Consisting of a dry bulb and a wet bulb thermometer with a crank handle. The muslin was moistened when in use. The sling psychrometer would be rotated for 2 to 3 minutes to generate airflow around the thermometers. This enables water in the muslin to evaporate and the temperature of wet bulb thermometer will be reduced. Based on the dry bulb and wet bulb temperature, the air humidity could be calculated using the conversion tables or humidity slide rule.