Which device is primarily used to measure the moisture content of air?

The device primarily used to measure the moisture content of air is a psychrometer. A psychrometer consists of two thermometers: one with a dry bulb that measures air temperature and another with a wet bulb that indicates the temperature of the air after it has been moistened. By using the readings from both thermometers, the relative humidity of the air can be calculated. This is crucial for various applications, such as HVAC systems, where monitoring the moisture level helps to maintain comfort and prevent issues like mold growth.

While other tools are used in relation to atmospheric conditions, they serve different purposes. A barometer measures atmospheric pressure, which does not directly gauge humidity levels. A thermometer measures temperature, and a hygrometer indeed measures humidity directly but employs different technologies and principles than a psychrometer. Therefore, the psychrometer stands out for its specific capability to assess both temperature and humidity, making it the correct choice in this context.