a psychrometer is a device that includes both a dry-bulb and a wet-bulb thermometer. a sling psychrometer requires manual operation to create the airflow over the bulbs, but a powered psychrometer includes a fan for this function. knowing both the dry-bulb temperature (dbt) and wet-bulb temperature (wbt), one can determine the relative humidity (rh) from the psychrometric chart appropriate to the air pressure.