Saturday, July 3, 2021

Wet and dry bulb hygrometer (a simple instrument for relative humidity test)

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Wet and dry bulb hygrometer (a simple instrument for relative humidity test)


The wet and dry bulb hygrometer is commonly used to measure the relative humidity in the testing laboratory and textile mill. This instrument has a very simple design. The relative humidity can be checked easily by using this hygrometer. This hygrometer doesn't need any kind of electric power supply. 

Structure of the wet and dry bulb hygrometer:

Basically, two mercury thermometers are used in this hygrometer. One thermometer feels the surrounding air temperature. This thermometer is called a dry bulb thermometer. Another mercury thermometer is used to measure the water temperature at the temperature of a dry bulb thermometer. This thermometer is called the wet-bulb thermometer. A sleeve of muslin cloth or wicking dip is fitted over the feeling end of the wet-bulb thermometer. The free end of this muslin sleeve dips in the reservoir or container filled with distilled water. The temperature measuring scale is mounted close to each thermometer. 
There are two types of wet and dry bulb hygrometers used in textile mills. 


1- Wall mounted wet and dry bulb hygrometer.


2 - Sling or whirling hygrometer( portable wet and dry bulb hygrometer)

Working principle of wet and dry bulb hygrometer: 


The working principle of both hygrometers is the same. A wall-mounted hygrometer gets mounted on the wall. The temperature may differ due to uneven airflow. Thus, the accuracy of the test of relative humidity affects. In this case, a hygrometer is mounted at each corner of the shed to obtain more accurate results. The wet and dry bulb thermometer readings are noted and differences between both thermometers temperatures are calculated. Now, the relative humidity table is consulted.


In the whirling hygrometer, the thermometers are mounted in a frame pivoted on the handle. This enables them to be whirled round at 2 or 3 revolutions per second so that the airspeed past the wet bulb is at least 15 feet per second. The whirling is stopped after 30 seconds. The wet-bulb temperature read immediately. This procedure is repeated three or four times until the minimum reading is obtained. Now, the difference between wet and dry bulb temperature is calculated and the relative humidity table is consulted.


Suppose that 
Dry bulb  thermometer temperature = 68 degree F
Wet-bulb thermometer temperature  = 61 degree F
Difference                                    = 7 degree F
Relative humidity from table             = 67 %

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