Psychrometers

A psychrometer is a hygrometer used to measure relative humidity by comparing temperatures recorded by two thermometers. One thermometer, the dry-bulb, is exposed to the surrounding air, while the second, the wet-bulb, has its bulb wrapped in a water-soaked wick. Evaporation from the wick cools the wet-bulb, and the rate of cooling depends on the air’s humidity. The difference between the wet-bulb and dry-bulb readings is related to the ambient moisture content.

There are two common forms. A sling psychrometer consists of the two thermometers mounted together and whirled rapidly in the air to promote evaporation. An aspirated or ventilated psychrometer uses a small fan to force a controlled flow of air over the bulbs, providing more stable readings. Maintaining a saturated wick and shielding the instruments from radiant heat improves accuracy, and readings are typically taken at standard conditions or with local air pressure taken into account.

Relative humidity is determined from the temperature difference using a psychrometric chart or corresponding equations that incorporate atmospheric pressure. From this, additional humidity parameters such as dew point and specific humidity can be derived. Psychrometers remain common in field meteorology, HVAC commissioning and diagnostics, greenhouse and agricultural monitoring, and education.

Advantages include simplicity and no reliance on electronics; disadvantages involve sensitivity to airflow, radiant heat, water quality, and the need for careful calibration and proper technique to obtain accurate results.