How Does a Hygrometer Work?

"Hygrometer" is a general term for an instrument used to measure humidity. There are different types of hygrometers that work on different principles. One kind is the so-called sling psychrometer.

The sling psychrometer consists of two identical thermometers mounted side by side. One of the thermometers is fitted with a wick type of material fastened around the thermometer's bulb. This bulb with the wick material is called the "wet bulb" because, to use the psychrometer, the wick material is soaked with water while the other bulb is left dry. After the wick is soaked on the wet bulb side, the unit on which the two thermometers are mounted is spun or slung around through the air by a handle with a swivel attaching it to the thermometer unit. The "sling" part of the name refers to the action involved in using the instrument.

What happens next is something intuitively very familiar. To understand the principle, all we have to do is think of swimming. We're at the water's edge and considering going in, and we feel pretty warm even though there's a breeze. We go in the water and come out, and suddenly we're cold. The only thing that's different is that we were dry and now we're wet.

The phenomenon involved here is called evaporative cooling. With dry skin, we feel one temperature, and then, when wet---especially if there is a breeze---we are cooler. Water evaporating from a surface is a physical change that has a cooling effect. Water molecules going from the liquid phase on the skin surface to the vapor phase literally take heat energy with them in the process.

It turns out that at a given temperature and wind speed, how much of a cooling effect we experience is determined by the level of humidity in the air. On a day when the air is very dry, when we come out of the water we will be cooler than on a humid day with the same temperature and wind speed.

It's exactly the same phenomenon that makes the psychrometer work. Slinging the unit around through the air creates a breeze that has an evaporative cooling effect on the wet bulb thermometer, but no cooling effect on the dry bulb. Once the spinning stops, the wet bulb thermometer will most often show a reading lower than that of the dry bulb. How much of a difference in temperature there is will be determined by the humidity in the air. The more humid the atmospheric conditions, the less of a difference in the two temperatures. At 100 percent humidity, the two readings will be the same. This is because the air is already saturated and has no capacity to receive water molecules evaporating from the wick on the wet bulb thermometer. No evaporation means no cooling effect.

Once the two temperature readings are obtained, there are established charts that translate the differences between the wet bulb and dry bulb temperatures into percent humidity.