Whirling Hygrometer

A whirling hygrometer, or sling psychrometer, is a simple yet reliable instrument used to measure relative humidity in various field conditions. It features two thermometers mounted side by side—one dry bulb and one wet bulb wrapped in a moist wick. When the device is manually whirled, airflow causes the wet bulb to cool through evaporation, creating a temperature difference between the two bulbs. This temperature gap, known as the wet-bulb depression, is then compared to psychrometric charts to determine the relative humidity. Compact, durable, and easy to use, it remains a trusted tool for coating inspectors, HVAC technicians, and environmental monitoring.