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The psychrometer (or Wet & Dry bulb hygrometer) is an important instrument used for measuring the water vapour content (Relative Humidity) per unit of air at a given temperature.

The instrument is made up of two indentical thermometers, one a wet bulb, and the other a dry bulb. The wet bulb thermometer has it's bulb wrapped in a tight fitting muslin cloth material (wicking) which is soaked in distilled water. When the thermometers are ventilated the wet bulb temperature will be lower than the dry bulb temperature.

To understand how the psychrometer works we need to know a little more about Latent Heat and Evaporation.

Latent heat is the energy taken up by the air as water changes phase. In the case of the psychrometer the change is from liquid to vapour (Evaporation). Latent heat is the key to understanding many aspects of weather, as it is the main source of energy for thunder storms and hurricanes.

In the psychrometer, moving air molecules collide with the liquid water molecules in the damp wicking. This gives some of the water molecules enough energy to speed up and break away from the liquid as water vapour and a cooling effect is obtained. This is the same effect seen in the cooling of the human body by perspiration. A cooling effect is felt as air moves over the perspiration - Like standing in front of a fan on a hot day.

The graphic (Left) illustrates what happens during evaporation - the phase change from liquid to vapour. Fast-moving, warm air molecules collide with liquid water molecules. The water could be falling rain drops, a pond or damp wicking on a hygrometer. The collision transfers energy from the air molecules to the water molecules giving some of the water molecules the energy required to break away from the liquid as water vapour

The dryer the surrounding air is, the greater the rate of evaporation and thus the lower the temperature will be of the wet bulb . The rate of evaporation decreases as the moisture content of the air increases and approaches saturation. In addition, the saturation point increases rapidly as the temperture of the air rises. Thus cold air, while its moisture content is quite low, may be almost saturated in relation to the maximum amount of water vapor it can hold (high relative humidity). Cold air with a high relative humidity "feels" colder than dry air of the same temperature. This is because high humidity in cold weather increases the rate at which heat is conducted away from the body. Conversely, hot air with high relative humidity "feels" warmer than it actually is because of an increase in conduction of heat to the body, combined with a reduction in the cooling effect afforded by evaporation.

There are a few different types of Psychrometer available ranging from the simplest stationary device, through whirling type, designed to be spun round, to the electronic type with built in fans etc.

Materials Testing Equipment can supply all the variants available, for further information on this and other types of atmospheric monitoring devices, please contact or sales team by the methods below.

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