What device is primarily used to measure relative humidity?

The device primarily used to measure relative humidity is a hygrometer. A hygrometer specifically detects and measures the moisture content in the air, giving readings that directly indicate the relative humidity. This is essential in various applications, including HVAC systems, where maintaining appropriate humidity levels is critical for comfort and operational efficiency.

Although a sling psychrometer can also measure humidity, it does so indirectly by assessing the temperature difference between two thermometers—one with a dry bulb and one with a wet bulb—which reflects the moisture in the air. While it is a useful tool for humidity measurement, the hygrometer is more straightforward and commonly used for directly measuring relative humidity levels.

The thermocouple serves a different purpose in HVAC systems, as it measures temperature changes rather than humidity. Similarly, a calorimeter is designed to measure heat transfer rather than humidity levels. Therefore, the hygrometer is the correct choice as it is tailored specifically for the task of measuring relative humidity directly.