How Does a Sling Psychrometer Work?

Relative Humidity

A psychrometer is a simple device for measuring the relative humidity of the air. At any temperature, the air is able to absorb a certain amount of water vapor. The relative humidity is a measurement of the portion of the total possible water vapor that is currently in the air. When the air is nearly completely filled with vapor, it feels muggy if the air is hot or damp if it is cold.

Principles

A sling psychrometer measures the humidity by measuring the cooling effect of evaporation. Water is constantly evaporating into the air and condensing out of it. When that water evaporates, it takes some of the heat from whatever it is evaporating off of with it, cooling that object down. That is why we sweat in the heat---to cool down. Water blowing past aids this evaporation, causing the object to cool more quickly when the air has a high relative humidity, however, water does not evaporate as easily. That is why humid days feel hotter than dry days---sweat doesn't evaporate off the body, and so the body doesn't cool down. The sling psychrometer calculates humidity by measuring how much evaporation contributes to cooling.

Process

A sling psychrometer consists of two thermometers. One is left dry to record the true temperature, while the other is covered in water. The wet thermometer is then swung around, causing some of the water to evaporate. The lower the relative humidity, the more water will evaporate and the more the wet thermometer will cool down. By comparing the temperatures of the wet and dry thermometers to a chart called a psychometric chart, a scientist can determine how humid the air is.