What instrument is used to measure the temperature of hair dryers?

The instrument used to measure the temperature of hair dryers is a thermometer. This device is specifically designed to gauge temperature, making it essential for ensuring that hair dryers operate within safe and effective heat levels. Thermometers can provide immediate readings of the air temperature being emitted by the dryer, which is crucial for avoiding damage to the hair and ensuring client safety during styling.

The other instruments listed serve different purposes; a hygrometer measures humidity levels, a manometer measures pressure, and a barometer is used to measure atmospheric pressure. None of these would provide the necessary temperature readings required for hair dryers, highlighting why a thermometer is the appropriate choice.