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Psychrometer is a device meant for determining relative humidity in the atmosphere by the reading and comparison of two thermometers: i.e. the dry-bulb and wet-bulb air temperatures. The psychrometer is a type of hygrometer. A psychrometer possesses many different uses both in the home and industry. Besides measuring relative humidity and weather forecasting, it can be also used for air conditioning maintenance and relative humidity levels for manufacturing. One must take necessary precautions during the use of a psychrometer. They must avoid heating the thermometer or placing it in extreme weather. This might result in giving a misleading reading of temperatures. Psychrometers come in two types:
Sling psychrometer – in this type of psychrometer, the thermometers are attached to a grip or span of rope and are whirled around in the air for some time. It is used for field computations. It can now be substituted via a more suitable electronic sensor.
Whirling psychrometer – in this type two thermometers are fixed vertically into an instrument that primarily looks like a ratchet or football rattle.
Psychrometers are made from stainless steels that are generally resistant to corrosion.
Working of Psychrometer
As the water present on the surface evaporates, it takes out heat, which in turn, cools down the surface significantly. While doing this, air holds only a certain amount of water, after which it becomes saturated, and at this point, water will not evaporate as rapidly.
The design of the psychrometer utilizes these principles in order to determine relative humidity. When the surrounding air is comparatively dry, then more moisture will try to evaporate from the wet bulb, and the drop in temperature will be higher. Whereas, if air is more humid, the rate of evaporation will decrease, and the drop in temperature will also be lowered. Thus, a psychrometer helps in determining the humidity by correlating temperature movement.
Psychrometer Design
Though each model will vary slightly, the fundamental sling psychrometer design is mainly composed of two thermometers contained within a plastic housing. A wick surrounds the bulb of one of the thermometers. The wick must be soaked in water before use. This thermometer is known as the ‘Wet-Bulb Thermometer.’ The other thermometer remains exposed to air and is referred to as the ‘Dry-Bulb Thermometer.’ At the end of the thermometer housing, a pivoting handle is present which is used to rotate the psychrometer.
Sling Psychrometer
A sling psychrometer comprises a wet-bulb thermometer and a dry-bulb thermometer. A dry-bulb thermometer is mainly an ordinary thermometer, whereas a wet-bulb thermometer is a thermometer that comprises its bulb wrapped in cloth and moistened by distilled water.
When a sling psychrometer is whirled around in the air, then moisture will evaporate from the wet-bulb thermometer, resulting in a reduction in its temperature based on the humidity of the air it is exposed to. The greater the humidity, the lesser the rate of evaporation and thus higher will be the recorded temperature. Wet-bulb temperatures are as same as the dry-bulb temperatures at a relative humidity of 100%, but otherwise it will be lesser than the dry-bulb temperatures because of the cooling effect of evaporation. The two thermometers might give steady-state readings after a few minutes.
A sling psychrometer can also be used in determining the physical and thermal properties of moist air via using standard tables and charts. Generally it is used to determine relative humidity. Some sling psychrometers also include mechanisms to allow relative humidity to be calculated directly without the need of referring to tables.
Psychrometric charts are complicated graphs that mainly represent the dry-bulb temperature, wet-bulb temperature, relative humidity, specific volume, dew point temperature, humidity ratio and enthalpy of moist air at known atmospheric pressure. The state of moist air can also be used in determining any two of these properties (like those of the wet-bulb temperature and dry-bulb temperature that can be read on a sling psychrometer) from which all other properties can then be determined.
Uses of Psychrometer
Psychrometers help in measuring the humidity in the general environment or specific areas (like a chamber or structure). Psychrometers are mainly used in laboratory settings in order to determine detailed measurements for experiments and chemical reactions.
A stationary psychrometer is the very common and is generally used in some forms of industrial equipment, whereas a mobile psychrometer, is referred to as a sling psychrometer, comprising of two thermometers attached to a handle and is whirled in the air for a few minutes to calculate the relative humidity in the field.
Conclusion
Many devices are so effective that there is no point in trying to improve them, and it is no doubt true for Sling Psychrometers. These instruments have a direct use, battery-free, and an inexpensive way to calculate the relative humidity. Here we come to an end of this topic. We hope that you were able to grasp a clear concept of psychrometer, sling psychrometer, their working, design and uses.
