A psychrometer is used to measure which property of air?

• A. Temperature
• B. Pressure
• C. Humidity
• D. Dew point

Answer: (C)

A psychrometer is designed to determine how much moisture is in the air by using evaporative cooling. It has two thermometers: a dry-bulb thermometer that measures the air’s actual temperature, and a wet-bulb thermometer whose bulb is wrapped in a wet wick. The rate of evaporation from the wet wick depends on how much moisture the air can still absorb. In drier air, evaporation happens more quickly, cooling the wet-bulb more and making the difference between the two readings larger. In moister air, evaporation slows down, the wet-bulb doesn’t cool as much, and the readings are closer together. That difference is used, often with a psychrometric chart or calculations, to determine relative humidity. From those humidity readings you can also derive dew point and other properties, but the instrument’s primary purpose is to measure humidity, not directly measure temperature, pressure, or dew point.