What instrument measures the moisture content of air or gas?

The instrument that measures the moisture content of air or gas is the hygrometer. This device is specifically designed to assess humidity levels, which is the amount of water vapor present in the air. Hygrometers can come in various types, including analog, digital, and even psychrometers, each utilizing different methods to measure humidity.

Understanding the importance of monitoring humidity is crucial, especially in substation environments where moisture levels can affect equipment efficiency, performance, and lifespan. Maintaining the proper humidity levels can also play a vital role in electrical safety and reliability.

The other instruments in the list serve different purposes: a thermometer measures temperature, a barometer measures atmospheric pressure, and a hydrometer measures the specific gravity of liquids, which includes the density of a liquid relative to water, not air or gas. Hence, their functions do not pertain to measuring moisture in the air.