

Other instruments measure different quantities: a hygrometer generally measures humidity directly, not by comparing two temperatures; a barometer measures air pressure; an anemometer measures wind speed.