What Is a Psychrometer

A psychrometer is a useful instrument to keep in the house. It measures the quantity of water vapor in the air or relative humidity. Relative humidity refers to the moisture content in the air compared to how much moisture the air can hold at a given temperature. Knowing the relative humidity is helpful in understanding the weather and what to expect. It is also helpful if you keep several indoor plants.

How a Psychrometer Works

Psychrometers use the principles of evaporation to work. When water evaporates from a surface, a cooling effect occurs on the surface. As the water evaporates, the surface cools because the evaporating water carries heat with it from the surface. Moreover, not all the water will evaporate from a given surface. This is because only a certain amount of water can be absorbed into the air. The more humid the air is, the greater the quantity of water that must be left on the surface. Low humidity is associated with high evaporation rates and a greater cooling effect on the surface. During high humidity periods, evaporation of water is reduced, and there is less of a cooling effect on the surface.

Design of the Psychrometer

The instrument consists of a dry-bulb and wet-bulb thermometer. The dry bulb measures temperature. It is left as it is, without any covering, for maximum exposure to air. The wet-bulb must be kept wet. It is covered with some wet cloth or dipped in water until it is required for use. At the desired time, you remove the wet bulb from the water but retain the wet cloth on it.

Readings of both bulbs are taken immediately and also at intervals of a few minutes. Note is made of the cooling temperatures on the wet-bulb. The temperature of the wet-bulb will always be lower than the dry-bulb due to the evaporating water which creates a cooling effect. A comparison of the two temperatures gives you the relative humidity of the air.

Sling Psychrometer

The sling psychrometer consists of a simple design. Two thermometers are secured onto a frame. They should be placed far apart so as to prevent collisions when the frame starts to swing. The frame is further secured onto a handle using a chain or some string. Prior to use, one of the thermometers must be covered with a wet cloth. The other thermometer is left exposed to measure room temperature. The frame is then subjected to circular motion for about 3 minutes. When the temperatures of both bulbs are read, the wet-bulb will register a lower reading. This is because of the cooling effect caused by evaporation.

Aspirating Psychrometer

The aspirating psychrometer has a more complicated design. It is a mechanized unit powered by battery. The unit incorporates a fan that generates cool air for the wet-bulb. You must also cover the wet-bulb with a wet cloth prior to use. When ready to use, you turn on the fan for about 3 minutes to assist evaporation. Readings are taken and then compared. This model generally gives more accuracy than the Sling type. This is partly because of less interference with body heat or moisture.